Chapter 16 — Digestion And Absorption

What is digestion and absorption? Digestion is the process by which complex food materials are broken down into smaller, absorbable molecules by mechanical and chemical means. Absorption is the subsequent uptake of these small molecules (nutrients) into the blood and lymph for distribution to body cells.

Types of digestion

• Mechanical (physical) digestion: Chewing in the mouth, churning in the stomach, and segmentation in the intestine that reduce particle size and increase surface area for enzymes.
• Chemical digestion: Enzyme-catalyzed hydrolysis of macromolecules — carbohydrates, proteins and lipids — into monosaccharides, amino acids and fatty acids + monoglycerides respectively.
• Extracellular vs intracellular: In humans digestion is mainly extracellular (occurs within lumen of alimentary canal) while some organisms also use intracellular digestion (within cells).

Major organs and their roles

• Mouth: Mastication (mechanical) and salivary amylase begin starch breakdown.
• Stomach: Acidic environment (HCl) unfolds proteins; pepsin begins proteolysis; muscular churning creates chyme.
• Small intestine (duodenum, jejunum, ileum): Major site of chemical digestion and nutrient absorption. Pancreatic enzymes (amylase, lipase, proteases) and bile (emulsification of fats) act here. Brush border enzymes (maltase, sucrase, lactase, peptidases) complete digestion.
• Large intestine: Water and mineral absorption; fermentation of some residues by gut bacteria.

How absorption happens

• Structural adaptations: Small intestine has circular folds (plicae circulares), villi, and microvilli (brush border) to massively increase absorptive surface area.
• Modes of transport:
• Simple diffusion — for small lipophilic molecules (some fatty acids).
• Facilitated diffusion — for some monosaccharides via carrier proteins.
• Active transport — energy-dependent uptake of glucose (SGLT co-transport with Na+), amino acids against concentration gradients.
• Endocytosis — e.g., neonatal absorption of immunoglobulins in some mammals.
• Fats: Emulsified by bile salts, hydrolysed by lipase to fatty acids and monoglycerides which form micelles, diffuse into enterocytes, are re-esterified to triglycerides, packaged as chylomicrons and transported into lacteals (lymph).

Why digestion and absorption are important

They supply the body with fuel (glucose, fatty acids), building blocks (amino acids) and essential nutrients (vitamins, minerals) for energy, growth, tissue repair and metabolic functions.

Clinical and physiological notes

• Enzyme activity depends on pH and temperature (e.g., pepsin optimal pH ~2, pancreatic enzymes ~7.5–8).
• Disorders: Lactose intolerance (deficiency of lactase), malabsorption (celiac disease), pancreatitis (reduced digestive enzymes), bile salt deficiency → fat malabsorption and steatorrhea.

Introduction
Digestion and absorption are the processes that convert ingested food into small, absorbable molecules and transfer them into the body's internal fluids for use in energy production, growth and repair. Digestion = mechanical + chemical breakdown of food. Absorption = uptake of digestion products across the intestinal wall into blood and lymph.

Major steps

• Ingestion: Taking food into the mouth; chewing reduces particle size and mixes food with saliva.
• Mechanical digestion: Mastication in mouth, churning in stomach, segmentation in small intestine — increases surface area for enzymes.
• Chemical digestion: Enzymatic hydrolysis of macromolecules (carbohydrates, proteins, lipids) into monomers (sugars, amino acids, fatty acids, glycerol).
• Absorption: Uptake of small molecules across the intestinal epithelium (mainly in small intestine) into blood (water-soluble) or lymph (lipid-soluble).
• Assimilation & egestion: Transported nutrients are used by cells (assimilation). Undigested residues are expelled as faeces (egestion).

Organs and their functions (brief)

• Mouth: Mechanical breakdown; salivary amylase starts starch digestion; saliva lubricates food.
• Oesophagus: Peristalsis moves bolus to stomach.
• Stomach: Protein denaturation and partial digestion by pepsin in acidic pH (1.5–3.5); mechanical churning produces chyme.
• Small intestine (duodenum, jejunum, ileum): Major site of chemical digestion and absorption. Pancreatic enzymes + bile act in duodenum; brush-border enzymes finish digestion; jejunum and ileum absorb nutrients.
• Large intestine: Water and mineral absorption; bacterial fermentation of some residues; formation of faeces.

Digestion of major nutrients

• Carbohydrates: Starch → (salivary & pancreatic amylase) → disaccharides (maltose) → (brush-border disaccharidases) → monosaccharides (glucose, fructose, galactose).
• Proteins: Proteins → (pepsin in stomach, trypsin/chymotrypsin from pancreas) → peptides → (peptidases on brush border & cytosol) → amino acids.
• Lipids: Triglycerides emulsified by bile salts → lipase action yields monoglycerides and free fatty acids → form micelles → diffuse into enterocytes → re-esterified to triglycerides → packaged into chylomicrons → transported via lacteals (lymph).

Absorption mechanisms

• Passive diffusion: For small lipids, water and some small solutes down their concentration gradient.
• Facilitated diffusion: Carrier proteins for e.g., fructose.
• Active transport: Energy-dependent uptake (e.g., glucose and galactose via SGLT co-transport with Na+; many amino acids via Na+-dependent carriers).
• Endocytosis: Limited uptake of some macromolecules (e.g., immunoglobulins in neonates).

Structural adaptations for absorption
The small intestine has folds (plicae circulares), villi (finger-like projections) and microvilli (brush border) on enterocytes — these multiply mucosal surface area dramatically (effective absorptive area ≈ 200 m² in adults). Each villus contains capillaries (absorb water-soluble nutrients) and a central lacteal (absorbs lipids as chylomicrons).

Transport after absorption

• Water-soluble nutrients (glucose, amino acids, small peptides, water-soluble vitamins, minerals): enter capillaries → portal vein → liver for processing.
• Fat-soluble nutrients (long-chain fatty acids, fat-soluble vitamins): reassembled into triglycerides → chylomicrons → lacteals → lymph → thoracic duct → blood (bypassing immediate liver first-pass).

Regulation
Neural (enteric & autonomic) and hormonal control coordinate secretion and motility. Key hormones: gastrin (stimulates acid/pepsinogen), secretin (stimulates pancreatic bicarbonate), cholecystokinin CCK (stimulates pancreatic enzymes and gallbladder contraction).

Clinical correlations (brief)
Lactose intolerance — deficiency of lactase (brush-border enzyme) leads to undigested lactose, osmotic diarrhoea and gas. Pancreatic insufficiency reduces fat digestion causing steatorrhoea. Celiac disease damages villi and causes malabsorption.

Typical physiological numbers

• pH: saliva ~6.5–7.5; stomach ~1.5–3.5; small intestine ~6–8.
• Transit times (approx.): stomach 2–6 hours; small intestine 3–6 hours; large intestine 12–48 hours.
• Effective absorptive area of small intestine in adult: order of 100–300 m² (textbook value ≈ 200 m²).

Summary
Digestion reduces complex food polymers to absorbable monomers via mechanical action and specific enzymes; absorption — aided by large surface area and transport mechanisms — moves these monomers into blood/lymph for use or storage.

The alimentary canal (digestive tract) is a continuous muscular tube extending from the mouth to the anus that processes food, digests it into absorbable molecules and absorbs nutrients and water. Though organs vary in specialised structure and function (mouth, oesophagus, stomach, small and large intestine), the basic wall plan (general structure) is common to most parts.

• Four basic layers (from lumen outward):
  • Mucosa — innermost layer: epithelium (absorptive and secretory), lamina propria (loose connective tissue with capillaries, immune cells, glands) and muscularis mucosae (thin smooth muscle). Functions: secretion (mucus, enzymes), absorption, protection.
  • Submucosa — dense connective tissue with blood and lymphatic vessels, nerves and sometimes glands (e.g. Brunner's glands in the duodenum). Contains Meissner's (submucosal) plexus controlling secretions and local blood flow.
  • Muscularis externa — usually two layers of smooth muscle: inner circular and outer longitudinal; responsible for peristalsis and segmentation. Between these layers lies the Auerbach (myenteric) plexus that controls motility.
  • Serosa / Adventitia — outer covering. Serosa (visceral peritoneum) on intraperitoneal organs; adventitia (connective tissue) on retroperitoneal parts (e.g. oesophagus).
• Specialisations along the canal
  • Mouth: stratified squamous epithelium, salivary glands (mucous/serous) — mechanical breakdown & salivary amylase secretion.
  • Oesophagus: mucous glands, muscularis with skeletal (upper) and smooth (lower) muscle; peristalsis moves bolus.
  • Stomach: rugae (folds) to allow expansion; gastric pits and glands (parietal cells secrete HCl, chief cells secrete pepsinogen); thick mucous layer for protection.
  • Small intestine: maximal absorption — plicae circulares (circular folds), villi (finger‑like projections of mucosa), and microvilli (brush border) on enterocytes. Intestinal crypts (Crypts of Lieberkühn) contain stem cells and secretory cells. Submucosal Brunner's glands in duodenum secrete alkaline mucus. Rich blood capillary and central lacteal (lymph) in villi for nutrient transport.
  • Large intestine: no villi, many goblet cells (mucus), teniae coli and haustra for segmental movement and water absorption; abundant gut-associated lymphoid tissue.
• Functional highlights
  • Movement: coordinated by muscularis externa (peristalsis propels food; segmentation mixes contents).
  • Secretion: enzymes, acid, bile (delivered into duodenum), mucus and hormones from enteroendocrine cells.
  • Absorption: structural specialisations (folds → villi → microvilli) increase surface area hugely to maximise nutrient and water uptake. Fat is absorbed into lacteals; amino acids and sugars enter capillaries.
  • Neurovascular supply: intrinsic enteric nervous system (Meissner's and Auerbach's plexuses) controls local functions; extrinsic ANS and hormones modulate activity.

Clinical / physiological notes: Loss or damage to villi (e.g., celiac disease) reduces absorptive area and causes malabsorption. Excess acid or impaired mucosal defence can cause peptic ulcers. Altered motility yields constipation or diarrhoea.

Overview
The alimentary canal (digestive tract) is a continuous muscular tube that starts at the mouth and ends at the anus. Its main functions are ingestion, mechanical and chemical breakdown of food, absorption of nutrients and water, propulsion of food, and elimination of undigested residues.

General structure (wall layers)

• Mucosa – epithelium, lamina propria and muscularis mucosae; site of secretion and absorption.
• Submucosa – connective tissue with blood/lymph vessels and glands; contains Meissner's plexus.
• Muscularis externa – inner circular and outer longitudinal smooth muscle; responsible for peristalsis and segmentation; contains Auerbach (myenteric) plexus.
• Serosa/adventitia – outer connective tissue layer.

Major regions and specific structures

• Mouth (Buccal cavity) – teeth (mastication), tongue (mixing, taste, swallowing), saliva (salivary amylase, lubrication).
• Pharynx and Oesophagus – passage and propulsion; oesophagus uses peristalsis; no digestion except minor lubrication.
• Stomach – cardia, fundus, body, pylorus; gastric glands secrete HCl (lowers pH ~1.5–3.5), pepsinogen (activated to pepsin for protein digestion), intrinsic factor (B12 absorption), mucus; mechanical churning produces chyme.
• Small intestine – duodenum, jejunum, ileum. Major site of chemical digestion and nutrient absorption. Intestinal wall has circular folds (plicae circulares), villi and microvilli (brush border) that massively increase surface area. Enterocytes secrete disaccharidases, peptidases and absorb nutrients; lacteals absorb fats.
• Large intestine – caecum (with appendix), colon (ascending, transverse, descending, sigmoid), rectum and anal canal. Absorbs water and electrolytes, compacts feces, houses gut microbiota that ferment undigested carbohydrates and synthesize some vitamins (e.g., K, some B vitamins).
• Accessory glands – salivary glands, pancreas (pancreatic juice: amylase, lipase, proteases like trypsinogen activated to trypsin), liver (bile production; bile salts emulsify fats), gall bladder (stores and concentrates bile).

Functions — stepwise

• Ingestion: taking in food (mouth).
• Mechanical digestion: chewing, churning in stomach, segmentation in intestines increase surface area for enzymes.
• Chemical digestion: enzymatic hydrolysis of carbohydrates, proteins, lipids and nucleic acids (salivary amylase, gastric pepsin, pancreatic enzymes, brush-border enzymes).
• Propulsion: swallowing and peristalsis move food along the tract.
• Absorption: monosaccharides, amino acids, short-chain fatty acids and water-soluble vitamins absorbed into blood; fatty acids and monoglycerides are reassembled into triglycerides and transported via lacteals as chylomicrons into lymph.
• Defecation: elimination of indigestible residues via rectum and anus.

