Chapter 5 — Human Physiology

Cell, Tissue, Organ and System: The human body is organised in levels. The basic unit is the cell, specialised for different functions. Cells with similar structure and function group to form tissues; there are four primary tissue types in humans: epithelial, connective, muscular and nervous. Organs are structures composed of two or more tissue types working together for a common function. Systems are groups of organs cooperating to perform broader physiological tasks, for example the digestive system or circulatory system.

Examples of hierarchical organisation: A muscle is made of muscle cells (fibres) arranged into fascicles and covered by connective tissue; the stomach is an organ with epithelial lining (for secretion and absorption), smooth muscle (for churning) and connective tissue (for support); the digestive system includes mouth, oesophagus, stomach, intestines, liver and pancreas.

Integration and homeostasis: Systems do not work in isolation. The circulatory and respiratory systems cooperate to supply oxygen to tissues; the nervous and endocrine systems coordinate responses. Homeostasis — the maintenance of a relatively constant internal environment — is achieved by feedback mechanisms that operate across levels of organisation. Cells carry receptors that detect internal changes; organs like kidneys adjust volume and composition of blood; hormones from endocrine glands alter metabolism at the cellular level.

Structural-functional relationship: At every level, structure supports function. The thin, flat shape of epithelial cells in alveoli increases surface area for gas exchange; the branching structure of neurons increases contact area for information transfer; the layered arrangement of the small intestine (mucosa, submucosa, muscularis) supports digestion and transport.

Clinical relevance: Diseases often affect multiple levels. A genetic mutation in a cellular protein can disrupt tissue organisation, weakening an organ and impairing the whole system. Understanding organisation helps to interpret symptoms, design treatments and appreciate why systemic effects arise from local lesions.

Cell structure and organelles: Every physiological process begins at the cellular level. A typical animal cell contains a nucleus that stores DNA and controls gene expression; mitochondria that generate ATP through oxidative phosphorylation; rough endoplasmic reticulum studded with ribosomes for protein synthesis; smooth endoplasmic reticulum for lipid synthesis and detoxification; Golgi apparatus for modifying and packaging proteins; lysosomes with hydrolytic enzymes for intracellular digestion; and a plasma membrane that forms a selective barrier. The cytoskeleton—microfilaments, intermediate filaments and microtubules—gives mechanical support, maintains shape and facilitates intracellular transport and cell division.

Membrane structure and transport: The plasma membrane is a fluid mosaic of lipids and proteins. Transport across this membrane is central to physiology. Simple diffusion moves small nonpolar molecules down concentration gradients. Facilitated diffusion uses carrier proteins or channels for polar molecules. Osmosis is the net movement of water across a semipermeable membrane toward higher solute concentration. For many ions and nutrients, active transport uses ATP to move substances against gradients; the sodium-potassium ATPase maintains low intracellular Na+ and high K+, crucial for volume regulation and electrical excitability.

Ion gradients and electrical properties: Ion pumps and selective permeability establish concentration and electrical gradients. These gradients underlie the resting membrane potential and are central to excitability in nerve and muscle cells. Small changes in membrane permeability to Na+ or K+ produce graded potentials; when threshold is reached an action potential is triggered. The cell’s ability to restore gradients after an action potential requires energy and intact organelle function.

Cell signalling and receptors: Cells communicate using chemical cues—neurotransmitters, hormones and local mediators. Membrane receptors detect signals and transduce them into intracellular responses via second messengers (cAMP, Ca2+, IP3) or kinase cascades, altering enzyme activity, ion channel states or gene transcription. Some signals act quickly by opening ion channels; others change gene expression and have long-term effects on cellular behaviour.

Metabolism and energy flow: Metabolic pathways convert nutrients into usable energy. Glycolysis in the cytosol breaks glucose to pyruvate, yielding ATP and NADH; in the presence of oxygen, pyruvate enters mitochondria for the Krebs cycle and electron transport chain producing more ATP. Anaerobic glycolysis yields lactate and less ATP. Cells store energy as glycogen or triglycerides and can mobilise these stores in response to hormonal signals such as insulin and glucagon.

Cell division, growth and apoptosis: Mitosis allows tissue growth and repair; controlled cell death (apoptosis) removes unwanted or damaged cells. Stem cells in many tissues renew specialised cells throughout life. Disruption of the balance between proliferation and death leads to disease—abnormal proliferation causes cancers; insufficient repair leads to degenerative conditions.

Overview of blood components: Blood is a specialised fluid connective tissue with two major parts: plasma (the liquid matrix) and formed elements (cells and cell fragments). Plasma is about 55% of blood in volume and consists mainly of water, electrolytes (Na+, K+, Cl−), plasma proteins (albumin, globulins, fibrinogen), nutrients, metabolic wastes, gases and hormones. Formed elements occupy roughly 45% of volume (haematocrit) and include erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes (platelets).

Red blood cells (erythrocytes): Erythrocytes are specialised for oxygen transport. Their biconcave shape increases surface area for gas exchange and confers flexibility to pass through narrow capillaries. Mature RBCs lack a nucleus and many organelles, maximising space for haemoglobin, the iron-containing pigment that binds oxygen reversibly. Haemoglobin’s affinity for oxygen depends on partial pressure of oxygen (PO2), pH, CO2 and temperature. Erythropoiesis—production of RBCs—occurs in bone marrow under control of erythropoietin from the kidneys, which rises in response to low oxygen levels.

White blood cells (leukocytes): Leukocytes are defenders of the body and are diverse. Granulocytes include neutrophils (phagocytose bacteria), eosinophils (defend against parasites and modulate allergic responses) and basophils (release histamine in inflammation). Agranulocytes include lymphocytes—B cells (which produce antibodies) and T cells (helper and cytotoxic roles)—and monocytes which differentiate into macrophages in tissues to phagocytose pathogens and present antigens. Leukocyte counts and types change during infection, inflammation and immune disorders.

