Chapter 7 — Structural Organisation In Animals

Overview

Living organisms show a structured hierarchy of organization — from molecules to the whole organism — that enables complexity, specialization and efficient functioning. Each higher level is made of lower-level units arranged and cooperating to produce new properties (emergent properties) not seen at lower levels. This hierarchical organisation allows division of labour, increased efficiency and adaptability.

Major Levels of Organisation

• Molecular and Chemical Level: Biomolecules (carbohydrates, proteins, lipids, nucleic acids), ions and small compounds. These provide structure, catalysis (enzymes) and information (DNA/RNA).
• Cellular Level: The cell is the basic structural and functional unit of life. Types include prokaryotic and eukaryotic cells; unicellular organisms (e.g., Amoeba, Paramecium) perform all life processes within one cell. In multicellular organisms, cells are specialised (nerve cells, muscle cells, epithelial cells) to perform particular functions.
• Tissue Level: A tissue is a group of similar cells working together to perform a common function. Animal tissues include epithelial, connective, muscular and nervous tissues. Example: cardiac muscle tissue composes heart walls.
• Organ Level: An organ is a structure composed of two or more tissue types organized to perform specific tasks. Examples: heart (pumps blood), lungs (gas exchange), liver (metabolism and detoxification).
• Organ System Level: Organs that work together form an organ system to carry out broad physiological roles. Examples: circulatory (heart, blood vessels), digestive (stomach, intestines, liver, pancreas), nervous (brain, spinal cord, nerves), respiratory (lungs, trachea).
• Organism Level: The complete living being capable of independent existence — sum of all organ systems functioning in integrated manner (e.g., human, dog, bird).

Key Principles

• Emergent properties: Higher levels show new properties (e.g., consciousness emerges at organism/organ system level, not at single-cell level).
• Division of labour and specialisation: Cells and tissues specialise for efficiency (e.g., red blood cells carry oxygen, neurons conduct impulses).
• Interdependence: Organs and systems depend on each other; dysfunction in one organ/system affects others.
• Cell differentiation: Multicellular development involves stem cells differentiating into specialised cell types under genetic and environmental control.

Unicellular vs Multicellular Organisation

• Unicellular organisms perform all life functions within one cell; they show organismal-level integration at cellular level.
• Multicellular organisms show cellular specialization, tissues and organs; they achieve greater size and complexity but require systems for communication and transport (nervous, hormonal, circulatory).

Relevance to Physiology and Adaptation

Levels of organisation explain how large organisms overcome limits such as diffusion distances (by developing circulatory systems), and how size affects physiology (surface area:volume constraints).

Animal Tissues — Introduction

Tissues are groups of similar cells that perform a common function. In animals, tissues form the next level of organization above cells and together make up organs. There are four primary types of animal tissues: epithelial, connective, muscular and nervous. Each type has characteristic cells, extracellular components and functions.

Epithelial Tissue

• Structure: Closely packed cells with little extracellular matrix. Cells rest on a basal lamina (basement membrane) and show polarity (apical and basal surfaces).
• Cell junctions: Tight junctions, adherens, desmosomes, gap junctions.
• Functions: Protection, absorption, secretion, filtration, sensory reception.
• Types: Simple (one cell layer) and stratified (many layers); shape: squamous, cuboidal, columnar; specialised forms: pseudostratified, ciliated epithelium, glandular epithelium.

Connective Tissue (CT)

• Structure: Few cells widely spaced in an abundant extracellular matrix (ECM) composed of fibres (collagen, elastic, reticular) and ground substance.
• Major cell types: Fibroblasts, macrophages, mast cells, adipocytes, chondrocytes, osteocytes, blood cells.
• Functions: Support, binding of tissues, protection, transport (blood), storage (adipose), repair.
• Types: Loose (areolar, adipose, reticular), dense (regular, irregular), specialized (cartilage, bone, blood, lymph).

Muscular Tissue

• Structure: Cells (muscle fibres) specialized for contraction; contain contractile proteins (actin, myosin).
• Types & features:
  • Skeletal: striated, multinucleated, voluntary — attached to bones for locomotion.
  • Cardiac: striated, branched, single central nucleus, involuntary, intercalated discs for coordinated contraction — heart.
  • Smooth: non-striated, spindle-shaped, involuntary — walls of gut, blood vessels, uterus.

Nervous Tissue

• Structure: Neurons (nerve cells) and neuroglia (supporting cells). Neuron parts: cell body (soma), dendrites (receive signals), axon (conducts impulses).
• Functions: Receive, conduct and process information; coordinate activities of the body; reflexes, sensation, thought.

Additional concepts

• Regenerative capacity: Epithelial and many connective tissues regenerate readily; skeletal muscle has limited regeneration; cardiac muscle and most neurons have poor regenerative ability (important for repair and clinical outcomes).
• Stem cells: Tissue-specific stem cells (e.g., epithelial stem cells, hematopoietic stem cells) are responsible for renewal and repair.
• Functional integration: Organs are made of combinations of tissues (e.g., stomach: mucosal epithelium, connective tissue, smooth muscle and nerves).

Why this matters: Understanding tissues explains how organs work, how wounds heal, how diseases (inflammation, fibrosis, cancer) affect the body, and the basis for therapies (stem cells, tissue engineering).

Introduction: Epithelial tissue (epithelium) is a sheet of closely packed cells with very little extracellular matrix that covers body surfaces, lines cavities, forms glands and interfaces for exchange. It shows polarity (apical and basal surfaces), is avascular, but innervated, and rests on a basement membrane.