Special adaptations for absorption

• Plicae circulares (folds) → villi (finger-like projections) → microvilli (brush border): multiply surface area for absorption.
• Rich blood supply to villi rapidly carries absorbed nutrients away, maintaining concentration gradients.
• Lacteals in villi transport absorbed lipids as chylomicrons into lymphatics.

Control and coordination
Enteric nervous system (myenteric and submucosal plexuses) coordinates local mixing and peristalsis; parasympathetic and hormonal signals (e.g., gastrin, secretin, cholecystokinin) regulate secretion and motility.

Important physiological facts

• Typical pH: saliva ~6.5–7.5, stomach 1.5–3.5, small intestine ~7–8.
• Bile salts emulsify fats; pancreatic enzymes complete digestion of carbs, proteins and fats in the duodenum.
• Brush-border enzymes on enterocyte surface finish carbohydrate and peptide digestion (e.g., maltase, sucrase, lactase; peptidases).

Overview: The mouth (oral cavity) or buccal cavity is the first part of the alimentary canal. It receives food, begins mechanical breakdown and chemical digestion, mixes food with saliva to form a bolus, and participates in taste, speech and swallowing.

Gross structure:

• Lips and vestibule (space between lips/cheeks and teeth).
• Oral cavity proper (space inside the dental arches).
• Floor: tongue. Roof: hard palate (anterior) and soft palate (posterior) including uvula.
• Gingiva (gums), teeth embedded in alveolar sockets, and openings of salivary glands.

Histology (brief): Lined mainly by stratified squamous epithelium (keratinized at hard palate and gingiva). Lamina propria contains salivary glands and blood vessels. Tongue surface has papillae with taste buds (gustatory receptors).

Tongue and taste: Muscular organ for manipulation of food, mixing with saliva, formation of bolus and initiating swallowing. Papillae types: filiform (mechanical), fungiform, circumvallate and foliate (contain taste buds). Lingual glands secrete mucus and sometimes lingual lipase (important in infants).

Teeth & dentition: Teeth are adapted for cutting, tearing and grinding. Each tooth has crown and root. Layers: enamel (hardest, outer), dentin (beneath enamel), pulp cavity (contains blood vessels & nerves), cementum (covers root) and periodontal ligament (anchors tooth). Humans are heterodont (different types) and diphyodont (two sets of teeth: deciduous and permanent).

Types of teeth: Incisors (cutting), canines (tearing), premolars and molars (grinding).

Salivary glands & saliva: Major glands: parotid (serous), submandibular (mixed serous + mucous), sublingual (mainly mucous). Minor salivary glands are scattered in the mucosa. Saliva (~1–1.5 L/day) is mostly water, with mucus, electrolytes, IgA, lysozyme and enzymes (salivary amylase/ptyalin).

Functions of mouth/buccal cavity:

• Ingestion and initial mechanical digestion by mastication (chewing).
• Chemical digestion: salivary amylase begins starch digestion to maltose and dextrins.
• Lubrication and bolus formation (mucus + saliva).
• Taste and sensory analysis of food; reflex stimulation of digestion (salivation).
• Speech and airway protection during swallowing (soft palate elevates to close nasopharynx).

Chemical digestion in the mouth: Salivary amylase acts optimally at near-neutral pH (~6.8–7.0) and body temperature (~37 °C). It hydrolyses starch (polysaccharides) into maltose and dextrins. Lingual lipase (minor role in adults) can begin lipid digestion, important in infants.

Buccal (oral) phase of swallowing: Voluntary stage: tongue presses bolus against hard palate and pushes it into oropharynx. Once the bolus enters the pharynx, involuntary reflex (pharyngeal phase) takes over.

Clinical relevance: Poor saliva production (xerostomia) reduces taste, mouth lubrication and increases dental caries. Dental caries arise when oral bacteria (e.g., Streptococcus mutans) ferment dietary sugars producing acids that demineralize enamel. Good oral hygiene and fluoride prevent decay.

Summary: The mouth/buccal cavity prepares food physically and chemically for further digestion, protects the airway during swallowing, and provides sensory feedback. It is a complex, multifunctional entry point to the digestive system.

Overview: Saliva is a watery secretion produced by the three paired major salivary glands (parotid, submandibular, sublingual) and numerous minor glands in the oral cavity. An average adult produces about 0.5–1.5 L of saliva per day. Saliva maintains oral hygiene, begins digestion, protects mucosa and teeth, and assists speech and swallowing.

Composition (qualitative & quantitative highlights):

• Water (~99%) — solvent and medium for dissolution of food and taste stimuli.
• Electrolytes — Na+, K+, Cl−, HCO3−, PO43−. Bicarbonate and phosphate provide buffering capacity (helps neutralize acids).
• Mucus (mucin) — glycoproteins that lubricate food and form bolus for swallowing.
• Enzymes
  • Salivary amylase (ptyalin): initiates starch hydrolysis to dextrins and maltose; optimum pH ≈ 6.7–7.0 and optimum temperature ≈ body temperature.
  • Lingual lipase (minor, more active in stomach) — begins lipid digestion in infants and continues in stomach.
• Antimicrobial and immune proteins — lysozyme, lactoferrin, secretory IgA, peroxidases — limit microbial growth and help immune defence.
• Other components — urea, ammonia, growth factors, epithelial cells and small amounts of serum proteins.

Functions:

• Lubrication and bolus formation: Mucins coat food to form a slippery bolus for safe swallowing.
• Initial digestion: Salivary amylase hydrolyses starch (amylose/amylopectin) into dextrins and maltose; lingual lipase starts limited lipid digestion.
• Buffering and pH maintenance: Bicarbonate and phosphate help maintain oral pH (~6.2–7.6), protecting enamel and controlling bacterial growth.
• Oral hygiene and antimicrobial action: Lysozyme and IgA reduce microbial colonization and dental caries risk.
• Taste and speech: Saliva dissolves taste substances and lubricates oral structures for articulation.
• Protection and repair: Growth factors and proteins in saliva assist mucosal repair and wound healing.
• Drug absorption: Some drugs (e.g., sublingual nitroglycerin) are rapidly absorbed across mucosa aided by saliva.

Control of secretion: Salivary secretion is under autonomic nervous system control. Parasympathetic stimulation (via facial and glossopharyngeal nerves) produces a large volume of watery saliva. Sympathetic stimulation produces a smaller, more viscous (mucous-rich) secretion. Salivary flow rate affects composition — higher flow increases bicarbonate concentration and raises pH.

Clinical relevance / disturbances:

• Xerostomia (dry mouth) — reduced saliva leads to difficulty swallowing, increased dental caries, oral infections.
• Sialolithiasis — salivary stones blocking ducts, producing pain and swelling during meals.
• Salivary gland infections / autoimmune disease (e.g., Sjögren's syndrome) reduce secretion and compromise oral health.

Key numeric values: typical pH 6.2–7.6; daily volume ~0.5–1.5 L; salivary amylase optimum pH ~6.7–7.0; temperature optimum ≈ 37 °C.

Concise biochemical note: Salivary amylase acts by hydrolysing α-1,4 glycosidic bonds in starch to produce dextrins and disaccharides (maltose); activity continues in the mouth and early stomach until inactivated by acidic gastric juice.

Deglutition (Swallowing)

Deglutition is the process by which food is moved from the mouth into the stomach. It is a complex sequence of events involving voluntary and reflex actions and is normally divided into three stages: oral (voluntary), pharyngeal (involuntary/reflex), and esophageal (involuntary).

1. Oral (Voluntary) Phase

• Bolus formation: Chewing mixes food with saliva to form a cohesive bolus.
• Tongue action: The tongue presses the bolus against the hard palate and pushes it posteriorly into the oropharynx. This phase is under conscious control.

2. Pharyngeal (Reflex) Phase

• Swallowing center: Sensory input from CN IX (glossopharyngeal) and CN X (vagus) to the medulla and lower pons activates the swallowing center.
• Protective closures: Soft palate elevates to close the nasopharynx; larynx elevates and the epiglottis covers the glottis to prevent aspiration.
• Upper esophageal sphincter (UES) relaxation: The UES (cricopharyngeus) relaxes, allowing the bolus to enter the oesophagus.
• Pharyngeal constrictor muscles contract in sequence to push the bolus downward.

3. Esophageal (Involuntary) Phase

• Primary peristalsis: A coordinated wave of circular and longitudinal muscle contraction (initiated by the pharyngeal swallow and mediated by vagal motor fibers) propels the bolus down the oesophagus.
• Secondary peristalsis: If residue remains, stretch receptors in the oesophagus trigger secondary peristaltic waves until the lumen is cleared.
• Lower esophageal sphincter (LES) and receptive relaxation: The LES relaxes in anticipation of the bolus (receptive relaxation) allowing entry into the stomach; this relaxation is mediated by vagal pathways and inhibitory neurotransmitters (NO, VIP).

Oesophagus: Structure and Function

The oesophagus is a muscular tube (~25 cm in adults) that conducts the bolus from the pharynx to the stomach. Key structural features:

• Mucosa: Non-keratinized stratified squamous epithelium (protects against abrasion).
• Submucosa: Contains mucous glands (secrete mucus to lubricate the bolus).
• Muscularis externa: Upper third = skeletal muscle (voluntary), middle third = mixed skeletal and smooth, lower third = smooth muscle (involuntary).
• Adventitia/Serosa: The oesophagus is mostly covered by adventitia; a short abdominal segment has serosa.
• Sphincters: UES (prevents air entry during respiration) and LES (prevents gastric reflux).

Physiological Functions

• Transport of food and liquids to the stomach by peristalsis.
• Secretion of mucus for lubrication.
• Prevention of reflux via LES tone; sphincter dysfunction leads to conditions such as GERD.

Neural Control

Swallowing is coordinated by a central swallowing center in the medulla and pons. Cranial nerves involved: V, VII, IX, X, XII. Vagus (X) plays a major role in esophageal motility. Intrinsic enteric neurons (Auerbach/myenteric plexus) coordinate local peristalsis and LES tone.

Clinical Correlates

• Dysphagia: Difficulty in swallowing—causes may be neurological (stroke), mechanical obstruction (tumor, strictures), or motility disorders (achalasia).
• Achalasia: Loss of myenteric ganglia → failure of LES to relax → bird-beak appearance on imaging, progressive dysphagia.
• GERD (heartburn): Incompetent LES allows reflux of acidic gastric contents causing oesophagitis.
• Aspiration/choking: Failure to protect the airway during swallowing can lead to inhalation of food/liquid into the lungs.

Summary

Deglutition is a safe, fast, and well-coordinated sequence: voluntary oral transport, reflexive pharyngeal closure and UES opening, and peristaltic esophageal transport with LES relaxation. Structural specializations of the oesophagus (epithelium, glands, muscle type, sphincters) support these functions.

Overview: The oesophagus (esophagus) is a muscular, tubular passage that connects the pharynx to the stomach and transports the food bolus by coordinated muscular contractions called peristalsis. Swallowing (deglutition) is the process that moves food from the mouth to the stomach and has three stages: oral (voluntary), pharyngeal (involuntary), and oesophageal (involuntary).

Anatomy & Histology:

• Length: about 25–30 cm in adults; lies posterior to the trachea in the neck and thorax and passes through the diaphragm (oesophageal hiatus) to the stomach.
• Wall layers (inner → outer): mucosa (stratified squamous non-keratinized epithelium), submucosa (containing oesophageal glands that secrete mucus), muscularis externa (inner circular and outer longitudinal layers), and adventitia (connective tissue).
• Muscle composition: upper third — skeletal (voluntary) muscle; middle third — mixed skeletal + smooth; lower third — smooth (involuntary) muscle.
• Sphincters: Upper oesophageal sphincter (UES) — prevents air entry; Lower oesophageal sphincter (LES) or gastro-oesophageal junction — prevents reflux of gastric contents.

Phases of Swallowing:

• Oral (buccal) phase — voluntary: food is chewed, mixed with saliva to form a bolus, tongue pushes the bolus posteriorly.
• Pharyngeal phase — involuntary reflex: soft palate elevates (preventing nasal regurgitation), larynx elevates, epiglottis covers the glottis to protect the airway, UES relaxes briefly and opens to receive the bolus.
• Oesophageal phase — involuntary: peristaltic wave(s) propels the bolus down the oesophagus; LES relaxes to allow entry into stomach then contracts to prevent reflux.

Neural Control: Swallowing is coordinated by the swallowing centre in the medulla oblongata and lower pons. Afferent input primarily via glossopharyngeal (CN IX) and vagus (CN X) nerves; efferent control uses CN X (smooth muscle control), CN IX, CN XII (tongue) and somatic motor fibres for the upper oesophagus.

Peristalsis: Sequential contraction of circular and longitudinal muscle layers produces a wave that pushes the bolus. There are primary peristalsis (continuation of the pharyngeal wave) and secondary peristalsis (local reflexes triggered by retained food). Liquids typically transit in ~1 s; solids take longer (several seconds).