Platelets and clotting: Platelets are small cell fragments derived from megakaryocytes, essential for haemostasis. On vascular injury platelets adhere, aggregate and form a plug. The coagulation cascade—complex series of enzyme activations—converts soluble fibrinogen into insoluble fibrin strands that stabilise the clot. Plasma proteins such as clotting factors and calcium are necessary. Regulation prevents inappropriate clot formation; however, excessive clotting can cause thrombosis and block blood vessels.

Plasma proteins and transport: Albumin maintains oncotic pressure, which holds water in the vascular compartment and prevents excessive tissue oedema. Globulins include antibodies (immunoglobulins) and transport proteins that carry hormones and lipids. Fibrinogen is essential for clot formation. Plasma also transports nutrients (glucose, amino acids, lipids), metabolic wastes (urea, creatinine) to excretory organs, and distributes hormones and heat, helping in thermoregulation.

Functions and clinical importance: Blood functions include transport (gases, nutrients, wastes, hormones), regulation (pH buffering, temperature control, osmotic balance) and defence (immune cells and clotting). Disorders such as anaemia (low haemoglobin or RBC count), polycythaemia (excess RBCs), leukopenia/leukocytosis (abnormal WBC numbers), haemophilia (clotting factor deficiency) and infections affect these functions. Laboratory tests like complete blood count, haemoglobin concentration, clotting time, and blood smears help diagnose conditions and guide treatment.

Gross anatomy of the heart: The human heart is a four-chambered muscular organ located in the thoracic cavity. It is roughly the size of a fist and lies between the lungs in the mediastinum. The chambers are two atria (thin-walled receiving chambers) and two ventricles (thicker-walled pumping chambers). The left ventricle has the thickest myocardium because it must generate high pressure to deliver blood through the systemic circulation. Valves maintain unidirectional blood flow: atrioventricular valves (tricuspid on the right and mitral or bicuspid on the left) separate atria from ventricles; semilunar valves (pulmonary and aortic) guard the exits of the ventricles.

Internal and external features: The heart has a fibrous skeleton that supports valves and electrically isolates atria from ventricles except via specialised conduction tissue. Coronary arteries arise from the aorta and supply oxygenated blood to the myocardium; blockage of these arteries leads to ischaemia and myocardial infarction. The pericardium, a double-walled sac, encloses the heart and produces lubricating fluid reducing friction during heart movements.

Blood flow sequence: Deoxygenated blood enters the right atrium from the superior and inferior venae cavae, flows through the tricuspid valve into the right ventricle, and is pumped via the pulmonary valve into the pulmonary trunk and arteries to the lungs. Oxygenated blood returns to the left atrium via pulmonary veins, passes through the mitral valve into the left ventricle and is ejected through the aortic valve into the aorta for systemic distribution.

The cardiac cycle: The cardiac cycle is the repeating sequence of contraction (systole) and relaxation (diastole) that ensures blood movement. Atrial systole tops up ventricular filling; ventricular systole increases intraventricular pressure, closing AV valves (producing the first heart sound) and forcing open semilunar valves to eject blood. During diastole, ventricles relax, semilunar valves close (second heart sound) and ventricles refill. Stroke volume is the blood volume ejected per beat; end-diastolic volume and end-systolic volume determine stroke volume.

Electrical conduction and ECG: Electrical activity originates in the sinoatrial (SA) node, the heart’s pacemaker, generating impulses that spread through atria causing contraction. Impulses reach the atrioventricular (AV) node which delays conduction, allowing ventricles to fill, then propagate along the bundle of His and Purkinje fibres to cause coordinated ventricular contraction. The electrocardiogram (ECG) records electrical events: P wave (atrial depolarisation), QRS complex (ventricular depolarisation), and T wave (ventricular repolarisation). ECGs help detect arrhythmias, conduction blocks and myocardial damage.

Regulation of heart function: Heart rate and contractility are modulated by the autonomic nervous system and circulating hormones. Sympathetic stimulation increases heart rate and force of contraction, while parasympathetic (vagal) activity decreases rate. Frank-Starling mechanism states that increased venous return stretches cardiac muscle fibres enhancing stroke volume. Pathologies include valve defects, heart failure, hypertension-induced hypertrophy and electrical conduction disorders; clinical assessment uses auscultation, ECG, echocardiography and angiography.

Classification and structure: Blood vessels form a closed circulatory system with arteries, arterioles, capillaries, venules and veins. Arteries have thick, elastic and muscular walls composed of an inner endothelium, a middle layer of smooth muscle and elastic fibres, and an outer connective tissue layer; this structure enables them to withstand and dampen the pulsatile high-pressure output from the heart. Arterioles have a proportionally larger smooth muscle layer relative to size and regulate flow into capillary beds via vasoconstriction and vasodilation. Capillaries are composed of a single layer of endothelial cells and a basement membrane, optimised for exchange. Venules and veins have thinner walls and larger lumens; veins contain valves to prevent backflow and rely on surrounding muscles and low-pressure gradients to return blood to the heart.

Circulatory circuits: The pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium. The systemic circuit distributes oxygenated blood from the left ventricle to tissues and returns deoxygenated blood to the right atrium. Additionally, portal systems—such as the hepatic portal system—connect two capillary networks in series, permitting substances absorbed from the gut to be processed by the liver before entering general circulation.

Microcirculation and exchange: Exchange between blood and tissues occurs primarily in capillaries. Mechanisms include diffusion (down concentration gradients for gases and small solutes), filtration driven by hydrostatic pressure and reabsorption driven by colloid osmotic pressure of plasma proteins. At the arterial end of capillary beds hydrostatic pressure favours fluid movement out; at the venous end osmotic pressure favours reabsorption. The lymphatic system collects excess interstitial fluid, transports dietary lipids as chyle and returns proteins and fluids to the venous circulation; lymph nodes filter lymph and are sites of immune surveillance.