• General characteristics
  • Polarity: apical surface (may have cilia/microvilli) and basal surface (attached to basement membrane)
  • Cell junctions: tight (occluding) junctions, adherens/desmosomes, gap junctions
  • Basement membrane: extracellular layer (basal lamina + reticular lamina) that anchors epithelium to connective tissue
  • Avascular: nutrients diffuse from underlying connective tissue
  • High regenerative capacity
• Classification (by number of layers)
  • Simple epithelium – single cell layer (for absorption, secretion, filtration)
  • Stratified epithelium – two or more layers (for protection)
  • Pseudostratified – appears multilayered but every cell contacts the basement membrane (often ciliated)
  • Transitional epithelium – special stratified epithelium that stretches (urinary tract)
• Classification (by cell shape on apical surface)
  • Squamous – flattened cells (thin for diffusion)
  • Cuboidal – cube-shaped (for secretion/absorption)
  • Columnar – tall column-shaped (absorption/secretion; often has microvilli/cilia)
• Common types with structure, function and locations
  • Simple squamous epithelium – single flat layer; function: rapid diffusion and filtration; locations: alveoli of lungs, glomerular capsule (Bowman’s capsule) in kidney, endothelium of blood vessels. Suggested diagram: thin flattened cells with nucleus bulging.
  • Simple cuboidal epithelium – single layer of cube-shaped cells; function: secretion and absorption; locations: kidney tubules, ducts of glands, thyroid follicles.
  • Simple columnar epithelium – single tall cells often with microvilli and goblet cells; function: absorption and secretion; locations: small intestine (microvilli), stomach and large intestine (goblet cells).
  • Pseudostratified ciliated columnar epithelium – appears stratified but all cells touch basement membrane; often has cilia and goblet cells; locations: trachea and upper respiratory tract; function: mucus secretion and movement by cilia.
  • Stratified squamous epithelium
    • Non-keratinized: moist surfaces (oral cavity, esophagus, vagina) for protection
    • Keratinized: epidermis of skin; outer layers have dead keratin-filled cells for waterproofing and abrasion resistance
  • Transitional epithelium (urothelium) – stratified cells that can stretch and change shape; location: urinary bladder, ureters; function: allows distension.
  • Glandular epithelium
    • Unicellular glands: goblet cells that secrete mucus
    • Multicellular glands: exocrine (with ducts) and endocrine (ductless, release hormones into blood)
• Modes of secretion by exocrine glands
  • Merocrine (eccrine): secretion by exocytosis (e.g., pancreatic acinar cells, sweat glands eccrine type)
  • Apocrine: apical portion pinches off carrying secretory products (e.g., mammary glands, some sweat glands)
  • Holocrine: whole cell disintegrates to release content (e.g., sebaceous glands)
• Functions of epithelial tissue
  • Protection (stratified epithelium)
  • Absorption (intestinal epithelium)
  • Secretion (glandular epithelium)
  • Diffusion and filtration (simple squamous epithelium)
  • Sensory reception (specialised epithelia, e.g., taste buds)
  • Excretion and transport (ciliated epithelia)
• Clinical relevance
  • Carcinomas: cancers arising from epithelial cells (e.g., squamous cell carcinoma)
  • Disorders of secretion (e.g., cystic fibrosis affects epithelial ion transport and mucus secretion)
  • Barrier dysfunctions lead to infections and fluid loss
• Observation and staining
  • Light microscopy with H&E stain is commonly used; specialized stains and electron microscopy reveal junctions, basement membrane and surface specializations (cilia, microvilli).

Summary: Epithelial tissues form continuous sheets that protect, absorb, secrete and allow exchange. Their form (layers and shape) reflects their function and location.

Definition: Connective tissue is a type of animal tissue that connects, supports, binds or separates other tissues and organs. It is characterised by abundant intercellular material (extracellular matrix, ECM) and relatively few cells.

Main components:

• Cells – fibroblasts/fibrocytes (produce fibres & ground substance), adipocytes (store fat), chondrocytes (cartilage), osteocytes (bone), blood cells (in blood), macrophages, mast cells, plasma cells.
• Fibres – collagen (strength), elastic (elasticity), reticular (fine supportive network).
• Ground substance (matrix) – amorphous gel of glycosaminoglycans (GAGs), proteoglycans and water; determines tissue compressibility and diffusion.

Classification and key features:

1. Connective tissue proper – contains fibres and ground substance; cells mainly fibroblasts.

• Loose (areolar): loosely arranged collagen & elastic fibres in abundant ground substance. Location: beneath epithelium, binds skin to muscles. Function: support, diffusion, immune defence.
• Adipose: adipocytes filled with lipid droplets. Location: hypodermis, around kidneys, omentum. Function: energy storage, insulation, cushioning.
• Reticular: network of reticular fibres (type III collagen). Location: lymph nodes, spleen, bone marrow. Function: soft internal skeleton (stroma).
• Dense connective tissue – tightly packed fibres.
  • Dense regular: parallel collagen bundles (tendons, ligaments) — high tensile strength along fibre axis.
  • Dense irregular: bundles in different directions (dermis) — resists multi-directional stress.

2. Cartilage – firm, flexible connective tissue with chondrocytes in lacunae and a semi-rigid ECM (collagens + proteoglycans). Avascular; heals slowly.

• Hyaline cartilage: glassy matrix, fine collagen (articular surfaces, tracheal rings, embryonic skeleton).
• Elastic cartilage: abundant elastic fibres (pinna of ear, epiglottis).
• Fibrocartilage: dense collagen bundles (intervertebral discs, pubic symphysis, menisci).

3. Bone (osseous tissue) – mineralised matrix (hydroxyapatite) with collagen; cells: osteoblasts, osteocytes, osteoclasts. Compact bone shows osteons (Haversian systems); spongy bone has trabeculae. Functions: mechanical support, protection, mineral reservoir, blood cell production (marrow).

4. Blood (fluid connective tissue) – liquid plasma matrix with suspended formed elements: erythrocytes (RBCs), leukocytes (WBCs), platelets. Functions: transport of gases, nutrients, wastes, hormones; immune defence; thermoregulation.

Functions of connective tissue:

• Support and structural framework (bone, cartilage).
• Binding and packing of tissues and organs (areolar tissue).
• Protection and cushioning (adipose, bone, cartilage).
• Transport of substances (blood).
• Storage of energy (adipose) and minerals (bone).
• Immune defence and repair (macrophages, mast cells, fibroblast-mediated wound repair).

Special notes:

• ECM largely determines mechanical properties: more fibres and mineral = stiffer (bone); more proteoglycans and water = compressible (cartilage).
• Cartilage is avascular; nutrients diffuse through matrix, so healing is slow.
• Bone remodels continuously under hormonal and mechanical control.

Microscopy / identification tips:

• Areolar: many cell types, loose fibres, lots of ground substance.
• Adipose: large empty-looking cells (lipid removed in histological prep) with peripheral nuclei.
• Hyaline cartilage: smooth, glassy matrix with chondrocytes in lacunae.
• Bone: concentric lamellae with osteocyte lacunae and central Haversian canal.
• Blood smear: RBCs (biconcave, without nucleus), WBCs (larger, nucleated), platelets (small fragments).

Clinical relevance / examples of disease: osteoporosis (loss of bone mass), osteoarthritis (degeneration of articular cartilage), scurvy (defective collagen synthesis), atherosclerosis (connective tissue changes in vessels), lipomas (benign adipose tumours).

Definition: Muscular tissue is a specialized contractile tissue composed of cells (muscle fibers) that can shorten and generate force to produce movement, maintain posture and pump fluids (blood, lymph).

Types of muscular tissue:

• Skeletal (striated voluntary) muscle: Attached to bones by tendons; multinucleated long fibres; shows clear striations (alternating A and I bands); under voluntary control by the somatic nervous system. Example functions: locomotion, posture, facial expressions.
• Cardiac muscle: Found only in heart; branched, uninucleated or binucleated cells; striated and connected by intercalated discs (gap junctions & desmosomes) for coordinated contraction; involuntary (autonomic control).
• Smooth (visceral) muscle: Spindle-shaped cells, no striations; found in walls of gut, blood vessels, iris, urinary bladder; involuntary control (autonomic & local factors); capable of slow, sustained contractions.

Microstructure of a skeletal muscle (key components):

• Muscle fibre (cell): Enclosed by sarcolemma; contains many myofibrils and multiple nuclei located peripherally.
• Myofibrils: Long cylindrical organelles made of repeating sarcomeres (the contractile unit).
• Sarcomere: Region between two Z-lines; composed of thin (actin) and thick (myosin) filaments. Visible pattern: A-band (dark, thick + overlapping thin), I-band (light, thin only), H-zone (thick only), M-line (middle).
• Sarcoplasmic reticulum (SR) & T-tubules: SR stores Ca2+; T-tubules carry action potentials deep into the fibre to trigger Ca2+ release.