Secretions and Lubrication: Oesophageal glands in the submucosa and mucosal surface secrete mucus to lubricate and protect the mucosa during passage of food.

Clinical notes (short): Dysfunction of UES/LES or peristalsis causes dysphagia (difficulty swallowing), reflux (GERD) when LES tone is low, and choking/aspiration when airway protection fails. Oesophagitis, strictures and achalasia are clinical conditions related to oesophageal motility or sphincter dysfunction.

Key points to remember:

• Swallowing: oral → pharyngeal → oesophageal.
• UES and LES are critical for preventing air entry and gastric reflux.
• Peristalsis is coordinated by central and local neural mechanisms.

Introduction: The stomach is a J‑shaped, distensible muscular sac of the upper abdominal cavity that acts as a temporary storage and digestive organ between the oesophagus and small intestine. It continues mechanical and chemical digestion begun in the mouth, especially of proteins.

Gross structure

• Regions: Cardia (around oesophageal opening), Fundus (dome-shaped upper part), Body (major central region), Pylorus (distal region leading to duodenum) with pyloric sphincter controlling emptying.
• Inner features: Rugae (mucosal folds) that allow expansion; gastric mucosa contains gastric pits leading to gastric glands.
• Wall layers: Mucosa (with glands), Submucosa, Muscularis externa (three layers: outer longitudinal, middle circular, inner oblique — unique to stomach), Serosa.

Microscopic structure — gastric glands and cell types

• Surface mucous cells & mucous neck cells: secrete protective alkaline mucus and bicarbonate.
• Parietal (oxyntic) cells: secrete hydrochloric acid (HCl) and intrinsic factor (required for vitamin B12 absorption).
• Chief (zymogenic) cells: secrete pepsinogen (inactive precursor of pepsin) and gastric lipase.
• Enteroendocrine (G) cells: secrete gastrin (hormone) which stimulates acid secretion and motility.

Secretions of the stomach

• Gastric juice composition: water, HCl (creates pH ~1.5–3.5), pepsinogen/pepsin, gastric lipase, mucus, intrinsic factor, and electrolytes.
• Typical volume: ~2–3 litres of gastric juice per day (varies with diet and stimulation).

Functions

• Storage: Stores ingested food and regulates delivery to the small intestine (pyloric sphincter).
• Mechanical digestion: Powerful muscular contractions (mixing and churning) convert food to a semi‑liquid chyme.
• Chemical digestion: Acid (HCl) denatures proteins and provides optimal pH for pepsin; pepsin begins proteolysis converting proteins into peptides; gastric lipase begins limited fat digestion.
• Secretion of intrinsic factor: Essential for absorption of vitamin B12 in ileum — necessary for RBC production.
• Protection: Acid kills many microbes; mucus and bicarbonate protect mucosa from autodigestion.
• Limited absorption: Alcohol, aspirin and some weak acids/bases are absorbed directly through stomach mucosa.

Regulation of gastric activity

• Neural: Vagus nerve (parasympathetic) stimulates secretion and motility; sympathetic input inhibits.
• Hormonal: Gastrin (from G cells) stimulates HCl and motility; other hormones (secretin, CCK) inhibit gastric activity when duodenum signals fullness or acidity.
• Phases of secretion: Cephalic (sight/smell/taste — vagal reflex), Gastric (food in stomach — local and hormonal), Intestinal (food in duodenum — mostly inhibitory).

Clinical correlations

• Peptic ulcers: Acid + pepsin damage mucosa; Helicobacter pylori infection and NSAIDs are common causes.
• Gastroesophageal reflux disease (GERD)/heartburn: reflux of acidic gastric contents into the oesophagus causes pain and inflammation.
• Pernicious anaemia: caused by autoimmune loss of parietal cells → intrinsic factor deficiency → impaired B12 absorption.

Summary: The stomach is a multifunctional organ — reservoir, mechanical mixer, and chemical digester — that initiates protein digestion, protects against pathogens, and controls rate of delivery of chyme to the intestines while supplying intrinsic factor for B12 absorption.

Overview
The stomach is a J-shaped, muscular enlargement of the alimentary canal between the oesophagus and small intestine. It stores food, mixes it with gastric secretions to form chyme, begins protein digestion and controls release of chyme into the duodenum.

Gross structure and parts

• Cardia – small region around the entry (cardiac orifice) where oesophagus opens.
• Fundus – dome-shaped superior part that often contains gas.
• Body (corpus) – main central region, major secretory area.
• Pylorus – distal funnel-shaped region with pyloric antrum and pyloric canal ending at the pyloric sphincter.
• Inner features – rugae (folds) allow expansion; mucosa forms gastric pits leading to gastric glands.

Wall layers (outside → inside)

• Serosa – outer covering.
• Muscularis externa – three layers of smooth muscle: outer longitudinal, middle circular, inner oblique (unique to stomach) — these generate strong churning movements.
• Submucosa – connective tissue with blood vessels and nerves.
• Mucosa – gastric epithelium with pits and glands; secretes acid, enzymes, mucus and hormones.

Gastric glands and cell types (mainly in fundus and body):

• Surface mucous cells – secrete mucus and bicarbonate; protect mucosa.
• Mucous neck cells – secrete mucus, transitional role.
• Parietal (oxyntic) cells – secrete HCl and intrinsic factor (for vitamin B12 absorption).
• Chief (zymogenic) cells – secrete pepsinogen (inactive) and gastric lipase.
• Enteroendocrine cells – G cells (gastrin), D cells (somatostatin), ECL cells (histamine) — regulate secretion and motility.

Gastric secretions: composition and action

• Gastric juice = water, HCl, mucus, pepsinogen, intrinsic factor, lipase, electrolytes and hormones.
• HCl (from parietal cells) provides low pH (≈1.5–3.5): activates pepsinogen → pepsin, denatures proteins, kills many microbes and helps iron absorption.
• Pepsin (active protease) begins proteolysis; optimal activity at low pH (~1.5–3).
• Intrinsic factor is essential for vitamin B12 absorption in the ileum; deficiency → pernicious anemia.
• Mucus–bicarbonate layer protects epithelium from acid and pepsin.

Mechanism of acid secretion
Parietal cells secrete H+ via the H+/K+ ATPase (proton pump). Carbonic anhydrase in the cell forms H+ and HCO3− from CO2 and H2O. H+ is pumped into lumen; HCO3− is exchanged into blood (alkaline tide after meals).

Regulation of gastric secretion

• Cephalic phase (30%): sight, smell, thought or taste of food → vagal stimulation → increases secretion.
• Gastric phase (60%): stomach distension, peptides, low acidity and gastrin release (G cells) further stimulate HCl and pepsinogen secretion.
• Intestinal phase (10%): initial intestinal signals may augment, later inhibitory signals (enterogastrones) reduce gastric secretion when chyme enters duodenum.
• Local reflexes, histamine (from ECL), acetylcholine (vagus) and gastrin are major stimulatory mediators; somatostatin inhibits secretion.

Protection and pathology (brief)
Mucus and bicarbonate protect mucosa. Disruption (e.g., H. pylori infection, NSAIDs) can cause gastritis and peptic ulcers. Excess acid → heartburn; anti-secretory drugs include antacids, H2 blockers and proton pump inhibitors (PPIs) like omeprazole which block H+/K+ ATPase. Lack of intrinsic factor → vitamin B12 malabsorption and pernicious anemia.

Important values
Normal gastric pH (fasting): ~1.5–3.5. Daily gastric juice volume: ~2–3 L in adults (varies with diet).

Concise functional summary: Stomach mechanically and chemically processes food: muscular mixing produces chyme; HCl and pepsin begin protein digestion; intrinsic factor enables vitamin B12 absorption; tight regulation balances digestion with mucosal protection.

Gastric enzymes and the chemistry of the stomach

The stomach performs initial chemical digestion by combining acidic secretions (HCl), enzymes (zymogens and active enzymes), mucus and intrinsic factor. Acid and enzymes act mainly on proteins and some lipids; the stomach also prepares chyme for intestinal digestion and absorption.

Major secretions and their sources

• Parietal (oxyntic) cells: secrete HCl and intrinsic factor (required for vitamin B12 absorption).
• Chief (peptic) cells: secrete pepsinogen (zymogen) and gastric lipase.
• Mucous (surface and neck) cells: secrete mucus and bicarbonate that protect the epithelium.
• G cells (antrum): secrete gastrin (hormone) which stimulates acid and enzyme secretion.

Chemistry of acid formation (basic steps)

• CO₂ produced in parietal cells reacts with H₂O via carbonic anhydrase to form carbonic acid:
• CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
• H+ is actively pumped into the gastric lumen by the H+/K+ ATPase (proton pump); Cl- follows into the lumen to form HCl. The base (HCO3-) is exchanged into blood causing the post‑prandial alkaline tide.
• Net luminal chemistry (simplified): H+ (lumen) + Cl- (lumen) → HCl

Key gastric enzymes

• Pepsin (active enzyme)
  • Source: activated from pepsinogen (chief cell zymogen) by H+ and autocatalysis.
  • Substrate: proteins - cleaves peptide bonds, especially near aromatic amino acids.
  • Product: large polypeptides and peptides (prepare substrate for pancreatic proteases).
  • Optimum pH: ~1.5–2.0 (strongly acidic).
  • Activation: pepsinogen --(HCl / pepsin)--> pepsin (autocatalytic activation).
• Gastric (acid) lipase
  • Source: chief cells.
  • Substrate: triglycerides (especially medium‑chain fats); acts on sn‑3 position.
  • Product: diglycerides, free fatty acids.
  • Optimum pH: ~3–6 (can act in acidic stomach and continues in duodenum).
• Rennin (chymosin) (important in infants)
  • Source: gastric mucosa in infants (sometimes present in young mammals).
  • Function: coagulates milk (casein) to retain milk longer in stomach, improving digestion.
  • Optimum pH: near neutral to mildly acidic depending on species (works under gastric conditions in infants).

Mechanisms & biochemical principles

• Hydrolysis of peptide bond (general): peptide + H2O --(pepsin)--> smaller peptides. Enzymes lower activation energy and position substrate for nucleophilic attack on the peptide bond.
• Zymogen activation (protection against autodigestion): pepsinogen is produced inactive to prevent damage to producing cells; low pH triggers conversion to pepsin, and pepsin can activate more pepsinogen (positive feedback/autocatalysis).
• pH and enzyme activity: each enzyme has a narrow pH optimum. Acidic pH unfolds some proteins and exposes peptide bonds for pepsin action. Mucus and bicarbonate protect epithelial cells from acid and enzymes.

Clinical and physiological notes

• Intrinsic factor (from parietal cells) is required for vitamin B12 absorption in the ileum; loss causes pernicious anemia.
• Excess acid or loss of mucosal protection (e.g., H. pylori infection, NSAIDs) can cause peptic ulcers.
• Antacids neutralize stomach acid (raise pH); proton pump inhibitors (PPIs) block H+/K+ ATPase and reduce HCl production; H2 blockers reduce histamine-mediated stimulation of parietal cells.

Summary

The stomach uses HCl to denature proteins and activate zymogens (pepsinogen → pepsin). Pepsin does most gastric proteolysis; gastric lipase digests some fats; rennin helps infants digest milk. Carbonic anhydrase and the H+/K+ ATPase drive acid production, and protective mucus/bicarbonate and intrinsic factor are key non-enzymatic chemical features of gastric physiology.

Overview
Gastric juice is the acidic secretion of the stomach that begins chemical digestion, especially of proteins. It is produced by gastric glands in the mucosa and contains hydrochloric acid (HCl), enzymes (in inactive and active forms), mucus and intrinsic factor.

Source and cell types

• Parietal (oxyntic) cells — secrete HCl and intrinsic factor (required for vitamin B12 absorption).
• Chief (zymogenic) cells — secrete pepsinogen (inactive precursor of pepsin) and gastric lipase.
• Mucous cells — produce alkaline mucus that protects gastric epithelium.
• G cells (in pylorus) — secrete gastrin (a hormone that stimulates HCl secretion).
• Enterochromaffin‑like (ECL) cells — release histamine (stimulates parietal cells).

Composition and properties

• Daily volume: about 2–3 L (varies with diet and physiology).
• pH: ~1.5–3.5 (highly acidic due to HCl).
• Main components: HCl, pepsinogen/pepsin, gastric lipase, mucus, intrinsic factor, electrolytes.

Functions of gastric juice

• Protein digestion: HCl denatures proteins and converts pepsinogen to pepsin; pepsin cleaves internal peptide bonds, producing polypeptides.
• Antimicrobial action: low pH kills many ingested microbes.
• Activation of zymogens: acid-mediated conversion of pepsinogen to pepsin.
• Facilitates iron absorption (Fe2+ formation) and, via intrinsic factor, enables vitamin B12 absorption in ileum.
• Gastric lipase begins fat digestion (more important in infants).