Blood pressure and its determinants: Arterial blood pressure depends on cardiac output and total peripheral resistance. Systolic pressure is the peak pressure during ventricular contraction; diastolic is the lowest during relaxation. Short-term control uses baroreceptors in carotid sinuses and aortic arch that adjust heart rate and vessel tone via the autonomic nervous system. Long-term regulation involves renal mechanisms adjusting blood volume, and hormonal systems such as the renin-angiotensin-aldosterone system (RAAS) that increase sodium and water retention and vasoconstriction when pressure is low.

Physiological and clinical relevance: Understanding vessel structure explains why arteries pulse and why veins are capacitive reservoirs of blood. Pathological changes include atherosclerosis (lipid deposition and plaque formation), which stiffens arteries and narrows lumina, raising blood pressure and risking thrombosis; varicose veins result from valve failure in superficial veins; shock results from inadequate tissue perfusion. Clinical measurement of blood pressure, pulse palpation, Doppler ultrasound and angiography are diagnostic tools. Therapeutic interventions such as vasodilators, ACE inhibitors and statins target specific circulatory mechanisms.

Anatomy of the respiratory tract: The respiratory system provides gas exchange necessary for aerobic metabolism. Air enters through the nose or mouth, is filtered by nasal hairs and mucosa, warmed and humidified, then passes through the pharynx and larynx into the trachea. The trachea divides into primary bronchi, bronchioles and terminal bronchioles that conduct air to the respiratory zone. Alveoli are tiny air sacs where gas exchange occurs; each alveolus is closely associated with pulmonary capillaries and has a thin respiratory membrane composed of alveolar epithelium, fused basement membranes and capillary endothelium to allow rapid diffusion.

Mechanics of breathing and ventilation: Ventilation depends on changing thoracic volume to alter intrapulmonary pressure. The diaphragm, a dome-shaped muscle, flattens during contraction increasing vertical thoracic volume. External intercostal muscles raise the ribs increasing anterior-posterior and lateral dimensions. This produces negative intrapulmonary pressure relative to atmospheric pressure and air flows in (inspiration). Expiration is passive at rest due to elastic recoil of lungs and chest wall; forced expiration involves abdominal and internal intercostal muscles. Pleural fluid in the pleural cavity keeps visceral and parietal pleurae apposed and maintains negative intrapleural pressure to prevent lung collapse.

Gas exchange and transport: Gas exchange follows Dalton’s and Fick’s principles: partial pressure differences drive diffusion and the rate depends on surface area and membrane thickness. Oxygen diffuses from alveoli (higher PO2) into blood (lower PO2), while CO2 diffuses opposite. In blood, oxygen is largely transported bound to haemoglobin; each haemoglobin molecule can bind four oxygen molecules. Carbon dioxide is transported in three forms: dissolved CO2, bound to haemoglobin as carbamino-haemoglobin, and most importantly as bicarbonate (HCO3−) formed when CO2 combines with water catalysed by carbonic anhydrase within red blood cells.

Regulation of breathing: Respiratory centres in the medulla oblongata and pons generate rhythmic breathing patterns and adjust rate and depth. Peripheral chemoreceptors in carotid and aortic bodies respond to low PO2, while central chemoreceptors in medulla respond to changes in pH driven by CO2 levels. Increased arterial CO2 (and the resulting drop in pH) is a powerful stimulus for increased ventilation. Voluntary control via motor cortex allows breath-holding and speech but is overridden when chemical drive requires oxygen or removes CO2.

Physiological responses and disorders: During exercise ventilation increases to match metabolic demands. Pathological conditions such as asthma (bronchoconstriction), chronic obstructive pulmonary disease (COPD, including emphysema and chronic bronchitis), pneumonia (alveolar infection) and pulmonary fibrosis (thickened alveolar membranes) impair ventilation or diffusion. Smoking damages cilia and alveoli and increases risk of infection and chronic disease. High altitude reduces ambient PO2 which can lead to hypoxia; acclimatisation includes increased ventilation, RBC production and haemoglobin changes to improve oxygen delivery.

Organisation of the digestive system: The digestive system transforms ingested food into small absorbable units and eliminates indigestible residues. Organs of the alimentary canal—mouth, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (colon), rectum and anus—work with accessory glands (salivary glands, liver, gall bladder and pancreas). Mechanical processes such as chewing, swallowing and peristalsis mix and move food; chemical digestion uses enzymes to break macromolecules into monomers.

Oral cavity and stomach functions: Digestion begins in the mouth where teeth mechanically break food and salivary amylase starts starch digestion. The tongue mixes food and helps form a bolus for swallowing. The oesophagus propels the bolus to the stomach by peristalsis. The stomach acts as a reservoir and performs mechanical churning while secreting gastric juice—hydrochloric acid that creates an acidic environment and pepsinogen (activated to pepsin) that begins protein digestion. The acidic environment also kills many microbes. Mucus protects the gastric lining from self-digestion.

Small intestine: digestion and major site of absorption: The acidic chyme entering the duodenum is neutralised by bicarbonate secreted by the pancreas. Pancreatic enzymes (amylase, lipase, proteases such as trypsin) and bile from the liver emulsify fats and complete digestion. Intestinal brush border enzymes (disaccharidases, peptidases) finish breakdown to monosaccharides, amino acids and small peptides. The small intestine is specialised for absorption: mucosal folds, villi and microvilli vastly increase surface area. Epithelial cells absorb nutrients—monosaccharides and amino acids enter capillaries and are transported via the hepatic portal vein to the liver; lipids are reassembled into chylomicrons and enter lacteals (lymphatic capillaries) for transport via the lymphatic system.

Large intestine and water balance: The large intestine absorbs water, electrolytes and vitamins synthesized by resident microbiota (e.g., vitamin K and some B vitamins). It compacts faeces and stores material until defecation. Gut microbiota play roles in digestion of some fibres, synthesis of vitamins, maturation of immune system and protection against pathogens.