Mechanism of contraction — Sliding Filament Theory (summary):

1. An action potential at the neuromuscular junction causes acetylcholine release → depolarisation of sarcolemma → AP travels down T-tubules.
2. Depolarisation triggers Ca2+ release from SR into sarcoplasm.
3. Ca2+ binds troponin on actin filaments → tropomyosin shifts, exposing myosin-binding sites on actin.
4. Myosin heads (energised by ATP hydrolysis) bind actin forming cross-bridges, then pivot (power stroke) pulling actin toward the centre of the sarcomere; ADP + Pi released.
5. New ATP binds myosin → myosin detaches from actin; ATP is hydrolysed to re-cock the myosin head for another cycle.
6. When Ca2+ is pumped back into SR, troponin-tropomyosin block is restored and muscle relaxes.

Properties of muscle tissue: Excitability (irritability), contractility, extensibility, elasticity.

Types of contraction:

• Isotonic: Muscle changes length while tension remains approximately constant (concentric = shortens; eccentric = lengthens under load).
• Isometric: Muscle generates tension without changing length (holding a weight stationary).

Functional notes & clinical relevance: Cardiac muscle has rhythmic autorhythmicity (pacemaker cells). Smooth muscle responds to hormones and local signals and can show tonic contractions. Satellite cells in skeletal muscle allow limited regeneration; severe damage or genetic disorders (e.g., muscular dystrophy) impair function.

Summary (one-line): Muscular tissue converts chemical energy (ATP) into mechanical work via regulated interactions between actin and myosin within sarcomeres; different muscle types are specialised for voluntary movement, rhythmic pumping and slow sustained contractions.

Definition and overview: Nervous tissue is a specialised tissue that forms the nervous system and is responsible for receiving stimuli, conducting impulses and coordinating body activities. It is composed mainly of two cell types: neurons (nerve cells) and neuroglial (glial) cells. Neurons transmit electrical signals; neuroglia support, protect and maintain the environment for neurons.

Structure of a neuron:

• Cell body (soma/perikaryon): Contains nucleus, Nissl bodies (rough ER) and organelles; site of metabolic activity.
• Dendrites: Short, branched processes that receive incoming signals and convey them toward the cell body.
• Axon: A single long process that conducts impulses away from the cell body to other neurons, muscles or glands. Axon hillock is the site of action-potential initiation.
• Axon terminal (synaptic terminal): Ends that make synapses and release neurotransmitters.
• Myelin sheath: Lipid-rich insulating layer formed by Schwann cells in PNS and oligodendrocytes in CNS. Gaps called Nodes of Ranvier enable saltatory conduction.

Types of neurons (structural): Unipolar (one process), bipolar (one axon + one dendrite), multipolar (many dendrites, one axon). Functional types: Sensory (afferent), Motor (efferent), Interneurons (relay/association).

Neuroglia (glial cells): Supporting cells that outnumber neurons. Major CNS types: astrocytes (metabolic support, blood–brain barrier contribution), oligodendrocytes (myelination in CNS), microglia (phagocytosis), ependymal cells (line ventricles). Major PNS types: Schwann cells (myelination, axon repair) and satellite cells (support ganglia).

Physiology — impulse generation and conduction:

• Resting membrane potential: Typically around −70 mV (inside negative) due to differential distribution of Na+, K+, Cl− and selective membrane permeability.
• Action potential: A rapid, stereotyped change in membrane potential: depolarization (Na+ influx), repolarization (K+ efflux), and often brief hyperpolarization. The action potential is an all-or-none event when membrane reaches threshold (~ −55 mV).
• Saltatory conduction: In myelinated axons, action potentials jump between Nodes of Ranvier, greatly increasing conduction velocity compared to unmyelinated fibres.
• Synaptic transmission: At chemical synapses, arrival of an action potential triggers Ca2+ influx, vesicle fusion and neurotransmitter release; neurotransmitter binds receptors on the postsynaptic cell causing excitatory or inhibitory postsynaptic potentials.

Histological features: Nissl bodies (rough ER) in soma and dendrites, absence of Nissl in axon hillock, distinctive staining of myelin in white matter vs grey matter in CNS.

Clinical and real-life relevance: Demyelinating diseases (e.g., multiple sclerosis) slow or block conduction; peripheral nerve injury and regeneration involve Schwann cells; reflexes (knee-jerk) illustrate rapid neural circuits; neuromuscular junction dysfunction (myasthenia gravis) affects muscle control.

Key functions summarized: reception of stimuli, generation and conduction of nerve impulses, integration and storage of information (learning and memory), coordination of body responses, metabolic and structural support by glia.

Definition of an organ: An organ is a structure composed of two or more types of tissues that work together to perform one or more specific functions. In animals, the four principal tissue types that form organs are epithelial, connective, muscular and nervous tissues.

Definition of an organ system: An organ system is a group of organs that cooperate to carry out major life processes and maintain the internal environment (homeostasis). Each organ system performs specific functions but is integrated with other systems.

Structural features:

• Organs show division of labour: different tissues specialise for particular subfunctions (e.g., secretion, support, contraction, conduction).
• An organ has a characteristic organisation: layers (e.g., stomach: mucosa, submucosa, muscularis, serosa) or chambers (e.g., heart).
• Organ systems are hierarchical: cells → tissues → organs → organ systems → organism.

Functional significance:

• Emergent properties: the organ performs functions that individual tissues alone cannot (e.g., coordinated pumping action of the heart).
• Integration and interdependence: organ systems cooperate to sustain life (e.g., respiratory and circulatory systems exchange and transport gases).
• Homeostasis: organ systems regulate internal conditions (temperature, pH, ion balance, blood glucose, water balance).

Examples of typical organs and their tissue composition:

• Heart: cardiac muscle tissue (myocardium), connective tissue (valves, fibrous skeleton), endothelium of blood vessels, and specialised conductive nervous tissue.
• Kidney: epithelial tissue (nephrons), connective tissue capsule, blood vessels and nerves.
• Liver: hepatocytes (parenchyma), connective tissue framework, rich blood supply.
• Stomach: epithelial lining (secretion), smooth muscle layers (mechanical mixing), connective tissue and nerve plexuses.

Major organ systems (with key functions):

• Digestive system – ingestion, digestion, absorption, egestion.
• Circulatory (cardiovascular) system – transport of gases, nutrients, wastes and hormones.
• Respiratory system – gas exchange (O2 uptake, CO2 release).
• Excretory/Urinary system – removal of metabolic wastes, osmoregulation.
• Nervous and endocrine systems – control and coordination via electrical and chemical signalling.
• Musculoskeletal system – support, movement and protection.
• Reproductive system – production of gametes and reproduction.
• Integumentary/skin system – protection, thermoregulation, sensory reception.