Key enzymes in gastric juice

• Pepsin — an endopeptidase formed from pepsinogen by HCl. Optimum pH ≈ 1.5–2.5. Acts on peptide bonds especially near aromatic amino acids (Phe, Tyr, Trp) to yield shorter polypeptides.
• Gastric lipase — secreted by chief cells; hydrolyses some triglycerides to diglycerides and fatty acids. Optimum pH ≈ 3–6; contributes notably to fat digestion in infants.
• Rennin (chymosin) — present in infants; coagulates milk by converting caseinogen to insoluble casein, slowing gastric emptying and aiding digestion of milk proteins.

Activation and regulation of secretion

• Pepsinogen + HCl → Pepsin (active)
• Regulatory phases of gastric secretion: cephalic (sight/smell/taste of food; vagal stimulation), gastric (distension, peptides, low acidity → gastrin release), and intestinal (initially stimulates then inhibits by enterogastrones).
• Major stimulators: vagal (ACh), gastrin, histamine. Inhibitors: somatostatin, low pH, secretin, cholecystokinin (CCK).

Physiology detail — H+ generation
Parietal cells use carbonic anhydrase to form H+ from CO2 and H2O:

CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3−

H+ is pumped into the gastric lumen by H+/K+ ATPase (proton pump); HCO3− is exchanged into blood (alkaline tide).

Clinical relevance / pathology

• Excess acid or mucosal damage → peptic ulcers (often associated with Helicobacter pylori or NSAID use).
• Achlorhydria (absence of gastric acid) impairs protein digestion, iron absorption and vitamin B12 uptake (can lead to pernicious anemia if intrinsic factor is lacking).
• Antacids neutralize excess HCl, and proton pump inhibitors (PPIs) block H+/K+ ATPase to reduce acid secretion.

Summary (concise)
Gastric juice provides an acidic environment and enzymes (pepsin, gastric lipase, rennin in infants) that start protein and some fat digestion, protect against microbes, and prepare chyme for further digestion and absorption in the small intestine. Its secretion is tightly regulated neurally and hormonally.

Gross anatomy
The small intestine is the longest part of the alimentary canal (about 6–7 m in an adult human). It is divided into three regions: the duodenum (≈25 cm, C‑shaped, receives bile and pancreatic juice), the jejunum (proximal ~2–3 m) and the ileum (distal ~3–4 m, ends at the ileocaecal valve). Its diameter is about 2.5–3 cm.

Main functions related to structure
The small intestine receives acidic chyme from the stomach and completes chemical digestion and most nutrient absorption. Structural specialisations maximise surface area and support digestion, enzymatic activity and immune defence.

Mucosal specialisations that increase absorptive efficiency

• Plicae circulares (Kerckring's folds): permanent circular folds of mucosa and submucosa that increase surface area and slow chyme flow to enhance contact time.
• Villi: finger‑like projections of mucosa (~0.5–1.6 mm long) covered by simple columnar epithelium; each villus contains a central lacteal (lymph capillary) and a dense capillary network for nutrient transport.
• Microvilli (brush border): microscopic cytoplasmic projections on enterocytes that further amplify surface area and contain membrane‑bound digestive enzymes (brush border enzymes).
• Combined effect: folding + villi + microvilli increase the effective absorptive surface area by up to ~600× compared with a smooth tube (textbook estimate).

Histology (layer details relevant to function)

• Mucosa: simple columnar epithelium with enterocytes (absorptive cells) and goblet cells (mucus). Enterocyte apical membranes form the brush border with enzymes (maltase, sucrase, lactase, peptidases, enterokinase).
• Intestinal glands (crypts of Lieberkühn): tubular glands at the base of villi that secrete intestinal juice (succus entericus) and contain stem cells, Paneth cells (antimicrobial agents like lysozyme and defensins) and enteroendocrine cells (hormones like secretin, CCK).
• Submucosa: contains blood vessels and lymphatics; in the duodenum it contains Brunner's glands which secrete alkaline mucus to neutralise acidic chyme and protect mucosa.
• Muscularis: inner circular and outer longitudinal smooth muscle layers responsible for segmentation (mixing) and peristalsis (propulsion).
• Immune tissue: Peyer’s patches (aggregated lymphoid nodules) are prominent in the ileum and provide mucosal immunity.

Transport pathways from lumen to circulation

• Water‑soluble nutrients (monosaccharides, amino acids, small peptides, water‑soluble vitamins): absorbed into enterocyte → capillaries of villus → portal vein → liver.
• Fats (long‑chain fatty acids, monoglycerides): form micelles with bile salts → diffuse into enterocytes → re‑esterified to triglycerides in ER → packaged into chylomicrons → exocytosed into lacteals → lymphatic circulation → thoracic duct → bloodstream.

Key cellular features
Brush border enzymes finish carbohydrate and peptide digestion (e.g., maltase, lactase, aminopeptidases). Enterokinase (enteropeptidase) activates pancreatic trypsinogen to trypsin, initiating pancreatic protease cascade.

Motility patterns
Segmentation (local contractions) mixes chyme with enzymes and increases contact with mucosa. Peristalsis propels residue toward the ileocaecal junction.

Clinical correlations (brief)
Damage to villi (e.g., celiac disease) reduces absorptive area → malabsorption and nutrient deficiencies. Lactase deficiency (brush border enzyme deficiency) causes lactose intolerance (osmotic diarrhoea). Oral rehydration therapy (ORS) exploits Na+–glucose cotransport in enterocytes to restore water and electrolytes.

Overview
The small intestine is the main site of chemical digestion and absorption in the human alimentary canal. It is a long tubular organ divided into three regions — duodenum, jejunum and ileum — each adapted for specific functions.

Gross structure

• Duodenum (first ~25–30 cm): receives acidic chyme from the stomach, bile from the liver/gall bladder and pancreatic juice via the hepatopancreatic ampulla; neutralises acid and continues digestion.
• Jejunum (middle part): major site of digestion and absorption of nutrients.
• Ileum (terminal part): absorbs bile salts, vitamin B12 and remaining nutrients; ends at the ileocaecal junction.

Histological layers and specialisations

• Mucosa: has simple columnar epithelium (enterocytes) with goblet cells. Key specialisations to increase absorptive efficiency are:
• Plicae circulares (circular folds): permanent folds of mucosa and submucosa that slow chyme flow and increase surface area.
• Villi: finger-like projections of mucosa (~0.5–1.6 mm long) containing a central lacteal (lymph capillary) and a capillary network to transport absorbed lipids and water-soluble nutrients respectively.
• Microvilli (brush border): apical membrane of enterocytes bearing dense microvilli that form the brush border and host membrane-bound digestive enzymes (e.g., disaccharidases, peptidases).
• Crypts of Lieberkühn: intestinal glands between villi that secrete intestinal juice and contain stem cells that renew the epithelium.
• Submucosa: contains blood vessels and (in duodenum) Brunner's glands that secrete alkaline mucus to protect mucosa and neutralise acid.
• Muscularis externa: inner circular and outer longitudinal smooth muscle layers that produce segmentation (mixing) and peristalsis (propulsion).
• Serosa: outer covering.

Immunological specialisations
Peyer's patches (aggregated lymphoid nodules) in the ileum detect antigens; M cells sample luminal antigens and deliver them to immune cells.

Functional specialisations and how they help absorption

• Huge effective surface area: plicae + villi + microvilli increase area enormously, maximizing contact between chyme and absorptive cells.
• Brush border enzymes: complete carbohydrate and peptide digestion at the epithelial surface so that monosaccharides and amino acids can be absorbed immediately.
• Rich blood and lymph supply: capillaries pick up water-soluble nutrients (glucose, amino acids) and lacteals carry lipids (in chylomicrons) into lymph.
• Specific transporters: Na+-dependent co-transporters, facilitative carriers and passive diffusion provide efficient, selective uptake of different solutes.
• Segmentation and slow transit: mixes chyme with enzymes and bile, increasing time for digestion and contact with absorptive surfaces.

Mechanisms of absorption — key steps

• Carbohydrates: polysaccharides are broken down to disaccharides and monosaccharides. Final digestion by brush border disaccharidases (lactase, sucrase, maltase). Glucose and galactose are absorbed by Na+-dependent cotransporter SGLT1 (secondary active transport using Na+ gradient); fructose enters by facilitated diffusion (GLUT5) and exits basolaterally via GLUT2 into blood.
• Proteins: proteins → peptides by pancreatic proteases → amino acids and small peptides by brush border peptidases. Amino acids use Na+-dependent carriers; di- and tri-peptides can be taken up by H+-dependent cotransporters and then hydrolysed intracellularly.
• Lipids: triglycerides emulsified by bile salts, digested by pancreatic lipase to free fatty acids and monoglycerides, which form micelles. Lipids diffuse into enterocytes, are re-esterified to triglycerides in the smooth ER, packaged into chylomicrons, and exported into lacteals (lymphatic transport) rather than blood capillaries.
• Vitamins, minerals and water: fat-soluble vitamins absorbed with lipids; water-soluble vitamins by specific carriers; vitamin B12 binds intrinsic factor and is absorbed in the ileum. Minerals like iron and calcium have specific transport and regulatory mechanisms. Water follows osmotic gradients (large amounts absorbed across the small intestine).

Important transport proteins and pumps
Na+/K+ ATPase on the basolateral membrane maintains a low intracellular Na+, driving secondary active transporters (e.g., SGLT1). Brush border enzymes complete digestion at the membrane, minimizing diffusion distance.

Clinical & physiological relevance (brief)

• Damage to villi (e.g., celiac disease) reduces absorptive surface area → malabsorption, steatorrhea, weight loss.
• Loss of ileum (surgery) impairs bile salt reabsorption → fat malabsorption.
• ORS (oral rehydration solution) exploits intact Na+-glucose co-transport in the small intestine to rehydrate patients with diarrhoea.

Summary: The small intestine is anatomically and histologically specialised (folds, villi, microvilli, enzymes, transporters, vascular and lymphatic supply) to maximise digestion and efficient absorption of nutrients into blood and lymph.

Overview: The pancreas is a mixed gland (both exocrine and endocrine) located behind the stomach. In the context of digestion, the exocrine pancreas secretes pancreatic juice into the duodenum, which contains water, electrolytes (especially HCO3–) and digestive enzymes that act on carbohydrates, proteins and lipids.

Structure (brief):

• Acinar cells: form acini and produce digestive enzymes (stored as zymogens).
• Ductal cells (including centroacinar cells): secrete bicarbonate-rich fluid that makes pancreatic juice alkaline.
• Islets of Langerhans (endocrine) are separate and secrete insulin, glucagon — mentioned here only for completeness.

Composition of pancreatic juice:

• Volume: ~1–1.5 L/day (varies with diet and stimulants).
• Appearance: clear, alkaline, isotonic.
• Major electrolytes: high HCO3–, also Na+ and K+ similar to plasma.
• pH: ~7.8–8.3 (alkaline) — important for enzyme activity and neutralizing acidic chyme from stomach.
• Enzymes (mostly secreted as inactive precursors, zymogens):

• Carbohydrate: Pancreatic (alpha) amylase — starch & glycogen → maltose, oligosaccharides.
• Protein (proteolytic zymogens):
  • Trypsinogen → (activated by enterokinase/enteropeptidase on duodenal brush border) → trypsin.
  • Trypsin activates other zymogens: chymotrypsinogen → chymotrypsin, procarboxypeptidase → carboxypeptidase, proelastase → elastase.
  • These proteases break proteins into peptides and amino acids.
• Fats: Pancreatic lipase (major), phospholipase A2, cholesterol esterase — act on emulsified triglycerides to produce free fatty acids and monoacylglycerols (which form micelles with bile salts for absorption).

Activation cascade (key steps):

• Enterokinase (enteropeptidase) on duodenal mucosa converts trypsinogen → trypsin.
• Trypsin activates other pancreatic zymogens (autocatalytic amplification).
• Storing enzymes as zymogens prevents autodigestion of the pancreas.

Control of secretion:

• Hormonal: Secretin (released by S cells of duodenum in response to acidic chyme) stimulates ductal cells to secrete HCO3–-rich fluid. CCK (cholecystokinin, from I cells in response to fats & proteins) stimulates acinar cells to release enzyme-rich secretion.
• Neural: Parasympathetic (vagus) stimulation increases pancreatic secretion.

Physiological roles:

• Neutralization of acidic chyme (HCO3– protects intestinal mucosa and provides optimal pH for pancreatic enzymes).
• Digestion of carbohydrates, proteins and lipids in the small intestine.
• Working together with bile: bile salts emulsify fat; pancreatic lipase hydrolyses triglycerides; products form micelles for absorption.