Regulation and clinical aspects: Digestive activity is regulated by neural (enteric nervous system, autonomic input) and hormonal signals (gastrin, secretin, cholecystokinin) that coordinate secretion and motility. Disorders include gastroesophageal reflux, peptic ulcers (often linked to Helicobacter pylori or NSAID use), malabsorption syndromes (e.g., coeliac disease), gallstones and pancreatitis. Nutrition, hygiene and timely medical care prevent many digestive illnesses.

Main roles of the excretory system: Kidneys maintain internal chemical and fluid balance, remove metabolic wastes (urea, creatinine), regulate blood volume and pressure, maintain electrolyte and acid-base balance, and secrete hormones such as erythropoietin. Paired kidneys filter blood, producing urine which passes via ureters to the bladder for storage and is expelled through the urethra.

Nephron structure and types: The nephron is the kidney’s functional unit. Each nephron has a renal corpuscle made of a glomerulus (capillary tuft) encased by Bowman's capsule, and a renal tubule segmented into the proximal convoluted tubule (PCT), loop of Henle (descending and ascending limbs), distal convoluted tubule (DCT) and collecting duct. There are cortical nephrons (short loops) and juxtamedullary nephrons (long loops reaching deep into the medulla) important for concentrating urine.

Filtration at the glomerulus: Blood pressure forces plasma across the glomerular filtration barrier into Bowman's capsule. The filtrate is essentially plasma without large proteins and cells. Glomerular filtration rate (GFR) depends on renal blood flow and glomerular hydrostatic pressure; autoregulation keeps GFR relatively stable over a range of arterial pressures. Some substances present in blood are filtered and then reabsorbed, while others are secreted into the tubule.

Reabsorption and secretion along the tubule: The PCT reabsorbs most of the filtered water, glucose, amino acids and ions via active and passive transport, returning them to peritubular capillaries. The loop of Henle creates a hyperosmotic medullary interstitium via counter-current multiplication: the descending limb is permeable to water but not salts, while the ascending limb pumps out Na+ and is impermeable to water. This gradient permits water reabsorption from the collecting ducts when antidiuretic hormone (ADH) is present, concentrating urine. The DCT and collecting duct fine-tune sodium, potassium and acid-base balance under hormonal control (aldosterone increases Na+ reabsorption and K+ secretion).

Regulation and clinical relevance: The juxtaglomerular apparatus senses perfusion and salt delivery and secretes renin initiating the renin-angiotensin-aldosterone system (RAAS) to raise blood pressure and sodium retention. Disorders include acute kidney injury, chronic kidney disease, glomerulonephritis, urinary tract infections, and kidney stones. Dialysis replaces filtration in renal failure. Monitoring urine volume, concentration, GFR and blood electrolytes is essential in clinical practice; maintaining hydration and avoiding nephrotoxic drugs supports renal health.

Overall organisation: The nervous system is divided into the central nervous system (CNS) comprising the brain and spinal cord, and the peripheral nervous system (PNS) made of nerves and ganglia. The PNS has a somatic division controlling voluntary skeletal muscles and an autonomic division controlling involuntary organs. The autonomic system further divides into sympathetic (mobilises energy, 'fight or flight') and parasympathetic (conserves energy, 'rest and digest') branches. These divisions integrate sensory input, central processing and motor outputs to maintain body function and behaviour.

Neuron structure and types: Neurons are specialised cells that transmit electrical signals. They generally consist of a cell body (soma) with a nucleus, dendrites that receive inputs, and an axon that conducts impulses away from the soma. Axons may be myelinated by Schwann cells in the PNS or oligodendrocytes in the CNS; myelin increases conduction velocity by insulating axon segments and enabling saltatory conduction between nodes of Ranvier. Neurons vary functionally as sensory (afferent), motor (efferent) and interneurons (integrative).

Membrane potentials and the basis of excitability: The resting membrane potential typically sits near −70 mV due to unequal ion distributions (high K+ inside, high Na+ outside) and selective permeability of the membrane. The sodium-potassium ATPase and leak channels help maintain this potential. When a depolarising stimulus raises the membrane potential to threshold, voltage-gated Na+ channels open, initiating an action potential: rapid Na+ influx (depolarisation) followed by K+ efflux (repolarisation). Voltage-gated channels have refractory periods that ensure unidirectional propagation and limit firing frequency.

Action potential propagation and myelination: In unmyelinated axons, action potentials propagate continuously; in myelinated axons, saltatory conduction leaps between nodes, speeding transmission and conserving energy. Conduction velocity increases with axon diameter and degree of myelination. Disorders that damage myelin—such as multiple sclerosis—slow conduction and impair function.

Synaptic transmission and plasticity: Chemical synapses convert electrical signals into chemical signals. Arrival of an action potential at the presynaptic terminal opens voltage-gated Ca2+ channels; Ca2+ triggers neurotransmitter release from synaptic vesicles into the synaptic cleft. Neurotransmitters bind postsynaptic receptors producing excitatory or inhibitory postsynaptic potentials. Termination mechanisms include enzymatic degradation, reuptake and diffusion. Synaptic strength can be modified by activity (long-term potentiation or depression), forming the basis of learning and memory.

Integration and higher functions: Neural circuits in spinal cord control reflexes and basic patterns; the brain integrates sensory information for perception, emotion, voluntary movement and cognition. The cortex, limbic system, basal ganglia and cerebellum each contribute distinct roles. Damage to specific neural pathways causes characteristic deficits, and clinical tests (reflexes, sensory mapping, imaging, electrophysiology) help localise lesions.

Principle of reflexes: Reflex actions are rapid, involuntary responses designed to protect the body and maintain homeostasis. A reflex arc typically involves a receptor that detects a stimulus, a sensory (afferent) neuron that transmits the signal to the central nervous system, an integration centre (often in the spinal cord) that processes the information, a motor (efferent) neuron that carries the command, and an effector (a muscle or gland) that produces the response. Reflexes are faster than voluntary responses because they can bypass conscious processing.