Integration and examples of inter-system cooperation:

• Exercise: skeletal muscles need more O2 → respiratory rate and cardiac output increase (respiratory + circulatory + muscular systems).
• Digestion and absorption: digestive enzymes break down food → absorbed nutrients enter blood and are carried to liver and other organs (digestive + circulatory + hepatic).
• Temperature regulation: skin, blood vessels and sweat glands adjust heat loss; hypothalamus coordinates responses (integumentary + circulatory + nervous).

Clinical and practical relevance:

• Damage to one organ affects the whole system (e.g., myocardial infarction reduces heart function and impairs circulation).
• Organ transplants and regenerative medicine rely on understanding tissue composition and organ function.

Summary: Organs are multi-tissue units specialised for tasks; organ systems are organised sets of organs that work together to maintain life and homeostasis. Understanding organs and organ systems reveals how complexity and division of labour produce integrated function in animals.

Brief definition: The digestive system is the organ system that breaks down food into smaller molecules that can be absorbed and used by body cells for energy, growth and repair. In animals digestion may be intracellular or extracellular; in higher animals it occurs mainly extracellularly in a digestive tract (alimentary canal) with accessory glands.

Types of digestion:

• Intracellular digestion: Occurs inside cells (e.g., sponges, some protozoa) where food vacuoles fuse with lysosomes.
• Extracellular digestion: Occurs in a cavity or lumen (e.g., alimentary canal of vertebrates). Food is processed outside cells then absorbed.

Human alimentary canal – major parts & functions:

• Mouth (oral cavity): Ingestion, mechanical digestion by teeth (mastication), chemical digestion begins using salivary amylase (ptyalin) which acts on starch. Tongue tastes and helps mix food to form bolus.
• Pharynx & Oesophagus: Transport of bolus by swallowing and peristalsis. No digestion occurs in oesophagus.
• Stomach: Mechanical churning and chemical digestion. Gastric glands secrete HCl (low pH ~1.5–3.5) and pepsinogen (activated to pepsin) to start protein digestion; mucous protects stomach lining.
• Small intestine (duodenum → jejunum → ileum): Principal site of chemical digestion and nutrient absorption. Duodenum receives bile (from liver/gallbladder) for emulsification of fats and pancreatic juice (enzymes + bicarbonate) for neutralization and digestion (pancreatic amylase, trypsin, chymotrypsin, lipase). Brush border enzymes (maltase, sucrase, lactase, peptidases) complete digestion to absorbable forms. pH ~7.4–8.
• Large intestine (colon): Absorbs water, electrolytes; forms feces. Gut flora ferment undigested carbohydrates, synthesize some vitamins (K, B-group).
• Rectum & Anus: Storage and egestion of feces.

Accessory organs and their roles:

• Salivary glands: Secrete saliva with amylase, mucous; begin starch digestion and lubricate food.
• Liver: Produces bile (no enzymes) which emulsifies fats into micelles, aiding lipase action; also processes absorbed nutrients and detoxifies.
• Gallbladder: Stores and concentrates bile; releases it into duodenum on CCK stimulus.
• Pancreas: Exocrine secretion of digestive enzymes (amylase, lipase, proteases) and bicarbonate; endocrine part secretes insulin/glucagon (metabolic regulation).

Major digestive enzymes and sites of action:

• Salivary amylase: starch → maltose (mouth)
• Pepsin (stomach): proteins → peptides
• Pancreatic amylase: starch → maltose (small intestine)
• Trypsin, chymotrypsin, carboxypeptidase: proteins → peptides → amino acids (pancreas/intestinal lumen)
• Pancreatic lipase: triglycerides → monoglycerides + fatty acids (small intestine, aided by bile)
• Brush-border enzymes (maltase, sucrase, lactase, peptidases): disaccharides/peptides → monosaccharides/amino acids (intestinal mucosa)

Stages of digestion: Ingestion → Mechanical digestion → Chemical digestion → Absorption → Assimilation → Egestion.

Regulation: Neural (cephalic phase via vagus nerve) and hormonal (gastrin: stimulates HCl & pepsinogen; secretin: stimulates pancreatic bicarbonate; cholecystokinin (CCK): stimulates pancreatic enzymes & gallbladder contraction).

Absorption details: Monosaccharides and amino acids are absorbed through villi into the blood capillaries; fatty acids and monoglycerides are re-esterified into triglycerides in enterocytes, packaged as chylomicrons and transported via lacteals (lymph).

Important physiological notes: Typical pH values — saliva ~6.8–7.0; stomach ~1.5–3.5; pancreatic juice ~8; small intestine ~7.4–8. Enzyme optima correspond to local pH. Transit time approximate: stomach 2–6 hours; small intestine 3–6 hours; large intestine 12–48 hours.

Common disorders (brief): Indigestion/dyspepsia, gastroesophageal reflux disease (GERD), peptic ulcer (H. pylori involvement), lactose intolerance (deficient lactase), pancreatitis, malabsorption (e.g., celiac disease).

Summary: Efficient digestion requires coordinated mechanical processes, specific enzymes, appropriate pH, bile-assisted emulsification for lipids, intact absorptive surface (villi and microvilli) and proper neural/hormonal control.

Definition & Overview: The circulatory system (also called the cardiovascular system) is the organ system that transports substances to and from cells to maintain homeostasis. In animals it typically consists of a pump (heart), a fluid medium (blood or hemolymph), and a network of vessels or sinuses through which the fluid flows.

Main Components: Blood (plasma + formed elements), heart, blood vessels (arteries, arterioles, capillaries, venules, veins) and the lymphatic system. Functions include transport of gases, nutrients, hormones and wastes; regulation of body temperature and pH; and protection through immune cells and clotting mechanisms.

Types of Circulatory Systems: Open (e.g., most arthropods — hemolymph bathes organs in sinuses) and closed (e.g., vertebrates — blood confined to vessels). According to circuit arrangement: single circulation (fishes: heart → gills → body → heart) and double circulation (amphibians/reptiles partial double; birds and mammals: complete double circulation with separate pulmonary and systemic circuits).

Heart Structure & Blood Flow (mammals): A four-chambered heart — two atria (receive blood) and two ventricles (pump blood). Right side handles deoxygenated blood: Right atrium → Right ventricle → pulmonary arteries → lungs. Left side handles oxygenated blood: Pulmonary veins → Left atrium → Left ventricle → aorta → systemic circulation. Valves (AV valves: tricuspid, bicuspid/mitral; semilunar valves: pulmonary, aortic) ensure unidirectional flow.

Cardiac Conduction & Cardiac Cycle: Electrical impulse originates at the sinoatrial (SA) node → atrioventricular (AV) node → Bundle of His → Purkinje fibres, causing coordinated contraction. Cardiac cycle phases: atrial systole (atria contract), ventricular systole (ventricles contract), diastole (relaxation & filling). Heart sounds "lub-dub" correspond to AV valve closure and semilunar valve closure respectively.

Blood Vessels: Arteries: thick muscular and elastic walls, carry blood away from heart (usually oxygenated). Arterioles: small branches that regulate flow via constriction/dilation. Capillaries: single-layer endothelium for exchange (gases, nutrients, wastes) between blood and tissues. Venules & veins: return blood to heart, valves in veins prevent backflow. Capillary exchange occurs by diffusion, filtration (hydrostatic pressure) and reabsorption (osmotic pressure).