Clinical correlations (short):

• Acute/chronic pancreatitis: autodigestion due to premature activation of zymogens; pain, elevated serum amylase/lipase.
• Cystic fibrosis: thick secretions block pancreatic ducts → insufficient enzyme delivery → steatorrhea, malabsorption, failure to thrive.
• Pancreatic exocrine insufficiency: causes steatorrhea; treated with pancreatic enzyme replacement therapy (enteric-coated enzymes).

Summary: The pancreas provides an alkaline, enzyme-rich juice crucial for final digestion in the duodenum. Bicarbonate neutralizes acid and creates the right pH for proteases, lipases and amylase; enzymes are secreted mainly as inactive zymogens to prevent self-digestion and are activated in the gut.

Overview
The liver is the largest internal organ and plays a central role in metabolism, detoxification and secretion of bile. Bile is an alkaline fluid produced by hepatocytes that helps in digestion and absorption of lipids. The gall bladder stores and concentrates bile and releases it into the duodenum when needed.

Structure & blood supply (brief)

• Functional unit: hepatic lobule — central vein surrounded by hepatocytes arranged in plates; portal triads at lobule corners contain a branch of hepatic artery, portal vein and bile ductule.
• Blood supply: hepatic artery (oxygenated) + portal vein (nutrient-rich) → hepatic sinusoids → central vein → hepatic veins → IVC. The liver receives roughly 20–30% of cardiac output.

Bile formation and flow

• Hepatocytes synthesize bile and secrete it into bile canaliculi between adjacent cells.
• Flow: bile canaliculi → bile ductules → intrahepatic ducts → right & left hepatic ducts → common hepatic duct. From here bile either flows via the cystic duct to the gall bladder for storage/concentration or down the common bile duct into the duodenum.
• At the duodenum, the sphincter of Oddi regulates bile entry. Cholecystokinin (CCK) stimulates gall bladder contraction and sphincter relaxation after a fatty meal; secretin increases biliary bicarbonate-rich secretion.

Composition of bile (major components and properties)

• Physical: yellow-green, alkaline (pH ≈ 7.6–8.6).
• Major constituents: water, bile salts (conjugated bile acids like glyco-/tauro-conjugates), bile pigments (mainly conjugated bilirubin), cholesterol, phospholipids (lecithin), and electrolytes.
• Bile salts are amphipathic molecules responsible for emulsification and micelle formation; they are key to lipid digestion and absorption of fat-soluble vitamins (A, D, E, K).

Functions related to digestion & absorption

• Emulsification: bile salts break large fat globules into fine droplets, increasing surface area for pancreatic lipase action (mechanical, not enzymatic).
• Micelle formation: bile salts and lecithin form micelles that solubilize fatty acids, monoglycerides and fat-soluble vitamins, enabling their diffusion across the intestinal mucosa.
• Excretion: bilirubin (a product of hemoglobin breakdown) and excess cholesterol are eliminated into bile for fecal excretion.
• Neutralization: biliary bicarbonate helps neutralize acidic chyme along with pancreatic secretions.

Enterohepatic circulation
Most bile salts are reabsorbed in the terminal ileum and returned via the portal vein to the liver for reuse. Typically ≈95% of bile salts are recycled multiple times per day (bile salt pool is reused several times after each meal).

Gall bladder: structure & role

• Hollow, pear-shaped organ lying on the inferior surface of the liver. Capacity ≈30–50 mL.
• Functions: stores bile, concentrates it (by absorbing water and electrolytes — up to ~10-fold concentration), and releases it when CCK is secreted after a fatty meal.

Clinical correlations (short)

• Cholelithiasis (gallstones): often cholesterol stones; can block cystic or common bile duct causing pain, obstructive jaundice or pancreatitis.
• Jaundice: accumulation of bilirubin — classified as pre-hepatic (hemolysis), hepatic (liver dysfunction), or post-hepatic/obstructive (blocked bile ducts).
• Cholestasis: impaired bile flow → fat malabsorption, deficiency of fat-soluble vitamins, pale stools and dark urine.

Summary points

• Hepatocytes produce ~500–1000 mL of bile per day (textbook range). Gall bladder stores ~30–50 mL but concentrates bile, releasing it on CCK stimulus.
• Bile salts emulsify fats and form micelles enabling absorption; most bile salts are recycled via enterohepatic circulation.
• Liver has multiple metabolic and detox functions beyond bile production, making it central to whole-body homeostasis.

Overview

The liver is the largest internal organ and performs many metabolic, synthetic and detoxification functions. In digestion it produces bile, a greenish-yellow alkaline fluid that helps in the digestion and absorption of fats. The gall bladder stores and concentrates bile and releases it into the duodenum in response to a fatty meal.

Synthesis and secretion of bile

• Liver hepatocytes continuously synthesize bile (≈ 600–1000 ml/day in adults). Bile is secreted into bile canaliculi → intrahepatic ducts → common hepatic duct.
• When no digestion is occurring, bile is diverted via the cystic duct to the gall bladder where it is concentrated (water and electrolytes absorbed) up to about 5–20× concentration.
• After a fatty meal, duodenal I-cells release CCK (cholecystokinin) and vagal stimulation causes gall bladder contraction and sphincter of Oddi relaxation, ejecting bile into the duodenum.

Composition of bile

Bile is a complex mixture. Major components include:

• Bile salts and bile acids (derived from cholesterol) — primary functional component for fat emulsification (e.g., cholic acid derivatives).
• Bilirubin (conjugated form) — pigment from haem breakdown responsible for bile colour.
• Phospholipids (mainly lecithin), cholesterol and electrolytes (Na+, K+, HCO3−).
• Water and small amounts of proteins and trace metals.

Functions of bile

• Emulsification of dietary fats: bile salts reduce surface tension and form mixed micelles so lipases can access triglycerides.
• Facilitates absorption of fat-soluble vitamins (A, D, E, K) by forming micelles.
• Excretion route for cholesterol, bile pigments (conjugated bilirubin) and some xenobiotics and drugs.
• Alkaline bile neutralises acidic chyme in the duodenum, aiding pancreatic enzyme activity.

Enterohepatic circulation

Most bile salts are reabsorbed in the terminal ileum and returned to the liver via the portal vein — this recycling conserves bile salts. A small amount (~5%) is lost in faeces and replaced by new synthesis from cholesterol.

Clinical correlations (brief)

• Gallstones (cholelithiasis): cholesterol or pigment stones form in the gall bladder. Typical presentation: biliary colic after fatty meal — severe right upper quadrant pain.
• Jaundice: elevated bilirubin in blood produces yellowing of skin and sclera. Causes include haemolysis, hepatic dysfunction (hepatitis), or obstructive jaundice (blocked bile duct).
• Cholecystectomy (gall bladder removal): bile flows directly from liver to intestine; fat digestion is usually adequate but some patients notice intolerance to very fatty meals.
• Ileal resection or disease: decreased bile salt reabsorption → bile salt deficiency → fat malabsorption and steatorrhea.

Key physical data: bile pH ~7.6–8.6; daily bile production ≈ 600–1000 ml; bile salt pool ~3–5 g with 12–18 g recycled per day.

Overview
The small intestine completes chemical digestion and carries out most nutrient absorption. Secretions involved here include intestinal juice (succus entericus) produced by intestinal glands (crypts of Lieberkühn) and membrane-bound brush border enzymes located on the microvilli of enterocytes. These together hydrolyse oligo- and disaccharides, peptides and nucleotides to absorbable units and activate pancreatic proenzymes.

Intestinal juice (Succus entericus)

• Composition: mostly water, electrolytes (Na+, K+, Cl-, HCO3-), mucus, and enzymes (peptidases, disaccharidases, nucleotidases). It is a neutral to slightly alkaline fluid (pH ~7–8) that moistens chyme and provides enzymes for final digestion.
• Source: secreted by crypts of Lieberkühn scattered along the small intestine epithelium; secretion increases with local irritation and parasympathetic stimulation.
• Major enzymatic functions: final hydrolysis of small peptides to amino acids, conversion of disaccharides to monosaccharides, digestion of nucleotides to bases, pentoses and phosphate.

Brush border enzymes
Brush border enzymes are integral membrane proteins anchored on microvilli (the ‘brush border’) of absorptive epithelial cells. Because they are membrane-bound they act on substrates at the epithelial surface and the products are immediately available for absorption.

• Key brush border enzymes and their actions:
  • Disaccharidases: sucrase (sucrase-isomaltase), maltase, lactase — convert sucrose, maltose, lactose to monosaccharides (glucose, fructose, galactose).
  • Peptidases (aminopeptidases, dipeptidases): split oligopeptides, dipeptides and tripeptides to free amino acids.
  • Enterokinase (enteropeptidase): activates trypsinogen (pancreatic zymogen) to trypsin which then activates other pancreatic proenzymes (cascade).
  • Nucleotidases and nucleosidases: degrade nucleic acids to bases and pentoses for absorption.
• Location advantage: Because these enzymes are membrane-bound, digestion occurs at the site of absorption reducing loss of nutrients and improving efficiency.

Activation cascade
Enterokinase on the brush border converts pancreatic trypsinogen to trypsin. Trypsin then activates other pancreatic proenzymes (chymotrypsinogen, proelastase, procarboxypeptidases), amplifying proteolysis.

Relation to absorption
Products formed by brush border enzymes are absorbed by specific transporters: monosaccharides and amino acids use carrier-mediated transport (some active, some facilitated). Important transport processes include:

• Glucose and galactose: absorbed by Na+-dependent cotransporter (SGLT1) at the apical side (secondary active transport using Na+ gradient), then exit to blood via GLUT2 on the basolateral side.
• Fructose: taken up by facilitated diffusion via GLUT5 (apical) and exits via GLUT2 (basolateral).
• Small peptides (di- and tri-peptides): taken up by H+-coupled cotransporters and further hydrolysed to amino acids inside enterocytes.

Physiological and clinical relevance — examples

• Lactose intolerance: deficiency of lactase (brush border enzyme) leads to undigested lactose in the lumen causing osmotic diarrhoea, bloating and gas due to bacterial fermentation.
• Celiac disease: immune-mediated damage to villi and microvilli reduces brush border enzyme activity leading to malabsorption of carbohydrates and proteins.
• Oral rehydration therapy (ORT): uses the Na+-glucose cotransport mechanism (SGLT1) to enhance water and Na+ absorption in diarrhoeal diseases — a direct practical application of brush border transport physiology.

Key points summary

• Intestinal juice provides enzymes for the final stages of digestion and a medium for absorption.
• Brush border enzymes are membrane-bound on microvilli — they complete carbohydrate and peptide digestion at the site of absorption.
• Enterokinase initiates activation of pancreatic proteases at the brush border.
• Damage to villi/brush border greatly impairs digestion and absorption; several diseases and genetic deficiencies illustrate this.

Definition & location: Intestinal juice (succus entericus) is a thin, watery secretion produced by glands in the mucosa of the small intestine (mainly crypts of Lieberkühn). It completes the enzymatic digestion started in the mouth and stomach and provides a medium for absorption. Brush border enzymes are membrane-bound enzymes located on the microvilli (brush border) of the intestinal epithelial cells (enterocytes) and carry out the final steps of chemical digestion.

Composition & properties:

• Volume: small (about 1–2 L/day, varies with diet and secretions from other glands).
• Appearance: watery, alkaline (pH ~7.5–8) due to bicarbonate.
• Contents: water, mucus, electrolytes, and digestive enzymes (mostly brush-border enzymes; some pancreatic enzymes also act in lumen).

Functions of succus entericus:

• Provides final enzymes for digestion of disaccharides and small peptides.
• Maintains alkaline pH to optimize intestinal enzyme activity.
• Helps solubilize nutrients and facilitates contact with absorptive surface.

Brush border structure & role: Microvilli on enterocytes increase surface area dramatically and carry the glycocalyx where brush border enzymes are immobilized. This arrangement allows digestion to occur right at the absorptive surface so products (monosaccharides, amino acids) can be absorbed immediately.

Important brush border enzymes (location: apical membrane/glycocalyx):

• Maltase: maltose → 2 glucose
• Sucrase (invertase): sucrose → glucose + fructose
• Lactase (β-galactosidase): lactose → glucose + galactose
• Isomaltase (α-dextrinase): α-limit dextrins → maltose/maltotriose → glucose
• Enterokinase (enteropeptidase): activates pancreatic trypsinogen → trypsin (key to activating other pancreatic proteases)
• Aminopeptidases / Dipeptidases / Oligopeptidases: break peptides into free amino acids or small peptides suitable for absorption

Sequence of carbohydrate and protein digestion (brief):

• Starch → (salivary & pancreatic amylase) → maltose, isomaltose, dextrins (in lumen)
• These → (maltase, isomaltase on brush border) → glucose (absorbed)
• Sucrose → (sucrase) → glucose + fructose (absorbed)
• Lactose → (lactase) → glucose + galactose (absorbed)
• Proteins → (pepsin, pancreatic proteases) → peptides → (brush border peptidases) → amino acids (absorbed)

Absorption linked to brush border: Monosaccharides are absorbed by specific transporters: SGLT1 (Na+-dependent glucose/galactose cotransporter) on the apical membrane and GLUT2 on the basolateral membrane. Na+/K+ ATPase on the basolateral membrane maintains the Na+ gradient that drives cotransport.