Monosynaptic and polysynaptic reflexes: The simplest reflex, the monosynaptic stretch reflex (such as the knee-jerk), has a direct synapse between a sensory neuron and a motor neuron. This provides a rapid response to maintain muscle tone and posture. Polysynaptic reflexes include one or more interneurons allowing more complex processing, modulation and coordination of multiple muscles, as seen in withdrawal reflexes where antagonistic muscles are inhibited (reciprocal inhibition) to allow a smooth action.

Sensory receptors and transduction: Sensory receptors convert a specific form of energy into electrical signals. Mechanoreceptors respond to pressure, touch and stretch; thermoreceptors detect temperature changes; nociceptors detect harmful stimuli perceived as pain; chemoreceptors respond to chemical stimuli (taste, smell, blood chemistry), and photoreceptors in the retina detect light. Receptor properties include specificity (sensitivity to particular stimuli), threshold (minimal stimulus to activate) and adaptation (phasic receptors rapidly decrease firing to a constant stimulus while tonic receptors sustain firing).

Examples of reflexes and their roles: The withdrawal reflex protects from injury by contracting flexor muscles and inhibiting extensors on the effected side, often coordinated with crossed extensor reflexes to maintain balance. The pupillary light reflex controls pupil diameter: bright light increases retinal photoreceptor signalling to brainstem nuclei, causing parasympathetic output to constrict pupils. Proprioceptive reflexes (muscle spindles and Golgi tendon organs) provide feedback on muscle length and tension, essential for posture and coordinated movement.

Modulation and clinical testing: Reflex activity is modulated by descending pathways from the brain, allowing voluntary suppression or enhancement. Clinically, testing reflexes (e.g., knee jerk, ankle jerk) and sensory modalities helps localise neurological damage. Absent, diminished or exaggerated reflexes point to lesions in peripheral nerves, spinal cord segments or upper motor neuron pathways. Understanding reflex circuitry aids in diagnosis and rehabilitation planning.

Organisation and major glands: The endocrine system consists of glands that secrete hormones into the bloodstream to regulate distant target cells. Major endocrine glands include the hypothalamus, pituitary, thyroid, parathyroids, adrenal glands, pancreas, gonads (ovaries and testes), and pineal gland. Hormones coordinate long-term processes such as growth, metabolism, fluid balance, reproduction and responses to stress.

Hormone classes and signalling mechanisms: Hormones can be peptides/proteins (insulin, glucagon), steroids derived from cholesterol (cortisol, aldosterone, oestrogens, testosterone) or amine derivatives (thyroxine, adrenaline). Peptide hormones bind to cell-surface receptors and activate second messenger systems (cAMP, IP3, Ca2+) leading to rapid cellular responses such as enzyme activation or ion channel modulation. Steroid hormones diffuse across membranes, bind intracellular receptors and regulate gene transcription, producing slower but longer-lasting effects. Thyroid hormones act intracellularly but are derived from tyrosine and regulate basal metabolic rate and development.

Hypothalamo-pituitary axis and feedback control: The hypothalamus integrates neural and hormonal signals and secretes releasing or inhibiting hormones to control the anterior pituitary. The anterior pituitary secretes tropic hormones (TSH, ACTH, FSH, LH) that regulate peripheral endocrine glands. Negative feedback is a central feature: rising levels of peripheral hormones (thyroxine, cortisol, sex steroids) inhibit hypothalamic and pituitary release to stabilise hormone levels. Positive feedback is less common but important in processes like the LH surge triggering ovulation. The posterior pituitary releases hormones (ADH and oxytocin) produced in the hypothalamus and transported along nerve axons.

Metabolic and homeostatic roles: Insulin and glucagon are key in glucose homeostasis: insulin promotes cellular uptake of glucose, glycogen synthesis and lipogenesis, lowering blood glucose; glucagon stimulates glycogenolysis and gluconeogenesis to raise blood glucose. Aldosterone increases renal sodium reabsorption raising blood volume and pressure; ADH increases water reabsorption in collecting ducts concentrating urine. Adrenal medullary hormones (adrenaline, noradrenaline) produce rapid sympathetic responses to stress.

Disorders, diagnosis and therapy: Endocrine disorders arise from hypo- or hypersecretion, receptor defects or tumours. Common conditions include diabetes mellitus (insulin deficiency or resistance), hypothyroidism and hyperthyroidism, Cushing’s syndrome (excess cortisol), Addison’s disease (adrenal insufficiency) and growth disorders. Diagnosis uses blood hormone assays and stimulation or suppression tests; treatments may include hormone replacement, receptor antagonists, surgery or lifestyle changes. Understanding hormone pathways is critical for targeted therapies and for interpreting systemic effects of endocrine dysfunction.

Types of muscle tissue and roles: Muscle tissues are classified into skeletal, cardiac and smooth. Skeletal muscle is striated, attached to bones, under voluntary control and responsible for locomotion and posture. Cardiac muscle is striated, involuntary, and forms the myocardium with specialised intercalated discs for synchronous contractions. Smooth muscle is non-striated, involuntary, and found in viscera and vessel walls where it controls diameter, motility and flow.

Organisation of skeletal muscle: A skeletal muscle is composed of bundles (fascicles) of long multinucleated muscle fibres (cells). Each fibre contains myofibrils composed of repeating sarcomeres—the structural and functional contractile units bordered by Z-lines. Sarcomeres contain overlapping thick (myosin) and thin (actin) filaments arranged to produce striations; the pattern of A bands, I bands, H zone and Z-lines corresponds to filament arrangement.

Sliding filament theory and cross-bridge cycle: Contraction occurs by sliding of actin past myosin filaments, shortening sarcomeres without changing filament lengths. The cross-bridge cycle involves myosin heads binding to actin, performing a power stroke that pulls actin inward, releasing on binding of ATP and re-cocking following ATP hydrolysis. Calcium released from the sarcoplasmic reticulum binds to troponin, causing tropomyosin to shift and expose myosin-binding sites on actin. Removal of calcium via active transport into the sarcoplasmic reticulum leads to relaxation.