Blood Composition & Functions: Plasma (water, proteins — albumin, globulins, fibrinogen — salts, nutrients, hormones) and formed elements: erythrocytes (RBCs, carry O2 via haemoglobin), leukocytes (WBCs, immune defense), platelets (thrombocytes, clotting). Functions: transport, regulation (temperature, pH, osmotic balance), protection (immune response, clot formation).

Lymphatic System: Network of lymph nodes and vessels that collects interstitial fluid (lymph) and returns it to blood, absorbs dietary fats (via lacteals) and participates in immune surveillance.

Regulation & Homeostasis: Short-term regulation via autonomic nervous system (sympathetic increases HR and contractility; parasympathetic decreases HR) and baroreceptor reflexes. Long-term regulation via hormones (e.g., adrenaline, ANP, ADH) and renal control of blood volume.

Common Disorders (brief): Atherosclerosis (plaque narrowing arteries), hypertension (high blood pressure), myocardial infarction (heart attack due to blocked coronary arteries), anemia (low RBC/haemoglobin), haemophilia (defective clotting), varicose veins (valve failure).

Educational Points for Class 11: Understand differences between open/closed and single/double circulation, structure of the mammalian heart and types of blood vessels, mechanism of cardiac cycle and valve action, and basic composition and functions of blood and lymph. Focus on diagrams and flowcharts to trace blood flow and exchange processes.

Overview

The respiratory system is the organ system that brings O2 into the body and removes CO2 produced by cellular respiration. It includes conducting passages that condition and transport air and the respiratory portion where gas exchange occurs.

Major structural components

• Upper respiratory tract: Nose/nasal cavity (filtering, warming, humidifying), paranasal sinuses, pharynx.
• Lower respiratory tract: Larynx (voice box), trachea, bronchi, bronchioles, terminal bronchioles, respiratory bronchioles, alveolar ducts and alveoli.
• Alveoli: Thin-walled sac-like structures with a large combined surface area (~70–100 m2 in adult humans) and extremely thin blood–gas barrier (alveolar epithelium + interstitial space + capillary endothelium) optimized for diffusion.

Histology and special features

Conducting airways are lined with ciliated pseudostratified columnar epithelium and mucus-secreting goblet cells (mucociliary escalator). Bronchioles lack cartilage and have smooth muscle allowing bronchoconstriction/bronchodilation. Alveoli are lined by type I pneumocytes (thin for gas exchange) and type II pneumocytes (surfactant secretion to reduce surface tension).

Mechanics of breathing

Breathing is a mechanical process driven by pressure differences produced by changes in thoracic volume.

• Inspiration: Diaphragm contracts and flattens, external intercostals raise ribs — thoracic cavity volume increases, intrapulmonary pressure falls below atmospheric pressure → air flows in. Mostly active.
• Expiration: Usually passive at rest — diaphragm relaxes, elastic recoil of lungs and thoracic cage decreases thoracic volume, intrapulmonary pressure rises above atmospheric → air flows out. Active expiration uses abdominal and internal intercostal muscles.

Boyle’s law (P ∝ 1/V) underlies the pressure–volume relationship in the chest.

Gas exchange and transport

• Gases move by diffusion down partial pressure gradients: O2 moves from alveolar air (high PO2) into pulmonary capillary blood (low PO2); CO2 moves in opposite direction because PaCO2 in blood is higher than in alveolar air.
• Blood transport: O2 is mainly carried bound to hemoglobin (Hb) as oxyhemoglobin; a small amount is dissolved in plasma. CO2 is transported dissolved, as bicarbonate (HCO3−) after hydration by carbonic anhydrase, and as carbamino compounds on Hb.
• Oxygen–hemoglobin dissociation curve: shows Hb saturation versus PO2; factors shifting the curve (temperature, pH, PCO2, 2,3-BPG) alter O2 unloading in tissues.

Regulation of respiration

Breathing is controlled by the respiratory centers in the medulla oblongata and pons. Central and peripheral chemoreceptors respond primarily to changes in CO2 (via H+ in CSF) and to a lesser extent to O2 (peripheral carotid and aortic bodies), adjusting rate and depth of breathing.

Lung volumes and capacities (typical adult resting values)

• Tidal volume (TV): ~500 ml
• Inspiratory reserve volume (IRV): ~1900–3100 ml
• Expiratory reserve volume (ERV): ~700–1200 ml
• Residual volume (RV): ~1100–1200 ml
• Vital capacity (VC = IRV + TV + ERV): ~3100–4800 ml
• Total lung capacity (TLC = VC + RV): ~4200–6000 ml
• Functional residual capacity (FRC = ERV + RV)

Clinical/relevant conditions

• Asthma: reversible airway obstruction due to bronchospasm and inflammation; treated with bronchodilators and steroids.
• Chronic obstructive pulmonary disease (COPD): chronic bronchitis and emphysema causing airflow limitation and decreased elastic recoil.
• Pneumonia: infection causing alveolar filling with exudate, impairing gas exchange.
• Pneumothorax: air in pleural space collapsing lung; disrupts negative intrapleural pressure.

Applications and artificial support

CPR and artificial ventilation (bag–valve mask, mechanical ventilators) create positive-pressure ventilation to force air into lungs. Oxygen therapy increases inspired O2 fraction to improve arterial oxygenation. Divers and high-altitude physiology illustrate adaptations and risks (hypoxia, decompression sickness).

Overview

The excretory system removes metabolic wastes and helps maintain water, electrolyte and acid–base balance. Different animal groups use different organs and nitrogenous waste forms depending on their habitat and evolutionary adaptations.

Excretory products and organs in animals

• Ammonotelic animals (most aquatic animals, e.g., many fishes): excrete ammonia directly — highly toxic but easily diluted in water.
• Ureotelic animals (most mammals, including humans): convert ammonia to urea (less toxic, water soluble) — conserves some water.
• Uricotelic animals (birds, many reptiles, terrestrial insects): excrete uric acid — relatively non-toxic, excreted as semisolid paste conserving water.

Human excretory system — main components

Major organs: paired kidneys, ureters, urinary bladder, urethra. Kidneys filter blood, form urine and regulate fluid/electrolyte/acid–base balance.

Kidney structure

External: bean-shaped with cortex (outer), medulla (inner pyramids) and pelvis (collecting area). Functional unit: nephron (about 1.2 million per human kidney). Two nephron types: cortical and juxtamedullary (long loops of Henle important for urine concentration).

Nephron — parts and functions

• Renal corpuscle (glomerulus + Bowman's capsule): blood filtration.
• Proximal convoluted tubule (PCT): bulk reabsorption of water, Na+, glucose, amino acids; secretion of some organic acids and drugs.
• Loop of Henle (descending and ascending limbs): countercurrent multiplier — creates medullary osmotic gradient; descending limb permeable to water, ascending limb pumps out Na+ and is impermeable to water.
• Distal convoluted tubule (DCT): regulated reabsorption (Ca2+, Na+) under hormones.
• Collecting duct: final adjustment of water and urea; permeability to water controlled by ADH (vasopressin); principal site for urine concentration.