Clinical/physiological notes:

• Lactose intolerance: lactase deficiency (congenital or acquired) leaves lactose undigested; it is fermented by gut bacteria producing gas (flatulence, bloating) and osmotically retains water causing diarrhea.
• Defects in enterokinase (rare) impair activation of pancreatic proteases causing protein maldigestion.
• Damage to brush border (e.g., rotavirus, celiac disease) causes carbohydrate malabsorption and diarrhea.
• Infants have high lactase activity (to digest milk); in many adults lactase decreases after weaning.

Key points to remember:

• Succus entericus provides the environment and brush border enzymes for final digestion.
• Brush border enzymes are membrane-bound — digestion occurs at absorption site.
• Enterokinase activates trypsinogen — a crucial step for protein digestion.
• Absorption uses carrier systems (SGLT1, GLUT2, amino acid transporters) driven by Na+ gradient.

Overview: Carbohydrates in food are mainly polysaccharides (starch, glycogen), disaccharides (sucrose, lactose) and monosaccharides (glucose, fructose, galactose). Digestion converts complex carbohydrates to monosaccharides which are absorbable. Digestion begins in the mouth, continues mainly in the small intestine, and involves both luminal (salivary and pancreatic) and membrane-bound (brush border) enzymes.

Stepwise process

• Mouth: Salivary amylase (ptyalin) hydrolyses starch (amylose, amylopectin) into dextrins and disaccharide maltose. Optimum pH ~6.7–7.0. Chewing increases surface area and starts enzymatic hydrolysis; this is why starchy foods can taste slightly sweet if chewed long enough.
• Stomach: Salivary amylase activity continues briefly but is inactivated by low gastric pH (<4.5). No significant carbohydrate digestion occurs in the acidic stomach environment.
• Small intestine (duodenum and jejunum):
  • Pancreatic amylase: Continues breakdown of starch/dextrins in the lumen to maltose, isomaltose and small oligosaccharides. Optimum pH ~7–8.
  • Brush border enzymes (intestinal mucosal lining): Membrane-bound disaccharidases complete digestion at the enterocyte surface: maltase (maltose → 2 glucose), isomaltase (α-limit dextrins → glucose), sucrase/invertase (sucrose → glucose + fructose), lactase (lactose → glucose + galactose). These enzymes act at the glycocalyx of enterocytes so products are released right next to transporters for absorption.
• Absorption across enterocytes:
  • Glucose and galactose are absorbed by secondary active transport via SGLT1 (sodium-dependent glucose transporter) at the apical membrane. SGLT1 uses the Na+ gradient established by Na+/K+ ATPase. Typical stoichiometry: 2 Na+ : 1 glucose (SGLT1) — this couples downhill Na+ entry to uphill glucose uptake.
  • Fructose is absorbed by facilitated diffusion via GLUT5 at the apical membrane.
  • All three monosaccharides exit the enterocyte at the basolateral membrane via GLUT2 (facilitated diffusion) into the interstitial fluid and then into the hepatic portal vein to the liver.

Energy and transporters: The Na+ gradient required for SGLT1 is maintained by Na+/K+ ATPase (3 Na+ pumped out, 2 K+ pumped in per ATP hydrolysed). Thus carbohydrate absorption is indirectly energy-dependent.

Physiological/regulatory and clinical points:

• Lactase activity is high in infants and often declines with age in many populations, causing lactose intolerance (undigested lactose in colon → fermentation, gas, osmotic diarrhea).
• Damage to intestinal mucosa (viral enteritis, celiac disease) reduces brush border enzymes causing carbohydrate malabsorption.
• Antacids or alkaline conditions can allow salivary amylase to be active longer; very acidic stomach pH quickly halts it.

Quick timeline: Mouth (minutes) → Stomach (minutes; mostly inactivation) → Duodenum/Jejunum (major digestion and absorption within hours following a meal).

Key take-home: Complex carbohydrates are broken down stepwise by amylases (luminal) and disaccharidases (brush border) to monosaccharides that are absorbed by specific transporters (SGLT1, GLUT5, GLUT2) into the blood.

Overview: Digestion is the enzymatic hydrolysis of large food molecules into small absorbable units. Major food types are carbohydrates, proteins, lipids (fats), and nucleic acids. Digestion begins in the mouth, continues in the stomach and small intestine, and finishes at the brush border of the small intestine. Absorption primarily occurs across the small intestinal epithelium (villi and microvilli).

1. Carbohydrates

• Major substrates: starch (amylose/amylopectin), glycogen, disaccharides (sucrose, lactose).
• Sites & enzymes:
  • Mouth: salivary alpha-amylase (ptyalin) — partial hydrolysis of starch to dextrins and maltose; optimum pH ≈ 6.5–7.
  • Stomach: salivary amylase is inactivated by low pH; no significant carbohydrate digestion by gastric enzymes.
  • Small intestine (pancreas): pancreatic alpha-amylase — further hydrolyses starch to maltose and limit dextrins (optimum pH ≈ 7–8).
  • Brush border (intestinal mucosa): disaccharidases — maltase, sucrase, lactase convert maltose, sucrose, lactose to monosaccharides (glucose, fructose, galactose).
• Products & absorption: monosaccharides (mainly glucose, plus fructose and galactose). Absorbed by enterocytes via SGLT1 (secondary active Na+-dependent uptake for glucose and galactose) and GLUT5 (facilitated diffusion for fructose); exit to blood via GLUT2.

2. Proteins

• Major substrates: dietary proteins (albumins, globulins, collagen), peptides.
• Sites & enzymes:
  • Stomach: pepsin (secreted as pepsinogen by chief cells; activated by HCl) — cleaves proteins into large peptides; optimum pH ≈ 1.5–3.
  • Small intestine (pancreas): trypsinogen (activated to trypsin by enteropeptidase), chymotrypsinogen (to chymotrypsin), elastase, carboxypeptidases — produce smaller peptides and free amino acids.
  • Brush border & cytosolic peptidases: aminopeptidases, dipeptidases — produce free amino acids.
• Products & absorption: free amino acids, di- and tripeptides. Amino acids absorbed mainly by Na+-dependent co-transporters; some di-/tripeptides absorbed by H+-dependent cotransport and further hydrolysed inside enterocytes. Amino acids are released into the portal blood to the liver.

3. Lipids (Fats)

• Major substrates: triglycerides (triacylglycerols), phospholipids, cholesterol esters.
• Sites & mechanisms:
  • Mouth & stomach: limited lingual and gastric lipase activity (important for infants and some TAGs).
  • Small intestine: bile salts (from liver) emulsify fat droplets, increasing surface area. Pancreatic lipase (with colipase) hydrolyses triglycerides to 2-monoacylglycerol and free fatty acids; pancreatic phospholipase A2 hydrolyses phospholipids; cholesterol esterase acts on cholesterol esters.
  • Micelle formation: bile salts form mixed micelles that solubilize fatty acids, monoacylglycerols, and lipid-soluble vitamins for transport to the brush border.
• Products & absorption: 2-monoacylglycerols, free fatty acids, glycerol, free cholesterol. These diffuse into enterocytes (facilitated by micelles). Inside enterocytes, long-chain fatty acids and monoacylglycerols are re-esterified into triglycerides, packaged with apolipoproteins into chylomicrons, and transported via lacteals (lymph) into the bloodstream. Short- and medium-chain fatty acids can enter portal blood bound to albumin.

4. Nucleic Acids

• Nucleic acids (DNA, RNA) are hydrolysed by pancreatic nucleases (DNase, RNase) to nucleotides, and brush border nucleotidases and nucleosidases break them down to nucleosides and free bases and sugars for absorption.

5. Large Intestine & Microbial contribution

• Indigestible carbohydrates (cellulose, resistant starch) are fermented by colonic microbiota to short-chain fatty acids (acetate, propionate, butyrate) and gases. SCFAs provide energy to colonocytes and have systemic effects.

Key physiological notes:

• pH environments: mouth ~6.5–7, stomach ~1.5–3, small intestine ~7–8 — enzyme optima reflect these.
• Surface area: villi and microvilli increase absorptive area enormously — critical for efficient absorption.
• Transport routes: monosaccharides and amino acids → portal vein → liver; chylomicrons (long-chain fats) → lymphatic system → systemic circulation.

Summary flow (simplified): Carbohydrates → mouth/pancreas/brush border → monosaccharides → blood. Proteins → stomach/pancreas/brush border → amino acids → blood. Fats → bile emulsification + pancreatic lipase → monoacylglycerols & FAs → re-esterified → chylomicrons → lymph.

Overview
Protein digestion is the enzymatic breakdown of dietary proteins into free amino acids (and small peptides) that can be absorbed by intestinal epithelial cells and transported to the blood. It is a staged process that begins in the stomach and is completed in the small intestine and at the intestinal brush border.

Stepwise process

1) Mouth and oesophagus: Mechanical breakdown (chewing) and mixing with saliva occur, but there is no significant chemical digestion of proteins in the mouth.

2) Stomach: Parietal cells secrete HCl (pH ~1.5–2), which denatures tertiary and quaternary structure of proteins, exposing peptide bonds. Chief (zymogen) cells release pepsinogen, which is activated by HCl (and by pepsin) to pepsin — an endopeptidase. Pepsin cleaves internal peptide bonds preferentially near aromatic amino acids (Phe, Trp, Tyr), producing large polypeptides, proteoses and peptones.

3) Small intestine — pancreatic phase: Acid chyme entering the duodenum is neutralized by pancreatic bicarbonate. Pancreatic acinar cells secrete zymogens: trypsinogen, chymotrypsinogen, proelastase and procarboxypeptidases. Enteropeptidase (enteropeptidase) on the duodenal brush border converts trypsinogen to active trypsin; trypsin then activates the other zymogens. Key specificities: trypsin cleaves at the carboxyl side of Lys/Arg; chymotrypsin prefers aromatic residues; elastase acts on small neutral residues; carboxypeptidases are exopeptidases removing C-terminal residues.

4) Brush border and cytosolic peptidases: Peptidases anchored on microvilli (aminopeptidases, dipeptidases, tripeptidases) and intracellular peptidases complete hydrolysis to free amino acids. Many di- and tripeptides are taken up intact and hydrolyzed inside enterocytes.

Absorption and transport
Amino acids are absorbed mainly by Na+-dependent co-transporters (secondary active transport driven by Na+ gradient maintained by Na+/K+ ATPase). Dipeptides and tripeptides are absorbed via H+-dependent cotransporters and then hydrolyzed inside the enterocyte to free amino acids. Amino acids exit the basolateral membrane by facilitated diffusion and enter the hepatic portal vein to the liver.

Physiological safeguards
Zymogens prevent autodigestion of the pancreas and gut lining. Acid in the stomach helps denature proteins and provides the acidic optimum for pepsin, while pancreatic bicarbonate raises pH to the alkaline optimum for pancreatic proteases (pH ~7–8).

Clinical relevance
Conditions that impair gastric acidity (long-term antacid/PPI use) or pancreatic enzyme secretion (chronic pancreatitis, cystic fibrosis) reduce protein digestion and cause malabsorption. Some raw legumes contain trypsin inhibitors that reduce protein digestibility until destroyed by cooking.

Overview
Absorption is the uptake of digested products, water, electrolytes and vitamins from the alimentary canal into the blood or lymph. In humans most absorption occurs in the small intestine (especially jejunum and ileum) because of large surface area provided by villi and microvilli (brush border).

Structural adaptations that aid absorption

• Villi and microvilli increase surface area enormously, shortening diffusion distances.
• Rich capillary network and lacteals maintain concentration gradients by rapidly removing absorbed substances.
• Brush border enzymes finish digestion at the epithelial surface facilitating immediate uptake.