Excitation-contraction coupling: Motor neurons release acetylcholine at the neuromuscular junction, producing an end-plate potential that triggers a muscle action potential. This propagates into the muscle fibre via T-tubules, causing calcium release from the sarcoplasmic reticulum. The amount of force produced depends on motor unit recruitment, frequency of stimulation (summation and tetanus), and muscle fibre length at the onset of contraction (length-tension relationship).

Energy sources and fatigue: ATP is required for cross-bridge cycling and calcium pumping. Immediate ATP comes from stored phosphocreatine; short-term high-intensity activity relies on anaerobic glycolysis producing lactate; prolonged activity depends on aerobic metabolism in mitochondria. Muscle fibres specialise: slow-twitch (type I) have many mitochondria and are fatigue-resistant for endurance; fast-twitch (type II) generate rapid force but fatigue quickly. Training induces physiological changes in fibre size, mitochondrial density and capillarisation. Disorders such as myasthenia gravis, muscular dystrophies and metabolic myopathies impair contraction and require clinical management.

Homeostasis and control systems: Homeostasis is the process by which stable internal conditions are maintained. Sensors (receptors) detect deviations from set points, control centres (often the hypothalamus) compare current values with set points and effectors (muscles, glands, blood vessels) bring about corrective responses. Negative feedback is the main mechanism; positive feedback occurs in special situations like childbirth where a process is amplified by its own outcomes.

Thermoregulation in humans: Humans are endotherms and maintain core temperature near 37°C despite environmental changes. Heat is produced by basal metabolic processes and increased by muscular activity (shivering). Heat is lost by radiation, conduction, convection and evaporation (sweating). The hypothalamus integrates thermal information from central and peripheral receptors and triggers appropriate responses to conserve or dissipate heat.

Responses to cold and heat: In cold environments, vasoconstriction of cutaneous vessels reduces blood flow to the skin and conserves heat; piloerection (hair standing) is limited in humans but present in other animals; shivering generates heat through involuntary muscle contractions. Behavioural responses—adding clothes, seeking shelter—are important. In heat, vasodilation increases skin blood flow facilitating heat loss; sweating enables evaporative cooling. If evaporative cooling is inadequate (high humidity), core temperature may rise producing heat exhaustion or heat stroke.

Metabolic and fluid homeostasis interactions: Thermoregulation links to fluid and electrolyte balance because sweating causes fluid and salt losses. Hormones such as ADH and aldosterone adjust renal water and sodium handling to maintain volume and osmolarity. Thyroid hormones influence basal metabolic rate and therefore heat production; sympathetic activation raises metabolic rate and heat generation. Fever is a regulated rise in hypothalamic set point due to pyrogens during infection, provoking heat-producing responses until the new set point is reached.

Adaptations, limits and clinical aspects: Acclimatisation to heat or cold occurs with physiological adjustments—e.g., altered sweat rate, plasma volume, and vascular responses. Age, hydration status, medical conditions and medications affect thermoregulation. Hypothermia (dangerously low body temperature) and hyperthermia (dangerously high temperature) are medical emergencies requiring rapid intervention. Understanding thermoregulation is essential for treating febrile illnesses, advising on exercise safety, and managing environmental exposures.

Innate immunity — first line of defence: Innate immunity provides immediate, non-specific protection. Physical barriers such as intact skin and mucous membranes prevent entry. Chemical barriers include acidic pH of stomach, antimicrobial peptides and enzymes like lysozyme in tears. Cellular components include phagocytes (neutrophils, macrophages) that engulf pathogens, natural killer cells that destroy infected cells, and dendritic cells that link innate and adaptive responses. Inflammation is a hallmark innate reaction: vasodilation, increased vascular permeability and leukocyte recruitment isolate and neutralise insults while promoting tissue repair. Complement proteins act in a cascade to opsonise pathogens, lyse membranes and recruit inflammatory cells.

Adaptive immunity — specificity and memory: Adaptive immunity is mediated mainly by B and T lymphocytes and is characterised by specificity and memory. B cells can differentiate into plasma cells that secrete antibodies specific to antigens; antibodies neutralise toxins, block pathogen entry, and tag microbes for phagocytosis (opsonisation). T cells include helper T cells (CD4+) that orchestrate immune responses and cytotoxic T cells (CD8+) that kill infected or abnormal host cells. Antigen presentation by major histocompatibility complex (MHC) molecules is essential for T cell activation. Following antigen exposure, memory B and T cells persist and mediate rapid, strong secondary responses on re-exposure.

Antibody structure and functions: Antibodies (immunoglobulins) are Y-shaped proteins with variable regions that bind antigens and constant regions that mediate effector functions. Classes such as IgM, IgG, IgA, IgE and IgD have different roles: IgM is effective in early responses, IgG is abundant and crosses the placenta, IgA protects mucosal surfaces, IgE mediates allergy and anti-parasite responses. Antibodies can neutralise pathogens, agglutinate cells, fix complement and facilitate phagocytosis.

Vaccination and immune memory: Vaccination exposes the immune system to non-pathogenic forms of antigens (inactivated, attenuated, or subunit vaccines), triggering adaptive responses and memory cell formation without causing disease. On subsequent exposure, memory cells produce faster and larger responses providing protection. Herd immunity occurs when a sufficient proportion of a population is immune, reducing spread and protecting vulnerable individuals.

Dysfunction and clinical considerations: Immune system malfunction leads to a range of problems: hypersensitivity reactions (allergies), autoimmunity (immune attack on self tissues), immunodeficiency (inability to mount effective responses) and chronic inflammation. Diagnostic tests include antibody titres, white blood cell counts, skin tests and specific immune assays. Therapies range from vaccines and antimicrobials to immunosuppressants and monoclonal antibodies. Understanding immune mechanisms underpins public health strategies and modern treatments such as immunotherapy for cancer.