Mechanisms of urine formation

1. Glomerular filtration: driven by hydrostatic and oncotic pressures (Starling forces). Filtrate is essentially plasma without most proteins.

2. Tubular reabsorption: selective movement of useful substances from tubular fluid back to blood (mostly in PCT).

3. Tubular secretion: selective transfer of substances from blood to tubular fluid (DCT and PCT) — helps eliminate wastes and regulate pH.

Net result: formation of urine that contains wastes (urea, creatinine, some ions) and excess water.

Regulation

• Antidiuretic hormone (ADH): increases water permeability of collecting ducts — concentrates urine and conserves water.
• Aldosterone: increases Na+ reabsorption and K+ secretion in DCT/collecting duct — influences water retention.
• Renin–angiotensin–aldosterone system (RAAS): adjusts blood pressure and Na+ balance.
• Atrial natriuretic peptide (ANP): reduces Na+ reabsorption and lowers blood volume/pressure.

Important physiological values

Approximate normal values: glomerular filtration rate (GFR) ≈ 125 mL/min (~180 L/day); renal plasma flow (RPF) ≈ 600 mL/min; filtration fraction ≈ 0.2; normal urine volume 1–2 L/day; urine osmolality range ≈ 50–1200 mOsm/kg depending on hydration.

Clinical relevance

Common disorders: kidney stones (nephrolithiasis), urinary tract infections, glomerulonephritis, acute/chronic renal failure (may require dialysis or transplant). Tests: urine analysis, serum creatinine, creatinine clearance (estimate of GFR), blood urea nitrogen (BUN).

Overview

The skeletal system provides a rigid framework for the body, protects internal organs, enables movement in conjunction with muscles, stores minerals (mainly calcium and phosphate), produces blood cells (haematopoiesis) and serves as a site for fat storage (yellow marrow).

Organisation of the Skeleton

The human skeleton is divided into:

• Axial skeleton: skull, vertebral column, ribs and sternum — supports and protects the central axis.
• Appendicular skeleton: limbs and girdles (pectoral and pelvic) — involved primarily in locomotion and manipulation.

Types of Bones

• Long bones (e.g., femur, humerus): tubular with a shaft (diaphysis) and two ends (epiphyses).
• Short bones (e.g., carpals, tarsals): roughly cube-shaped for stability and some movement.
• Flat bones (e.g., skull bones, sternum, ribs): protection and broad surface for muscle attachment.
• Irregular bones (e.g., vertebrae, some facial bones): complex shapes.
• Sesamoid bones (e.g., patella): embedded in tendons to reduce wear and improve mechanical advantage.

Structure of a Typical Long Bone

• Periosteum: fibrous covering with blood vessels and osteogenic cells.
• Compact (cortical) bone: dense outer layer made of osteons (Haversian systems) for strength.
• Spongy (cancellous) bone: trabeculae with spaces filled by red bone marrow (haematopoiesis).
• Medullary (marrow) cavity: contains yellow marrow (fat) in adults.

Bone Tissue & Cells

• Osteoblasts: bone-forming cells that secrete osteoid (collagen) and initiate mineralization.
• Osteocytes: mature bone cells within lacunae; maintain matrix and communicate via canaliculi.
• Osteoclasts: multinucleated cells that resorb bone (bone remodelling).
• Haversian system (osteon): concentric lamellae of mineralized matrix around a central Haversian canal containing blood vessels and nerves; Volkmann's canals connect osteons.

Bone Formation (Ossification)

Two types:

• Intramembranous ossification: direct bone formation from mesenchyme (skull bones).
• Endochondral ossification: replacement of a cartilage model by bone (long bones).

Joints (Articulations)

Joints are connections between bones and are classified structurally and functionally.

Structural classification

• Fibrous (no cavity) — bones joined by fibrous tissue (e.g., sutures of skull).
• Cartilaginous (no cavity) — bones joined by cartilage (e.g., intervertebral discs, pubic symphysis).
• Synovial (has a synovial cavity) — freely movable joints with a joint capsule (e.g., knee, shoulder).

Functional classification

• Synarthrosis: immovable (e.g., skull sutures).
• Amphiarthrosis: slightly movable (e.g., vertebral joints).
• Diarthrosis: freely movable (majority of synovial joints).

Structure of a Synovial Joint

• Articular cartilage: hyaline cartilage covering bone ends to reduce friction and absorb shock.
• Joint (articular) capsule: fibrous outer layer and inner synovial membrane that secretes synovial fluid.
• Synovial fluid: viscous fluid that lubricates and supplies nutrients to cartilage.
• Ligaments: connect bone-to-bone and stabilize joints.
• Menisci and bursae: shock-absorbing fibrocartilage pads and fluid-filled sacs to reduce friction.

Types of Synovial Joints and Movements

• Hinge (uniaxial): flexion/extension — e.g., elbow, knee.
• Ball-and-socket (multiaxial): flexion/extension, abduction/adduction, rotation — e.g., shoulder, hip.
• Pivot (uniaxial rotation): e.g., atlanto-axial joint (head rotation), proximal radioulnar joint.
• Saddle (biaxial): e.g., carpometacarpal joint of thumb (opposition).
• Condyloid/ellipsoid (biaxial): e.g., wrist joint (radiocarpal).
• Plane/gliding (multidirectional but limited): e.g., intercarpal joints.

Common movements: flexion/extension, abduction/adduction, rotation, circumduction, supination/pronation, inversion/eversion.

Bone & Joint Health

Common disorders: osteoporosis (reduced bone density, fracture risk), osteoarthritis (degeneration of articular cartilage), rheumatoid arthritis (autoimmune inflammation), gout (urate crystal deposition), sprains and bursitis. Prevention: balanced diet (calcium, vitamin D), weight-bearing exercise, avoiding smoking and excessive alcohol, early diagnosis and physiotherapy.

Overview

The musculo-skeletal mechanism is the integrated system of bones (or other supporting structures), joints, muscles, tendons and ligaments that gives shape to the body, supports weight, protects organs and produces movement (locomotion). Movement results from forces generated by muscles acting on a skeleton arranged as levers.

Types of Skeleton

• Hydrostatic skeleton: fluid-filled cavity with muscles around it (e.g., earthworm). Movement by changing pressure/shape.
• Exoskeleton: rigid external covering (e.g., arthropods). Muscles attach on the inside; movement at joints.
• Endoskeleton: internal skeleton of bone or cartilage (e.g., vertebrates). Provides internal support and sites for muscle attachment.

Bone and Cartilage Structure

• Bone components: periosteum, compact bone, spongy (cancellous) bone, bone marrow (red for hematopoiesis, yellow for fat).
• Microscopic structure: osteons (Haversian systems) in compact bone; matrix of collagen and mineral (calcium phosphate) gives strength and slight flexibility.
• Cartilage: hyaline, fibrocartilage, elastic — flexible support found in joints, nose, ear, intervertebral discs.

