Human Physiology

Digestion and absorption

Digestion breaks complex food into smaller molecules that can be absorbed. Mechanical processing increases surface area, while enzymes catalyse chemical breakdown. Salivary amylase begins starch digestion in the mouth. In the stomach, hydrochloric acid creates an acidic environment and helps activate pepsinogen to pepsin for protein digestion.

The small intestine is the main site of digestion and absorption. Pancreatic enzymes and intestinal enzymes complete digestion. Bile salts emulsify fats and help the formation of micelles; bile itself is not a digestive enzyme. Villi and microvilli provide a large absorptive surface.

• Monosaccharides and amino acids enter blood capillaries within intestinal villi.
• Much absorbed lipid is packaged into chylomicrons that enter intestinal lymphatic vessels called lacteals.
• The large intestine absorbs remaining water and electrolytes and contributes to faeces formation.

Breathing and exchange of gases

Ventilation moves air in and out of the lungs. During normal inspiration, the diaphragm contracts and thoracic volume increases. Pressure within the lungs falls below atmospheric pressure, drawing air inward. Quiet expiration is largely passive as the diaphragm relaxes and elastic tissues recoil.

Gas exchange occurs across the thin alveolar–capillary surface. Oxygen and carbon dioxide move down their partial-pressure gradients. Oxygen is transported mainly bound to haemoglobin. Most carbon dioxide is carried as bicarbonate; smaller amounts are dissolved or bound to haemoglobin.

• Breathing is air movement; cellular respiration is a set of metabolic reactions that releases usable energy.
• High carbon dioxide concentration, lower pH and increased temperature favour oxygen unloading from haemoglobin in active tissues.
• Carbonic anhydrase in red blood cells accelerates conversion between carbon dioxide and carbonic acid.

The heart, blood and circulation

The four-chambered heart supports double circulation: pulmonary circulation connects the heart and lungs, and systemic circulation supplies the rest of the body. Arteries carry blood away from the heart and veins carry it toward the heart. This definition depends on direction, not oxygen content.

The sinoatrial node normally initiates the heartbeat. Electrical activity spreads through the atria, is delayed at the atrioventricular node, and then reaches the ventricles through conducting fibres. Valves limit backward flow. Systole means contraction and diastole means relaxation.

Trace the route

Body → venae cavae → right atrium → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium → left ventricle → aorta → body. The pulmonary artery carries deoxygenated blood; the pulmonary veins carry oxygenated blood.

Kidneys and water balance

A nephron forms urine through filtration, reabsorption and secretion. Blood pressure drives filtration from glomerular capillaries into Bowman’s capsule. Normally, cells and most large proteins remain in the blood. Useful solutes and much water return to the blood by selective reabsorption. Secretion adds certain substances from blood to the tubular fluid.

The proximal convoluted tubule carries out most bulk reabsorption of filtered water and sodium, and normally reabsorbs almost all filtered glucose and amino acids. The loop of Henle contributes to the medullary concentration gradient. The distal tubule and collecting duct adjust the final composition of urine.

• ADH increases water permeability in parts of the distal nephron, particularly the collecting ducts, helping conserve water.
• Aldosterone promotes sodium reabsorption and potassium secretion in the distal nephron.
• Countercurrent arrangements in the loop of Henle and vasa recta support urine concentration.
• Filtration is a bulk process; reabsorption and secretion provide selective adjustments.

Movement and muscle contraction

Skeletal muscle contains myofibrils organised into sarcomeres. Thin filaments contain actin and regulatory proteins, while thick filaments contain myosin. In the sliding-filament model, filaments retain their length but slide past one another, shortening the sarcomere.

A nerve impulse stimulates calcium release from the sarcoplasmic reticulum. Calcium binds troponin, shifting tropomyosin so that myosin can interact with actin. ATP binding detaches a cross-bridge; ATP hydrolysis prepares the myosin head for another cycle. Removal of calcium from the cytosol permits relaxation.

• The A band remains approximately constant during contraction. The I band and H zone become smaller.
• ATP is needed both for cross-bridge cycling and for pumping calcium back into storage.
• Tendons connect muscle to bone; ligaments connect bone to bone.

Nervous coordination

Neurons receive, integrate and transmit signals. At rest, ion gradients and selective membrane permeability produce a negative potential inside the neuron relative to outside. During an action potential, voltage-gated sodium channels open to produce rapid depolarisation, followed by potassium-mediated repolarisation.

Myelin speeds conduction by insulating axons, with action potentials regenerated at nodes of Ranvier. At a typical chemical synapse, neurotransmitter is released from the presynaptic terminal and binds receptors on the postsynaptic membrane. Reflex arcs connect receptors, sensory pathways, processing centres, motor pathways and effectors.

• A stronger stimulus is commonly represented by a higher frequency of action potentials, rather than a larger action potential.
• The central nervous system comprises the brain and spinal cord. Peripheral nerves connect it to the rest of the body.

Hormones and feedback control

Endocrine glands release hormones into the circulation. Only cells with appropriate receptors respond. Negative feedback reduces the initial disturbance: for example, higher thyroid hormone levels suppress signals that stimulate further thyroid hormone production.

Pancreatic beta cells release insulin when blood glucose rises. Insulin favours glucose uptake in responsive tissues and storage as glycogen. Alpha cells release glucagon, which generally increases blood glucose. The hypothalamus and pituitary coordinate several endocrine pathways.