Male reproductive anatomy and function: The male reproductive system includes testes, where spermatogenesis occurs within seminiferous tubules and testosterone is produced by Leydig cells. Sperm mature in the epididymis and travel through the vas deferens to join secretions from seminal vesicles, prostate and bulbourethral glands, forming semen that is ejaculated via the urethra. Testosterone drives development of male secondary sexual characteristics, libido and supports spermatogenesis under hypothalamic-pituitary regulation (GnRH → LH/FSH).

Female reproductive anatomy and function: The female reproductive system includes ovaries that produce ova and secrete oestrogen and progesterone. Each month follicles develop in the ovary; typically one matures and ovulates an oocyte into the fallopian tube where fertilisation can occur. The uterus provides a site for implantation and fetal development; the endometrium cycles in response to hormonal changes. The cervix and vagina are involved in sperm entry and birth. Oestrogen and progesterone regulate the menstrual cycle in concert with FSH and LH from the anterior pituitary under hypothalamic control.

Gametogenesis: Spermatogenesis produces numerous sperm continuously from puberty under FSH and testosterone influence. Oogenesis begins prenatally with oogonia undergoing meiosis; at puberty, cyclic changes resume with one oocyte completing meiosis I each cycle to form a secondary oocyte arrested in metaphase II until fertilisation. This results in a limited number of gametes and a cyclic pattern in females.

Menstrual cycle and fertilisation: The typical 28-day cycle has follicular, ovulatory and luteal phases. Rising oestrogen during the follicular phase leads to a surge in LH causing ovulation. The ruptured follicle becomes the corpus luteum, secreting progesterone to maintain the endometrium; if pregnancy does not occur, the corpus luteum degenerates and menstruation follows. Fertilisation usually occurs in the fallopian tube; the zygote divides to form a blastocyst that implants in the uterine wall. The placenta forms to exchange nutrients and wastes between maternal and fetal blood without mixing their circulations and secretes hormones (hCG, progesterone) that maintain pregnancy.

Embryonic development and birth: Early development includes cleavage, blastulation, gastrulation (formation of three germ layers) and organogenesis. Organ systems develop in critical windows; teratogens can disrupt development. Labour involves hormonal signals (oxytocin, prostaglandins) that induce uterine contractions, cervical dilation and delivery. Postnatal development includes neonatal adaptation, growth and puberty when reproductive capacity is attained.

Contraception and reproductive health: Methods include barrier methods, hormonal contraception (combined pill, progestin-only), intrauterine devices, and sterilisation. Sexually transmitted infections affect reproductive health and require prevention and treatment. Reproductive physiology knowledge supports family planning, antenatal care and management of infertility and genetic counselling.

Essential nutrients and their roles: Human nutrition provides substrates for growth, maintenance and energy. Carbohydrates mainly supply glucose for ATP production; proteins supply amino acids for tissue synthesis and enzymatic function; fats provide dense energy, essential fatty acids and structural lipids for membranes; vitamins and minerals act as cofactors for metabolic enzymes, antioxidants and structural components; water is essential as solvent, for transport, and for thermoregulation. Dietary fibre supports gut motility and microbiota health.

Digestion, absorption and utilisation: Macronutrients are digested to absorbable units: carbohydrates to monosaccharides, proteins to amino acids and peptides, and fats to fatty acids and glycerol. These are absorbed and enter metabolic pathways: glucose can be oxidised via glycolysis and the Krebs cycle for ATP, stored as glycogen in liver and muscle, or converted to fat. Amino acids can be deaminated and used for energy or gluconeogenesis. Lipids are stored in adipose tissue and mobilised during fasting by lipolysis. Vitamins and minerals are needed in small amounts but deficiency affects specific enzymes and physiological processes.

Energy balance and body weight: Energy homeostasis is governed by energy intake (food calories) and expenditure (basal metabolic rate, physical activity, thermogenesis). Basal metabolic rate depends on body size, composition and hormones (notably thyroid hormones). Positive energy balance leads to weight gain and increased adiposity; prolonged imbalance contributes to obesity, insulin resistance and cardiovascular risk. BMI provides a simple index of body mass status but has limitations; body composition measures (fat vs lean mass) are more informative clinically.

Metabolic regulation by hormones: Insulin, glucagon, adrenaline and cortisol coordinate metabolic states. After a meal insulin promotes glucose uptake, glycogenesis and lipogenesis. During fasting glucagon and adrenaline mobilise glycogen and increase gluconeogenesis, and cortisol supports longer-term metabolic adjustments. Thyroid hormones upregulate overall metabolic activity. Disruption of these systems leads to metabolic diseases such as diabetes mellitus, hyperthyroidism or hypothyroidism.

Malnutrition and public health: Undernutrition (deficiencies of calories, protein or micronutrients) causes stunting, weakened immunity and developmental problems; overnutrition increases prevalence of obesity, type 2 diabetes and cardiovascular disease. Balanced diets tailored for life stage and activity, public health interventions (fortification, supplementation) and education are essential. Understanding metabolism helps interpret dietary guidance, manage metabolic disorders and design policies to improve population health.

Concepts of ageing at cellular and organismal levels: Ageing is a progressive decline in physiological integrity leading to impaired function and increased vulnerability to disease. At the cellular level changes include telomere shortening with repeated divisions, accumulation of DNA and protein damage, mitochondrial dysfunction, altered intercellular signalling and reduced autophagy. At the tissue and organ level these changes translate to decreased regenerative capacity, reduced elasticity (e.g., in blood vessels and lungs), and accumulation of senescent cells that secrete pro-inflammatory factors (senescence-associated secretory phenotype) contributing to chronic inflammation.

Repair mechanisms and limitations: Tissues have differing regenerative abilities. Epithelial tissues, bone and liver regenerate well; skeletal muscle, cartilage, cardiac muscle and neurons have limited regenerative potential. Repair often proceeds by inflammation, proliferation and remodelling phases. When functional cells are extensively lost, fibroblasts produce collagen leading to scar formation which restores structural integrity but not original function. Stem cells in adult tissues provide renewal, but their numbers and potency decline with age, reducing repair efficiency.