Joints (Articulations)

• Fibrous (immovable), e.g., skull sutures.
• Cartilaginous (slightly movable), e.g., intervertebral discs.
• Synovial (freely movable) — have synovial cavity, articular cartilage, capsule and ligaments. Types: hinge (elbow), ball-and-socket (hip, shoulder), pivot (atlas-axis), saddle, plane, condyloid.

Muscle Types and Structure

• Skeletal muscle: voluntary, striated, long multinucleated fibres organized into fascicles. Attached to bone by tendons.
• Cardiac muscle: striated, involuntary, branched, intercalated discs.
• Smooth muscle: non-striated, involuntary, in gut, blood vessels.

Muscle fibre ultrastructure: myofibrils contain repeating sarcomeres (Z-line to Z-line) with thin (actin) and thick (myosin) filaments. The arrangement of filaments produces striations.

Mechanism of Contraction — Sliding Filament Theory

1. At a motor end plate, a nerve impulse causes release of acetylcholine, depolarizing the muscle fibre membrane and causing action potential propagation.
2. Action potential triggers Ca2+ release from sarcoplasmic reticulum into cytosol.
3. Ca2+ binds troponin, causing tropomyosin to move and expose myosin-binding sites on actin.
4. Myosin heads (with ADP + Pi) form cross-bridges with actin; power stroke releases ADP + Pi, pulling actin toward centre of sarcomere; ATP binds myosin to detach it; ATP hydrolysis re-cocks the head.
5. Cycle repeats while Ca2+ and ATP are available — sarcomeres shorten, muscle contracts.

ATP is essential for cross-bridge cycling and for pumping Ca2+ back into sarcoplasmic reticulum to relax the muscle.

Antagonistic Pairs and Coordination

Most skeletal actions use antagonistic muscle pairs: one muscle contracts (agonist) while the opposite relaxes (antagonist). Example: biceps (flexion) and triceps (extension) at elbow. Synergists assist, fixators stabilize origin.

Mechanical Principles: Levers and Locomotion

The musculo-skeletal system uses bones as levers and joints as fulcrums. Levers change magnitude and direction of forces and are classified by the relative positions of load (resistance), effort (muscle force) and fulcrum:

• First-class lever: fulcrum between load and effort (e.g., neck extension when looking up).
• Second-class lever: load between fulcrum and effort (e.g., standing on toes — calf muscles lifting body).
• Third-class lever: effort between fulcrum and load (e.g., flexion of forearm by biceps) — most common in body, favors speed and range over mechanical advantage.

Key mechanical relationships: torque (moment) = force × perpendicular distance from fulcrum. Mechanical advantage (MA) = effort arm / load arm. Biological levers often sacrifice MA for speed and range of motion.

Locomotion in Different Animals

• Protozoa (e.g., amoeba): use cytoplasmic streaming and pseudopodia.
• Hydrostatic animals (e.g., earthworm): peristaltic waves produced by circular and longitudinal muscles working against fluid-filled coelom and aided by setae for anchorage.
• Cnidarians and flatworms: muscle-like contractile cells and body-wall contractions for simple movements.
• Arthropods: rigid exoskeleton with joints; muscles attach internally and pull on segments; jointed appendages allow precise locomotion (walking, running, flying in insects).
• Vertebrates: endoskeleton with paired appendages; muscles arranged for complex limb movements; specialized modes include running (quadrupeds/humans), hopping (kangaroo), flying (birds and bats — wings with modified forelimbs and powerful flight muscles), swimming (fish use axial muscle undulations; cetaceans use flukes).

Functions Summarized

• Support and shape
• Protection of internal organs
• Movement and locomotion
• Storage of minerals (e.g., calcium) and blood cell formation in bone marrow

Clinical/Functional notes: Tendon injuries, ligament sprains, arthritis (joint cartilage degeneration), muscle fatigue (ATP depletion, lactic acid in anaerobic work) affect movement.

Overview: The nervous system is the body’s communication and control network. It is divided anatomically into the Central Nervous System (CNS) — brain and spinal cord — and the Peripheral Nervous System (PNS) — all nerves and ganglia outside the CNS. Functionally it carries out sensory input, integration, and motor output.

Basic unit: the neuron

• Structure: dendrites (receive), cell body (soma, integrates), axon (conducts), axon terminals (transmit at synapses). Many neurons are wrapped by myelin produced by Schwann cells (PNS) or oligodendrocytes (CNS).
• Types: sensory (afferent), motor (efferent), interneurons (within CNS).

Central Nervous System (CNS)

• Brain: major parts — forebrain (cerebrum for higher functions; diencephalon containing thalamus and hypothalamus), midbrain (visual/auditory reflexes), hindbrain (cerebellum for coordination; pons and medulla for basic life functions).
• Spinal cord: conducts impulses to/from brain, contains reflex centres. White matter (myelinated tracts) surrounds grey matter (neuronal cell bodies). Spinal nerves exit via dorsal (sensory) and ventral (motor) roots.

Peripheral Nervous System (PNS)

• Composed of cranial and spinal nerves, ganglia, and sensory receptors.
• Divisions: somatic nervous system (voluntary control of skeletal muscles) and autonomic nervous system (ANS) — involuntary control subdivided into sympathetic (fight-or-flight) and parasympathetic (rest-and-digest). Enteric nervous system controls gut motility.

How signals travel

• Resting membrane potential: typically around −70 mV (inside negative). Set by ion gradients (Na+, K+) and selectively permeable membranes.
• Action potential: stimulus → threshold reached → rapid depolarization (voltage-gated Na+ channels open, Na+ in) → repolarization (K+ channels open, K+ out) → hyperpolarization → return to rest via Na+/K+ ATPase and leak channels.
• Saltatory conduction: in myelinated axons, action potentials jump between Nodes of Ranvier, greatly increasing conduction velocity compared to unmyelinated fibres.

Reflex arcs: A simple neural pathway for rapid involuntary responses. Components: receptor → sensory neuron → (may pass through) interneuron in spinal cord → motor neuron → effector (muscle/gland). Example: knee-jerk (stretch) reflex — monosynaptic, very fast.

Clinical and physiological relevance: Damage to CNS (spinal cord injury) can cause paralysis below lesion; demyelinating diseases (multiple sclerosis) slow conduction and cause symptoms like weakness, sensory loss. Local anesthetics block Na+ channels to prevent action potentials and cause loss of sensation.

Key numbers (typical): resting potential ≈ −70 mV; action potential peak ≈ +30 to +40 mV; threshold ≈ −50 to −55 mV; absolute refractory period ≈ 1 ms; conduction velocity: unmyelinated fibres ≈ 0.5–2 m/s, myelinated fibres up to ≈ 100–120 m/s.

Overview
Reproduction is the biological process by which organisms produce new individuals (offspring). In animals it occurs mainly by sexual reproduction (fusion of gametes) or, less commonly, by various forms of asexual reproduction. Human and most vertebrate reproduction is sexual and involves specialised reproductive organs, gametogenesis, fertilisation and embryonic development.

