Human physiology

1. Physiological organisation and homeostasis

Human physiology studies function at the levels of cells, tissues, organs and organ systems. Cells obtain usable energy mainly through the production of adenosine triphosphate, or ATP. Cellular respiration is distinct from breathing: breathing moves air into and out of the lungs, whereas cellular respiration releases energy from nutrients. Most aerobic ATP production occurs in mitochondria. The respiratory, digestive and circulatory systems supply the oxygen and nutrients needed for these reactions.

Homeostasis maintains variables such as body temperature, blood glucose, pH and water balance within workable limits. A typical control mechanism comprises a receptor that detects change, a coordinating centre and an effector that produces a response. Negative feedback opposes the initial disturbance. When body temperature rises, sweating and increased blood flow through skin help dissipate heat; when temperature falls, shivering increases heat production.

Positive feedback amplifies an initial change until a specific endpoint is reached. During childbirth, stretching of the cervix stimulates oxytocin release, which strengthens uterine contractions and causes further stretching. Positive feedback is therefore useful in particular processes but is not the usual mechanism for maintaining steady internal conditions. The hypothalamus links nervous and endocrine regulation and participates in temperature control, thirst and biological rhythms.

• Diffusion is movement down a concentration gradient; osmosis is water movement across a selectively permeable membrane.
• Active transport requires energy, directly or indirectly, and can move substances against their concentration gradient.

2. Digestion, absorption and nutrition

Digestion converts complex food into absorbable molecules. Salivary amylase begins starch digestion in the mouth. In the stomach, hydrochloric acid creates an acidic environment, helps activate pepsin and kills many ingested microorganisms. Pepsin begins protein digestion, while mucus helps protect the stomach lining. The stomach is not the principal site of nutrient absorption.

The small intestine receives bile from the liver and digestive secretions from the pancreas. Bile is stored and concentrated in the gallbladder; it contains no digestive enzymes. Bile salts emulsify fats, increasing the surface available for lipase action, and assist the absorption of fats and fat-soluble vitamins. Pancreatic bicarbonate helps neutralise acidic material arriving from the stomach, allowing intestinal enzymes to function effectively.

Villi and microvilli greatly increase the absorptive surface of the small intestine. Monosaccharides and amino acids generally enter blood capillaries and reach the liver through the hepatic portal circulation. Most absorbed long-chain dietary fats are packaged into chylomicrons and enter intestinal lymphatic vessels called lacteals. The large intestine absorbs remaining water and electrolytes and supports a diverse microbial community.

Nutritional deficiency can impair physiological function. Iron deficiency may cause anaemia, iodine deficiency can impair thyroid hormone production, vitamin A deficiency can cause night blindness, and vitamin D deficiency can cause rickets in children. Vitamin B12 absorption requires intrinsic factor produced by gastric parietal cells. Vitamins supply no energy themselves, although many enable essential metabolic reactions.

3. Respiration, circulation and immunity

During normal inspiration, the diaphragm contracts and flattens, increasing thoracic volume and lowering pressure within the lungs relative to atmospheric pressure. Quiet expiration is largely passive. Gas exchange occurs across the thin alveolar-capillary membrane by diffusion along partial-pressure gradients. Most oxygen travels bound to haemoglobin, while most carbon dioxide is transported as bicarbonate ions. Carbon monoxide binds strongly to haemoglobin and reduces effective oxygen delivery.

The four-chambered human heart maintains double circulation. Pulmonary circulation carries blood from the right ventricle to the lungs and back to the left atrium; systemic circulation carries it from the left ventricle to body tissues and back to the right atrium. Pulmonary arteries carry deoxygenated blood, while pulmonary veins carry oxygenated blood. Valves promote one-way flow. The sinoatrial node normally initiates the heartbeat.

Blood plasma transports proteins, nutrients, hormones and wastes. Red blood cells carry respiratory gases, white blood cells provide immune defence, and platelets participate in haemostasis. Lymph returns excess tissue fluid to the blood, transports absorbed fats and supports immune surveillance. Blood pressure depends on cardiac output, vascular resistance and other factors; a single elevated reading does not necessarily establish chronic hypertension.

Innate immunity includes skin, mucous membranes, phagocytes and other rapid defences. Adaptive immunity involves antigen-specific responses by B and T lymphocytes. Plasma cells derived from B cells secrete antibodies. Vaccination generates active immunity and immunological memory; administration of preformed antibodies provides passive protection that is usually temporary.

4. Excretion, water balance and hormonal control

The kidneys remove metabolic wastes and regulate water, electrolytes and acid-base balance. Each nephron contains a renal corpuscle and a tubule. Glomerular filtration produces a filtrate from blood plasma while normally retaining blood cells and most proteins. Selective tubular reabsorption returns useful substances to blood, while tubular secretion transfers additional substances into the tubular fluid. Urine formation is therefore more than simple filtration.

Antidiuretic hormone, also called vasopressin, increases water reabsorption in the collecting ducts. It is synthesised in the hypothalamus and released through the posterior pituitary. Aldosterone, produced by the adrenal cortex, promotes sodium reabsorption and potassium secretion in parts of the distal nephron. The kidneys also release renin, produce erythropoietin and help activate vitamin D.

Endocrine glands secrete hormones into blood, unlike exocrine glands, whose secretions pass through ducts. The pancreas performs both functions. Its beta cells secrete insulin, which promotes glucose uptake in muscle and adipose tissue and supports energy storage. Alpha cells secrete glucagon, which promotes hepatic glucose release. Type 1 diabetes involves severe insulin deficiency, usually following autoimmune beta-cell destruction; type 2 involves insulin resistance with inadequate compensatory insulin secretion.

Thyroid hormones require iodine and help regulate metabolism, growth and development. Calling the pituitary the master gland is useful shorthand, but its activity is substantially regulated by the hypothalamus and feedback from target-gland hormones. Endocrine control operates through networks rather than an entirely one-directional hierarchy.

5. Nervous coordination and reproductive physiology

The central nervous system comprises the brain and spinal cord; the peripheral nervous system connects it with the rest of the body. Neurons transmit electrical signals along their membranes and commonly communicate across synapses using neurotransmitters. Myelin accelerates impulse conduction. Reflex pathways enable rapid responses: withdrawal from a painful stimulus can be organised through spinal circuits before conscious awareness is fully established.

The autonomic nervous system regulates involuntary functions. Sympathetic activity generally prepares the body for exertion by increasing heart rate and mobilising energy, while parasympathetic activity generally supports digestion and recovery. These divisions do not always act as simple opposites. The cerebellum coordinates movement and balance; the medulla contains centres important for breathing and cardiovascular regulation.

Reproductive physiology depends on hypothalamic, pituitary and gonadal hormones. Follicle-stimulating hormone and luteinising hormone regulate ovarian and testicular functions. A surge in luteinising hormone triggers ovulation. Fertilisation usually occurs in the ampullary region of the uterine tube, followed by implantation in the uterine lining. The placenta enables exchange between maternal and fetal circulations without their blood normally mixing directly.

Human chorionic gonadotropin supports the corpus luteum in early pregnancy and is detected by pregnancy tests. Prolactin promotes milk production, whereas oxytocin causes milk ejection. Colostrum, the early milk, supplies antibodies and other protective factors, illustrating the connection between reproduction, nutrition and passive immunity.

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