Main mechanisms of absorption

• Simple (passive) diffusion: Movement down a concentration gradient without carriers or energy. Typical for lipid-soluble molecules (fatty acids, monoglycerides, some vitamins like A, D, E, K) and small non-polar molecules (alcohol). Rate ∝ concentration gradient and surface area.
• Facilitated diffusion: Carrier or channel proteins permit passive movement of polar molecules down their concentration gradient (example: GLUT transporters for monosaccharides across membranes). This is saturable and exhibits carrier kinetics.
• Active transport (primary and secondary):
  • Primary active transport uses ATP directly (e.g., Na+/K+ ATPase on the basolateral membrane pumps 3 Na+ out and 2 K+ in). This maintains low intracellular Na+, creating an electrochemical gradient.
  • Secondary active transport uses the Na+ gradient to drive uptake of nutrients against their concentration gradients (symporters). Examples: SGLT1 (Na+-glucose co-transporter) transports glucose and galactose with Na+; many amino acids are taken up by Na+-dependent co-transporters.
• Osmosis: Water moves across epithelium following osmotic gradients created by solute absorption. Large-scale water absorption occurs in small and large intestine.
• Endocytosis / Receptor-mediated uptake: Some substances are internalized by pinocytosis or receptor-mediated endocytosis. Important example: vitamin B12 binds intrinsic factor in stomach, then the B12–IF complex is taken up by receptor-mediated endocytosis in the ileum.
• Lipid absorption: Lipids are emulsified by bile salts into micelles, diffuse into enterocytes, get re-esterified to triglycerides in the endoplasmic reticulum, packaged into chylomicrons and secreted into lacteals (lymph). Chyle in lacteals gives a milky appearance after a fatty meal.

Physiological notes
Because many transporters are carrier-mediated, absorption shows saturation (maximum rate Vmax) and can be competitively inhibited. The continuous removal of absorbed material by blood and lymph preserves gradients which favour ongoing absorption.

Sites of absorption

• Stomach: little — some water, alcohol, certain drugs.
• Duodenum and jejunum: main site for carbohydrates, amino acids, iron, calcium.
• Ileum: bile salts, vitamin B12 (with intrinsic factor).
• Large intestine: water, Na+, Cl− and some vitamins produced by gut flora (vitamin K, some B vitamins).

Clinical and practical importance
Examples: Oral rehydration solution (ORS) exploits Na+-glucose co-transport to enhance water and salt absorption in diarrhoea. Damage to villi (coeliac disease) or lack of intrinsic factor (pernicious anaemia) lead to malabsorption.

Overview
Absorption is the transfer of digested food from the intestinal lumen into the blood or lymph. Different nutrients use different mechanisms — simple diffusion, facilitated diffusion, active transport, co-transport and endocytosis — depending on their chemistry and solubility. Most absorption occurs in the small intestine (duodenum, jejunum, ileum); the colon absorbs water, electrolytes and some short-chain fatty acids.

Carbohydrates
Dietary polysaccharides and disaccharides are digested to monosaccharides (mainly glucose, galactose, fructose). Glucose and galactose are absorbed across the apical membrane of enterocytes by sodium-dependent co-transporters (SGLT1) — this is secondary active transport driven by the Na+ gradient maintained by Na+/K+–ATPase on the basolateral membrane. Fructose enters by facilitated diffusion via GLUT5 on the apical side and exits the cell to the blood by GLUT2 on the basolateral side. Absorbed monosaccharides enter the hepatic portal vein and go to the liver.

Proteins
Dietary proteins are hydrolysed to amino acids, dipeptides and tripeptides. Free amino acids are taken up by multiple Na+-dependent carrier systems (apical) and exit the cell to blood via facilitated diffusion or antiporters on the basolateral side. Most small peptides (di- and tripeptides) are absorbed via H+-dependent cotransporter PepT1 and are often hydrolysed to amino acids inside enterocytes before entering the portal circulation. The Na+/K+–ATPase is again essential to maintain ionic gradients.

Lipids (fats)
Dietary triglycerides are first emulsified by bile salts and then hydrolysed by pancreatic lipase to 2-monoglycerides and free fatty acids (FFAs). These hydrophobic products and fat-soluble vitamins form mixed micelles with bile salts that diffuse to the brush border. FFAs and monoglycerides diffuse across the apical membrane into enterocytes (passive diffusion). Inside enterocytes they are re‑esterified into triglycerides in the smooth endoplasmic reticulum, packaged with cholesterol and apolipoproteins into chylomicrons, and secreted by exocytosis into lacteals (intestinal lymph). Chylomicrons bypass the hepatic portal vein and enter the venous circulation via the thoracic duct.

Water, Electrolytes and Minerals
Water follows osmotic gradients created by solute absorption (mostly Na+ cotransported with nutrients). Na+ is absorbed by multiple mechanisms (channels, co-transport with glucose/amino acids, exchangers). Cl- and HCO3- fluxes balance charges. K+ is largely absorbed passively. Calcium and iron use specialized transporters and regulated pathways (e.g., vitamin D–dependent Ca2+ absorption in the duodenum).

Fat‑ and Water‑Soluble Vitamins
Fat‑soluble vitamins (A, D, E, K) are absorbed with dietary lipids via micelles and packaged into chylomicrons (lymph route). Water‑soluble vitamins (B complex, C) are absorbed by diffusion or specific carriers; B12 requires intrinsic factor and is absorbed in the terminal ileum by receptor-mediated endocytosis.

Transport destinations
Most water‑soluble nutrients (monosaccharides, amino acids, small peptides, water-soluble vitamins, minerals) are transported via the hepatic portal vein to the liver. Lipids packaged as chylomicrons enter intestinal lymph (lacteals) and then the systemic circulation.

Clinical relevance / examples
- Oral rehydration therapy (ORS) exploits Na+–glucose co-transport (SGLT1) to enhance water and electrolyte absorption in diarrhoea.
- Bile salt deficiency (cholestasis) or pancreatic insufficiency causes fat malabsorption (steatorrhea).
- Diseases that damage villi (e.g., celiac disease) reduce surface area and impair absorption of multiple nutrients.

Key points to remember
- Carbohydrates: SGLT1 (glucose/galactose + Na+), GLUT5 (fructose), GLUT2 (basolateral).
- Proteins: Na+-dependent amino acid transporters, PepT1 for peptides.
- Lipids: micelle formation → diffusion → re‑esterification → chylomicron → lacteals.
- Na+/K+–ATPase on basolateral membrane is essential to maintain ion gradients that drive secondary active transport.

Overview: Lipids (mainly triacylglycerols, TAGs) are hydrophobic molecules that require emulsification and enzymatic hydrolysis to be absorbed. Digestion begins minimally in the mouth and stomach, but most lipid digestion and absorption occur in the small intestine with the help of bile salts and pancreatic enzymes.

Stepwise process:

• Mouth: Mechanical mixing; lingual lipase (secreted by glands of tongue) begins limited hydrolysis of short- and medium-chain TAGs. Activity is low compared with intestinal digestion.
• Stomach: Gastric lipase (from chief cells) continues partial hydrolysis of TAGs (especially in neonates and for milk fat). Acidic pH allows continued activity of lingual/gastric lipases, producing di- and mono-glycerides and free fatty acids (FFAs).
• Duodenum — Emulsification: Arrival of acidic chyme stimulates secretin and cholecystokinin (CCK). CCK causes gallbladder contraction (releasing bile) and pancreatic enzyme secretion. Bile salts (amphipathic molecules synthesized from cholesterol and stored in gallbladder) emulsify large fat droplets into many small droplets, increasing surface area.
• Duodenum — Enzymatic hydrolysis: Pancreatic juice contains pancreatic lipase (with colipase), phospholipase A2 and cholesterol esterase. Pancreatic lipase (active at near-neutral pH) hydrolyzes TAGs primarily at the sn-1 and sn-3 positions to yield two free fatty acids and one 2-monoacylglycerol (2-MAG). Colipase anchors lipase to the lipid droplet surface when bile salts would otherwise displace it.
• Micelle formation: Bile salts, 2-MAG, FFAs, cholesterol and lysophospholipids form mixed micelles — tiny soluble aggregates that ferry lipid digestion products to the enterocyte (brush border) surface.
• Absorption into enterocytes: Short-chain and medium-chain fatty acids (≤ ~12 carbons) can cross enterocyte membranes and enter portal blood (bound to albumin). Long-chain fatty acids, 2-MAGs and cholesterol are delivered in micelles to the brush border and taken up by diffusion and transport proteins.
• Intracellular re-esterification and chylomicron formation: Inside the smooth endoplasmic reticulum of enterocytes, 2-MAG and FFAs are re-esterified (via acyl-CoA intermediates and enzymes MGAT/DGAT) to form TAGs. TAGs are packaged with cholesterol esters, phospholipids and apolipoproteins into chylomicrons (lipoprotein particles) in the Golgi. Chylomicrons are released by exocytosis into lacteals (lymphatic capillaries) because they are too large for blood capillaries.
• Lymphatic transport and blood entry: Chylomicrons travel in lymph and enter the bloodstream via the thoracic duct into the left subclavian vein. In blood, lipoprotein lipase (activated by ApoC-II) hydrolyzes chylomicron TAGs, supplying FFAs to tissues (adipose, muscle). Chylomicron remnants are taken up by the liver.

Hormonal and regulatory factors: CCK stimulates pancreatic enzyme secretion and gallbladder contraction; secretin stimulates pancreatic bicarbonate secretion (neutralizes acid, optimal for pancreatic enzymes).

Clinical correlations (brief): Bile salt deficiency (e.g., cholestasis) or pancreatic insufficiency (e.g., cystic fibrosis) leads to poor lipid digestion and steatorrhea (fatty, foul-smelling stools). Drugs like orlistat inhibit pancreatic lipase, reducing fat absorption and causing oily stools.

Summary: Effective lipid digestion requires emulsification by bile salts, hydrolysis by pancreatic lipase (with colipase), formation of micelles, absorption and intracellular reassembly of TAGs into chylomicrons for lymphatic transport.

Overview
Nucleic acids (DNA and RNA) present in food are broken down in the small intestine into their building blocks — nitrogenous bases, pentose sugars (ribose or deoxyribose) and phosphate — which can be absorbed and reused or further degraded.

Where digestion occurs
Little or no nucleic acid digestion occurs in the mouth or stomach. The main events take place in the duodenum and jejunum: pancreatic secretions provide endonucleases and exonucleases, and the intestinal (brush border) enzymes complete the process so the products can be absorbed by enterocytes.

Enzymes and steps

• Pancreatic nucleases (RNase, DNase) and phosphodiesterases: act in the lumen of the small intestine to break nucleic acids into oligonucleotides and nucleotides by cleaving phosphodiester bonds.
• Intestinal (brush border) nucleotidases / phosphatases: remove the phosphate group from nucleotides to form nucleosides.
• Nucleosidases / nucleoside phosphorylases: split nucleosides into free nitrogenous bases and pentose (usually as ribose-1-phosphate).
• Transport into enterocytes: most absorption is of nucleosides (and some free bases) via specific carrier-mediated (often active) transport systems. Free nucleotides are not efficiently absorbed unless dephosphorylated first.

End products and fate
Final products available to the body are: free nitrogenous bases (purines and pyrimidines), pentose sugars (ribose or deoxyribose) and inorganic phosphate. Inside cells these can be:

• Reconverted to nucleotides (via phosphorylation of nucleosides) and used for synthesis of DNA, RNA and ATP (salvage pathways).
• Degraded: purine bases are ultimately catabolized to xanthine and then uric acid (excreted), while pyrimidines are broken down to soluble products that enter central metabolism.

Physiological importance
Digestion and absorption of dietary nucleic acids supply materials for rapidly dividing cells (gut lining, immune cells) and for recovery of nucleotides without the energetic cost of de novo synthesis. The liver and many tissues also carry out de novo nucleotide synthesis, but the salvage of dietary nucleosides is energetically economical.

Important notes for Class 11 level
Remember the sequence: nucleic acids → oligonucleotides → nucleotides → nucleosides → free bases + pentoses. Pancreatic enzymes act in the lumen; brush-border enzymes on the intestinal surface; absorption is mainly of nucleosides.

After digestion, the small intestine absorbs end-products (monosaccharides, amino acids, fatty acids, monoglycerides, glycerol, vitamins, minerals, and water). Their route and fate depend on solubility and cell handling.

Routes of transport

• Water-soluble products (glucose, fructose, galactose, amino acids, small peptides, salts): absorbed into the capillaries of intestinal villi → blood → hepatic portal vein → liver. From liver they enter systemic circulation.
• Fat-soluble products (long-chain fatty acids, monoglycerides): taken up by enterocytes, re-esterified to triglycerides, packaged into chylomicrons → released into lacteals (intestinal lymph) → thoracic duct → left subclavian vein → systemic circulation (bypass liver initially).
• Vitamins and minerals: water-soluble vitamins (B, C) follow blood route; fat-soluble vitamins (A, D, E, K) follow chylomicron/lymph route. Iron and calcium have regulated active transport and binding proteins.