Types of reproduction

• Sexual reproduction: involves two parents (usually) and fusion of haploid gametes (sperm and egg) to form a diploid zygote. Advantages: genetic variation.
• Asexual reproduction: single parent, offspring genetically identical (examples: budding, fragmentation, parthenogenesis in some animals).

Male reproductive system (basic anatomy & function)

• Testes: paired gonads producing sperm and testosterone. Inside are seminiferous tubules where spermatogenesis occurs.
• Epididymis: site of sperm maturation and storage.
• Vas deferens: conducts sperm to the ejaculatory ducts.
• Accessory glands (seminal vesicles, prostate, bulbourethral glands): add seminal fluid (nutrients, alkaline medium) to form semen.
• Urethra and penis: deliver semen into the female tract (internal fertilisation in mammals).

Spermatogenesis (key points)

• Occurs in seminiferous tubules from puberty throughout life.
• Sequence: spermatogonium (2n) → primary spermatocyte (2n) → meiosis I → two secondary spermatocytes (n) → meiosis II → four spermatids (n) → differentiate into spermatozoa.
• Results in many motile, small haploid sperm with minimal cytoplasm.

Female reproductive system (basic anatomy & function)

• Ovaries: paired gonads producing ova (eggs) and hormones (estrogen, progesterone). Follicles in ovaries develop and release an ovum.
• Fallopian tubes (uterine tubes): collect the ovulated ovum and are usual site of fertilisation.
• Uterus: implantation and embryonic development (endometrium lining thickens each cycle).
• Cervix and vagina: delivery and copulatory canal.

Oogenesis (key points)

• Begins before birth: oogonia → primary oocytes (2n) that arrest in prophase I until puberty.
• Each menstrual cycle one primary oocyte completes meiosis I → a large secondary oocyte (n) + small polar body.
• Secondary oocyte arrests in metaphase II and completes meiosis II only if fertilisation occurs → ovum (n) + second polar body.
• Oogenesis yields one functional ovum per primary oocyte (and polar bodies).

Fertilisation and early development

• Fertilisation: fusion of haploid sperm and ovum to form a diploid zygote (restores 2n chromosome number).
• Fertilisation can be external (many fish, amphibians) or internal (mammals, birds).
• After fertilisation, cleavage (mitotic divisions) produces a multicellular embryo which implants into the uterine wall (in placental mammals).

Hormonal regulation — hypothalamo–pituitary–gonadal axis

• Hypothalamus releases GnRH → stimulates anterior pituitary to secrete FSH and LH.
• In males: FSH stimulates spermatogenesis (via Sertoli cells); LH stimulates Leydig cells to produce testosterone.
• In females: FSH stimulates follicle growth and estrogen secretion; LH triggers ovulation and formation of corpus luteum which secretes progesterone (and some estrogen).
• Sex hormones exert feedback on hypothalamus and pituitary to regulate the cycle.

Menstrual (human) cycle — phases (approx. days)

• Menstrual phase (day 1–5): shedding of endometrium.
• Follicular / proliferative phase (day 6–13): follicle growth, rising estrogen, endometrium rebuilds.
• Ovulation (around day 14): LH surge causes release of ovum.
• Luteal / secretory phase (day 15–28): corpus luteum secretes progesterone; endometrium becomes secretory to support implantation; if no pregnancy corpus luteum degenerates → menstruation.

Reproductive patterns in animals

• Oviparity: eggs laid outside (birds, many reptiles, most fishes).
• Viviparity: live birth after internal development (most mammals).
• Ovoviviparity: eggs hatch inside mother and then young are born (some sharks, reptiles).
• Parthenogenesis: development from unfertilised egg in some invertebrates and a few vertebrates (e.g., some insects, certain reptiles).

Key points to remember

• Gametes are haploid (n); somatic cells are diploid (2n). In humans 2n = 46, n = 23.
• Spermatogenesis produces many small motile sperm; oogenesis produces a few large nutrient-rich eggs.
• Fertilisation restores diploid state and creates genetic variation (independent assortment, crossing over, random gamete fusion).

Overview: Integration and homeostasis describe how animals maintain a stable internal environment despite external changes. Integration is achieved primarily by the nervous and endocrine systems working together; homeostasis is maintained by feedback mechanisms that detect deviations from set points and activate effectors to restore balance.

Key components of homeostatic systems:

• Receptor – senses a physical or chemical stimulus (e.g., thermoreceptors, chemoreceptors, baroreceptors).
• Integrating (control) centre – processes information and compares it to the set point (e.g., hypothalamus, medulla, endocrine glands).
• Effector – brings about a response to correct the deviation (e.g., muscles, sweat glands, pancreas, kidneys).
• Feedback – information about the effect of the response returned to the control centre; can be negative or positive.

Types of feedback:

• Negative feedback (most common): the response counteracts the change and restores the set point. Example: regulation of blood glucose by insulin and glucagon.
• Positive feedback (amplifying): the response increases the change until a specific outcome is reached. Example: oxytocin-induced uterine contractions during childbirth.

Nervous vs Endocrine integration:

• Nervous system: fast, short-lived responses using electrical impulses and neurotransmitters (e.g., withdrawal reflex, baroreceptor reflex controlling heart rate).
• Endocrine system: slower, longer-lasting regulation using hormones released into the blood (e.g., thyroid hormones regulating metabolism, insulin regulating blood glucose).
• These systems often interact (e.g., stress → hypothalamus → ACTH → adrenal cortex releases cortisol; immediate sympathetic activation releases adrenaline).

Important homeostatic processes (applied concepts):

• Thermoregulation: maintaining body temperature by vasodilation/vasoconstriction, sweating, shivering, behavioural responses. Controlled mainly by the hypothalamus.
• Osmoregulation and excretion: kidneys regulate water, salts and excrete nitrogenous wastes; ADH controls water reabsorption in collecting ducts to concentrate urine.
• Blood glucose regulation: insulin (lowers glucose by promoting uptake and glycogen synthesis) and glucagon (raises glucose by promoting glycogen breakdown and gluconeogenesis).
• Acid–base balance: buffers in blood, respiratory control of CO2, and renal H+ secretion/HCO3- reabsorption maintain pH ~7.35–7.45.
• Cardiovascular regulation: baroreceptor reflex adjusts heart rate and vessel tone to maintain blood pressure; cardiac output = heart rate × stroke volume.

General steps to analyze a homeostatic problem:

1. Identify the variable (e.g., core temperature, blood glucose).
2. Determine the receptor and the integrating centre.
3. Identify the effector(s) and the expected response (opposite for negative feedback).
4. Decide if response restores the set point (negative feedback) or amplifies change (positive feedback).

Clinical/applied examples: understanding these concepts explains clinical treatments like insulin therapy for diabetes, dialysis for renal failure (replacing filtration and regulation functions of kidneys), antipyretics for fever (reset hypothalamic set point), and use of beta-blockers to reduce heart rate and blood pressure.

Summary: Integration (nervous + endocrine) detects and processes internal/external cues and coordinates effectors. Homeostasis relies mainly on negative feedback to maintain stability; applied concepts translate this understanding into medical interventions and everyday explanations for how the body copes with stress, temperature change, fluid shifts and metabolic demands.