Wound healing and factors affecting repair: Wound healing involves haemostasis and clot formation, an inflammatory phase where neutrophils and macrophages remove debris and pathogens, a proliferative phase with fibroblast activity, angiogenesis and extracellular matrix deposition, and a remodelling phase that strengthens tissue over time. Nutrition, oxygenation, blood supply, age, systemic diseases (diabetes), infection and medications (e.g., steroids) influence healing outcomes. Chronic wounds occur when normal progression is disrupted.

Physiological changes in ageing and clinical impact: Ageing affects cardiovascular function (reduced cardiac reserve), renal function (declining GFR), pulmonary function (reduced elastic recoil), musculoskeletal system (loss of bone density and muscle mass), and immune competence (immune senescence), increasing susceptibility to infections and impairing vaccine responses. Cognitive changes, sensory decline (vision, hearing) and endocrine shifts (menopause, decreased growth hormone) alter quality of life. Preventive measures such as exercise, adequate nutrition, vaccination and management of chronic disease can slow functional decline.

Regenerative medicine and rehabilitation: Research in stem cell therapy, tissue engineering and gene therapy aims to restore lost functions. Clinical rehabilitation uses physiotherapy, occupational therapy and assistive devices to maximise independence. Ethical and social considerations include equitable access to elder care, chronic disease management and support systems. Understanding repair mechanisms and ageing informs medical care, public health planning and individual lifestyle choices that promote healthy ageing.

Physiological demands of exercise: Exercise requires coordinated changes across multiple systems to supply working muscles with oxygen and nutrients and to remove metabolic wastes. Cardiovascular adjustments include increased heart rate and stroke volume raising cardiac output; redistribution of blood flow favours contracting muscles and skin for thermoregulation while reducing flow to less essential organs. Respiratory rate and tidal volume increase to enhance oxygen uptake and CO2 removal. Metabolic shifts favour increased glucose uptake by muscle (insulin-independent transport during activity), enhanced aerobic ATP production and, during intense effort, greater reliance on anaerobic glycolysis producing lactate.

Acute responses and fatigue mechanisms: Short-term responses to exercise include sympathetic activation releasing adrenaline that increases heart rate, dilates bronchioles and mobilises energy stores. Fatigue arises from multiple factors: depletion of energy substrates (glycogen), accumulation of metabolites (H+, inorganic phosphate), impaired calcium handling, and central (neural) fatigue. Thermoregulatory strain, dehydration and electrolyte imbalance can limit performance and cause heat illness.

Training adaptations: Regular endurance training induces cardiovascular and muscular adaptations: increased capillary density, mitochondrial number and oxidative enzyme activity in muscle fibres, and sometimes increased left ventricular stroke volume. Strength training increases muscle fibre cross-sectional area and neural efficiency. These adaptations improve oxygen utilisation (VO2 max) and performance while reducing risk of cardiovascular disease.

Stress responses and hormonal coordination: Stress—physical, emotional or environmental—activates the sympathetic nervous system and the hypothalamo-pituitary-adrenal (HPA) axis. Sympathetic outflow releases adrenaline and noradrenaline producing immediate cardiovascular and metabolic changes. The HPA axis releases cortisol which promotes gluconeogenesis, mobilises amino acids and modulates immune responses for longer-term adaptation. Chronic stress can dysregulate these systems leading to hypertension, impaired immunity, metabolic disturbances and mental health effects.

Practical applications and safety: Designing exercise programmes requires understanding intensity, duration and recovery to avoid overtraining. Hydration, balanced nutrition and progressive overload optimise adaptation. Recognising signs of heat exhaustion, hypoglycaemia, cardiac ischemia or severe asthma is important for safety. Stress management techniques (relaxation, exercise, sleep hygiene) reduce chronic physiological burden. Integrative understanding of how systems work together aids in prescribing exercise and managing stress for health and rehabilitation.

Common physiological measurements and instruments: Basic measurements used in physiology include pulse rate, blood pressure (measured by sphygmomanometer), respiratory rate, body temperature (thermometer), blood glucose (glucometer), and urine characteristics (colour, specific gravity, dipstick tests). Advanced tools include electrocardiography (ECG) for cardiac electrical activity, spirometers for lung volumes and flows, and peak flow meters for airway function. Laboratory analyses such as complete blood count, blood chemistry panels, and hormone assays provide information about internal physiological states.

Principles of accurate measurement: Valid measurements require standardised procedures, calibrated instruments and controlled conditions. For blood pressure, the subject should be seated and rested; cuff size must match arm circumference and readings should be averaged over multiple measurements. Pulse should be measured at rest and following standardised activity to assess reactivity. For spirometry, maximal efforts and proper technique yield reproducible volumes such as forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1).

Designing and conducting simple experiments: Classroom practicals may include measuring resting versus post-exercise pulse rate, effects of posture on blood pressure, or breath-holding times. Important experimental practices include defining variables (independent, dependent, controlled), using appropriate sample sizes, recording observations accurately, and applying simple statistics (mean, percentage change). Ethical considerations require informed consent from volunteers, non-invasive procedures, and privacy of personal data.

Safety, ethics and consent: When students work with human volunteers, safety is paramount. Procedures should be low risk, supervised and accompanied by emergency plans. Participants must give informed consent and may withdraw at any time. Data must be anonymised for reports and stored securely. Respectful conduct includes not performing invasive tests without qualified personnel and refusing any activity that may harm a volunteer.

Data handling and interpretation: After data collection, results should be organised in tables, plotted on graphs and interpreted in context of normal reference ranges. Comparing pre- and post-intervention values illustrates physiological responses; for example, a post-exercise increase in pulse reflects sympathetic activation. Understanding variability, outliers and possible measurement errors teaches critical analysis. These skills are foundational for further study and careers in health sciences.