Fate in the liver and tissues

• Liver is the metabolic hub: it receives blood from hepatic portal vein and modifies, stores, detoxifies and redistributes absorbed substances.
• Carbohydrates: monosaccharides arriving at liver may be (a) oxidized for energy (glycolysis → TCA cycle), (b) converted to glycogen (glycogenesis) for storage, or (c) converted to fatty acids if in excess (lipogenesis).
• Proteins/amino acids: used for protein synthesis (plasma proteins, enzymes), or deaminated; amino groups enter urea cycle to form urea for excretion; carbon skeletons can be used for energy or converted to glucose (gluconeogenesis) or fat.
• Lipids: chylomicrons deliver triglycerides to adipose and muscle (lipoprotein lipase releases fatty acids). Liver synthesizes VLDL, converts lipids into cholesterol and phospholipids, and assembles HDL/LDL for lipid transport and redistribution.
• Detoxification and excretion: liver detoxifies ammonia (urea), drugs and toxins; stores certain vitamins and iron; modifies hormones.

Physiological regulation & transport mechanisms

• Transport across enterocyte membrane: passive diffusion (for lipids and some small solutes), facilitated diffusion (GLUT transporters for monosaccharides), and active transport (sodium-dependent amino acid and glucose transporters).
• Hormonal control: insulin promotes glucose uptake by tissues and glycogen synthesis in liver and muscle; glucagon promotes glycogen breakdown and gluconeogenesis.

Clinical/relevance points

• If lymph flow is obstructed, fat absorption is impaired and lipids may appear in stool or tissues (lymphedema, steatorrhea).
• Liver disease disrupts metabolism (reduced plasma proteins, impaired detoxification, abnormal lipid and carbohydrate handling).

Summary: Water-soluble nutrients go to the liver via portal blood; fat-soluble nutrients travel via lymph as chylomicrons and enter the blood before reaching the liver. The liver determines metabolic fate: immediate energy, storage (glycogen, fat), synthesis (proteins, lipoproteins), or detoxification/excretion.

Definition: Absorption is the process by which the end products of digestion (monosaccharides, amino acids, fatty acids, glycerol), water, electrolytes and vitamins move from the lumen of the small intestine into the blood and lymph.

Primary site: Small intestine (especially jejunum and ileum) — large surface area due to folds (plicae circulares), villi and microvilli (brush border).

Structural basis for efficient absorption:

• Villi: finger-like projections with a central capillary network and a lacteal (lymph vessel).
• Enterocytes with microvilli: increase surface area and contain transporters and brush-border enzymes.
• Thin epithelial lining and rich blood supply: maintain steep concentration gradients.

Mechanisms of absorption:

1. Simple (passive) diffusion: Movement down a concentration gradient across the epithelial membrane without transport proteins or energy. Lipid-soluble molecules (e.g., short-chain fatty acids, some drugs) and gases follow this route.

2. Facilitated diffusion: Solute moves down its electrochemical gradient via specific carrier proteins or channels (no direct energy). Example: some monosaccharides (in specific steps) and water-soluble vitamins may use facilitated carriers.

3. Active transport: Movement against concentration/electrochemical gradient using energy (ATP). Two forms:

• Primary active transport — direct use of ATP (e.g., Na+/K+ ATPase on basolateral membrane maintains low intracellular Na+, enabling secondary transport).
• Secondary active transport (co-transport/coupled transport) — energy from an ion gradient (usually Na+) drives uptake of another substance.

Key example — Sodium–glucose co-transport (SGLT1) (secondary active transport): On the apical membrane of enterocytes, SGLT1 uses the inward Na+ gradient to bring glucose or galactose into the cell against its concentration gradient. Glucose then exits the cell across the basolateral membrane via GLUT2 (facilitated diffusion) into blood.

4. Cotransport of amino acids and peptides: Most amino acids are absorbed by Na+-dependent amino acid transporters. Small peptides (di-/tri-peptides) are often taken up by H+-dependent peptide transporters (PEPT1) and hydrolyzed inside the cell to amino acids.

5. Osmosis (water absorption): Water moves passively across epithelium following osmotic gradients set up by solute absorption (mainly Na+). Hence reabsorption of solutes drives water uptake.

6. Endocytosis / pinocytosis / receptor-mediated endocytosis: Uptake of large molecules or complexes. Examples: vitamin B12 bound to intrinsic factor is absorbed in the ileum by receptor-mediated endocytosis; occasional uptake of macromolecules in infants by pinocytosis.

Fat absorption (special pathway):

• Bile salts emulsify dietary triglycerides and aid formation of micelles containing free fatty acids (FFA), monoglycerides, cholesterol and fat-soluble vitamins.
• FFAs and monoglycerides diffuse across the apical membrane into enterocytes.
• Inside enterocytes they are re-esterified to triglycerides, combined with cholesterol and apoproteins to form chylomicrons.
• Chylomicrons are too large for capillaries and enter lacteals (lymph); they are transported via lymph and enter the blood via the thoracic duct (giving milky chyle appearance after fatty meal).

Absorption of specific nutrients and factors:

• Monosaccharides (glucose, galactose): SGLT1 (apical) + GLUT2 (basolateral).
• Fructose: GLUT5 (apical, facilitated diffusion) and GLUT2 (basolateral).
• Amino acids: Na+-dependent transporters; di-/tri-peptides via PEPT1.
• Fat-soluble vitamins (A, D, E, K): with fats in micelles and absorbed like lipids.
• Water-soluble vitamins (B, C): specific transporters or diffusion; vitamin B12 requires intrinsic factor and ileal receptors.
• Electrolytes: Na+ actively absorbed (major driver), Cl- follows passively or via channels, Ca2+ regulated by vitamin D and absorbed via channels.

Physiological importance and clinical relevance:

• Oral rehydration therapy (ORT) exploits Na+-glucose co-transport: glucose + Na+ in ORS promotes water absorption even when secretory diarrhoea persists.
• Pernicious anaemia: lack of intrinsic factor → inability to absorb vitamin B12 in ileum → megaloblastic anaemia.
• Fat malabsorption (steatorrhea) occurs when bile secretion is impaired (e.g., cholestasis) or pancreatic lipases are deficient.

Summary: Absorption is a multimechanistic process using passive and active transport, specialized structures (villi/microvilli), and distinct pathways for fats (via lymph) versus water-soluble nutrients (via blood). Efficient absorption depends on maintained gradients, transporter proteins and surface area.

Overview
Movements of the alimentary canal are coordinated contractions and relaxations of smooth (and some skeletal) muscle that mix, propel and control passage of food, chyme and feces. These movements are produced by the muscularis externa (inner circular and outer longitudinal smooth muscle), controlled locally by the enteric nervous system (myenteric and submucosal plexuses) and modulated by the autonomic nervous system and gastrointestinal hormones.

Main types of movements

• Peristalsis: A propulsive, wave‑like contraction. Circular muscle contracts behind the bolus and relaxes ahead, longitudinal muscle shortens the segment — this creates a high‑pressure zone behind and pushes content forward. Occurs in oesophagus, stomach (antral peristalsis), small intestine and colon (slow waves). Function: propulsion and gastric emptying.
• Segmentation (rhythmic contractions): Alternating contractions of circular muscle at adjacent segments that do not produce net forward movement but mix chyme with digestive secretions and increase contact with mucosa. Prominent in small intestine.
• Pendular movements: Longitudinal muscle contractions that produce to-and‑fro motion of intestinal contents, enhancing mixing and exposure to absorptive surfaces.
• Tonic contractions: Sustained contractions of sphincters (lower oesophageal sphincter, pyloric sphincter, ileocaecal valve, anal sphincters) that control passage between compartments.
• Mass movements: Strong, long colonic contractions that move fecal matter over large distances towards the rectum (often after meals; gastrocolic reflex).
• Migrating Motor Complex (MMC): A cyclical pattern of activity in the fasting state (phases I–III) that sweeps residual undigested material through the small intestine.

Mechanism at cellular level
Smooth muscle in the gut shows electrical slow waves (basal electrical rhythm) generated by interstitial cells of Cajal. When slow waves reach threshold (often aided by neural/hormonal input), action potentials and Ca2+‑dependent contractions occur. Myogenic activity plus enteric reflexes coordinate local and long‑range patterns.

Regulation
Neural: intrinsic enteric reflexes (short reflexes) and extrinsic autonomic input (parasympathetic generally excitatory, sympathetic inhibitory). Hormonal: gastrin, motilin (stimulates MMC and gastric motility), cholecystokinin (CCK) modulates gastric emptying, secretin influences motility indirectly.

Physiological significance
These movements ensure mechanical digestion, mixing with enzymes and bile, exposure to absorptive surfaces, controlled transit for absorption, and timely elimination of waste.

Clinical correlations
Disorders: achalasia (impaired oesophageal peristalsis/LES relaxation), gastroparesis (reduced gastric motility), intestinal obstruction (absent distal peristalsis, increased proximal peristalsis and pain), irritable bowel syndrome (altered motility), diarrhoea (increased transit), constipation (reduced colonic motility).

Overview
The small intestine (duodenum, jejunum, ileum) is the primary site for nutrient absorption. Its internal surface is enormously increased by plicae circulares (Kerckring folds), villi and microvilli (brush border) to maximise contact between chyme and absorptive epithelium.

Surface structure and cells

• Plicae circulares: permanent circular folds that slow down chyme and increase surface area.
• Villi: finger-like mucosal projections containing capillary loops and a central lacteal (lymphatic vessel).
• Microvilli: epithelial cell (enterocyte) membrane projections forming the brush border; contain digestive enzymes and transporters.
• Enterocytes, goblet cells and enteroendocrine cells: enterocytes absorb nutrients; goblet cells secrete mucus; enteroendocrine cells release hormones (e.g., CCK, secretin) that regulate digestion.

Mechanisms of absorption

• Simple diffusion: small nonpolar molecules and short-chain fatty acids move down their concentration gradients across the membrane.
• Facilitated diffusion: carrier proteins (e.g., GLUT transporters for monosaccharides) allow movement down a gradient without ATP.
• Active transport: carrier-mediated transport against a concentration gradient driven indirectly by Na+ gradient (maintained by basolateral Na+/K+‑ATPase). Example: Na+-coupled amino acid and glucose uptake.
• Co-transport (symport): e.g., SGLT1 (Na+-glucose cotransporter) uses Na+ gradient to take up glucose and galactose into enterocytes.
• Endocytosis / transcytosis: uptake of large molecules or immunoglobulins (important in neonates for antibodies) and receptor‑mediated uptake (e.g., vitamin B12–intrinsic factor complex in the ileum).

Absorption of major nutrients

• Carbohydrates: Dietary polysaccharides are digested to monosaccharides. Glucose and galactose enter enterocytes via SGLT1 (Na+‑dependent); fructose enters via GLUT5 (facilitated diffusion). All exit the basolateral side to blood via GLUT2. Absorbed monosaccharides go to the liver via the hepatic portal vein.
• Proteins: Digested to amino acids, di- and tripeptides. Amino acids use specific Na+-dependent carriers. Di- and tripeptides are taken up by PepT1 (H+-coupled) and hydrolysed to amino acids inside the cell; amino acids exit to blood and enter the portal circulation.
• Lipids: Emulsified by bile salts, digested by pancreatic lipase into free fatty acids (FFAs) and monoglycerides which form micelles. FFAs and monoglycerides diffuse into enterocytes, are re-esterified to triglycerides in smooth ER, packaged into chylomicrons in Golgi and secreted into lacteals (lymph). Chylomicrons enter systemic circulation via thoracic duct, bypassing the hepatic portal vein initially.
• Vitamins and minerals: Fat-soluble vitamins (A, D, E, K) are absorbed with lipids via micelles → lacteals. Water-soluble vitamins (B group, C) use diffusion or specific carriers. Vitamin B12 binds intrinsic factor and is absorbed in the ileum by receptor-mediated uptake. Calcium absorption is transcellular and regulated by vitamin D; iron is absorbed via DMT1 and exported by ferroportin (regulated by hepcidin).
• Water and electrolytes: Water follows osmotic gradients (bulk absorption). Na+ absorption is critical and is coupled to nutrient uptake; Cl- and HCO3- exchange maintain electroneutrality.

Pathways to circulation
Most monosaccharides, amino acids, water-soluble vitamins and small water-soluble products enter blood capillaries of villi → hepatic portal vein → liver. Fat-soluble products (chylomicrons) enter lacteals → lymphatic system → systemic circulation via thoracic duct.

Factors affecting absorption

• Surface area (reduced in diseases like celiac/surgical resection → malabsorption)
• Transit time: increased motility (diarrhea) reduces absorption; slowed transit increases absorption
• Integrity of brush border enzymes and transporters
• pH and presence of bile/pancreatic secretions

Clinical and physiological notes
Examples of clinical relevance include lactose intolerance (undigested lactose remains in lumen → osmotic diarrhoea), steatorrhea when fat absorption is defective (pancreatic insufficiency, bile salt deficiency), vitamin B12 deficiency (pernicious anemia, ileal disease), and the basis of oral rehydration therapy (ORT) which exploits intact Na+-glucose cotransport to promote water and electrolyte absorption during diarrhoea).