Chapter 3 — Human Anatomy and Physiology

Levels of organisation
The human body is organised into successive levels that build complexity while allowing division of labour. The smallest functional unit is the cell. Cells with similar structure and function group together to form tissues. Different tissues combine to make organs. Related organs form organ systems which collaborate to maintain life. This hierarchical organisation brings efficiency and specialisation.

Cells
Each cell is a living unit that performs basic life functions: obtaining nutrients, producing energy, growing, responding to stimuli and reproducing. For example, muscle cells shorten to produce movement, while nerve cells transmit electrical impulses. The shape and internal machinery of a cell suit its role.

Tissues
Tissues are collections of similar cells and the matrix they produce. The four primary tissue types are epithelial (covers surfaces and lines cavities), connective (supports and protects), muscular (produces movement) and nervous (controls and coordinates). Each tissue type has a characteristic arrangement and function that supports organ tasks.

Organs
An organ is a structure composed of at least two tissue types that work together for a specific function. For example, the stomach contains muscular tissue for churning, epithelial tissue for secretion and absorption, and connective tissue for support. Knowing the tissue composition of an organ helps explain how it performs its job.

Organ systems
Organ systems are groups of organs with coordinated roles. The digestive system breaks down food and absorbs nutrients; the circulatory system transports substances; the respiratory system enables gas exchange; the excretory system removes wastes; the nervous and endocrine systems coordinate activities. Systems communicate through blood, nerves and chemical messengers, so a problem in one organ may affect several systems.

Integration and clinical relevance
Understanding organisation helps in diagnosing disease. For instance, infection in the lungs (respiratory system) can affect oxygen delivery (circulatory system) and cause systemic effects like fever. When learning anatomy, always link structure to function and trace how an organ fits into higher levels. Practice labelling diagrams and explaining how change at one level impacts higher levels.

Cell structure relevant to function
Human cells are specialised for their tasks. Most have a plasma membrane that controls entry and exit of materials, a nucleus that holds genetic instructions, cytoplasm where biochemical reactions occur and organelles such as mitochondria for energy production, ribosomes for protein synthesis and endoplasmic reticulum and Golgi apparatus for processing and transport. These structures explain how cells grow, make proteins and respond to signals.

Four primary tissue types
The body’s tissues are grouped into four main categories. Epithelial tissue covers external and internal surfaces, forming protective barriers and sites of absorption and secretion. Examples include skin surface and lining of the gut. Connective tissue supports and binds structures; it ranges from soft (fat, loose connective tissue) to firm (bone) and fluid (blood). It contains cells in an extracellular matrix made of fibres and ground substance.

Muscle tissue
Muscle tissue produces movement and force. Skeletal muscle attaches to bones and is under voluntary control; it has long multinucleated fibres with visible striations. Cardiac muscle makes up the heart, has striations and intercalated discs for coordinated beating. Smooth muscle lines internal organs and blood vessels; it lacks striations and works involuntarily to move substances through hollow organs.

Nervous tissue
Nervous tissue detects stimuli, processes information and transmits signals. Neurons have a cell body, dendrites to receive input and an axon to send impulses. Neuroglia are supporting cells that nourish neurons, remove debris and maintain the environment. The arrangement of neurons and glia allows rapid communication across the body.

Specialised epithelial forms
Simple epithelium (single layer) is suited to absorption and filtration, such as in the small intestine and lungs. Stratified epithelium (many layers) protects against wear, seen in skin. Glandular epithelium secretes substances; glands may be unicellular (mucus cells) or multicellular (salivary glands).

Functional examples
Bone tissue provides strength and mineral storage; cartilage cushions joints; blood transports gases and nutrients. Recognising tissue type in an organ explains its mechanical and physiological properties. When studying tissues, focus on cell types, extracellular material and the function these create.

Overview
The digestive system converts food into small molecules the body can absorb and use. It consists of the alimentary canal (a continuous tubular passage from mouth to anus) and accessory organs (salivary glands, liver, gall bladder and pancreas) that secrete substances into the canal to aid breakdown of food. The process includes ingestion, mechanical and chemical digestion, absorption and elimination.

Mouth, teeth and saliva
Digestion begins in the mouth where teeth tear, cut and grind food into smaller pieces. The tongue mixes food and forms a bolus for swallowing. Saliva from salivary glands contains water, mucus and the enzyme amylase which starts the breakdown of starch into simpler sugars. Saliva also moistens food and contains antibacterial compounds that help oral hygiene.

Oesophagus and stomach
The bolus passes down the oesophagus by coordinated muscular contractions (peristalsis). The stomach stores food, mixes it with gastric juice and begins protein digestion. Gastric juice contains hydrochloric acid (which provides an acidic environment to activate enzymes and kill many microbes) and pepsin, an enzyme that breaks proteins into peptides. The stomach’s muscular walls churn the food to form chyme, a semi-fluid mixture ready for the small intestine.

Small intestine and accessory glands
Most digestion and most nutrient absorption occur in the small intestine, which has three parts: duodenum, jejunum and ileum. The duodenum receives bile from the liver and gall bladder and pancreatic juice from the pancreas. Bile emulsifies fats, creating small droplets for more efficient enzymatic action. Pancreatic enzymes include amylase (carbohydrates), proteases (proteins) and lipase (fats). Intestinal glands secrete enzymes that complete digestion. The inner wall of the small intestine is folded into villi and microvilli that greatly increase surface area for absorption of amino acids, monosaccharides, fatty acids and glycerol.

Large intestine and bacteria
Undigested material passes into the large intestine (colon), which absorbs water and minerals and compacts the waste into faeces. Beneficial gut bacteria ferment some indigestible fibre, producing short-chain fatty acids and synthesising vitamins such as vitamin K and some B vitamins. This microbial community contributes to digestion and health.

Elimination and health
Faeces are stored in the rectum and expelled via the anus. Healthy digestion requires a balanced diet, sufficient fibre, hydration and regular physical activity. Disorders include indigestion, ulcers, diarrhoea and constipation; some arise from infection, diet or stress and require medical attention.

Essential nutrients and their roles
The body needs a variety of nutrients: carbohydrates, proteins, fats, vitamins, minerals, water and fibre. Carbohydrates and fats are the main energy sources; proteins provide amino acids for growth, repair and enzymes; vitamins and minerals support metabolic reactions and structural roles; water is vital for transport and chemical reactions; fibre aids bowel movement and supports gut health.

Chemical digestion and end products
Digestive enzymes break large food molecules into small absorbable units: carbohydrates to monosaccharides (like glucose), proteins to amino acids and peptides, and fats to fatty acids and glycerol. Enzymes act at specific sites: salivary and pancreatic amylases for starch, pepsin and pancreatic proteases for proteins, and lipase for fats. Brush border enzymes in the small intestine complete digestion into units ready for absorption.

Absorption mechanisms
Absorption occurs mainly in the small intestine. The mucosa has villi and microvilli that amplify surface area. Glucose and amino acids are absorbed into blood capillaries by active transport or facilitated diffusion; they pass via the hepatic portal vein to the liver for processing. Fatty acids and glycerol are reassembled into triglycerides within enterocytes, packaged into chylomicrons and transported through lacteals of the lymphatic system into the bloodstream.

Metabolism: catabolism and anabolism
Metabolism includes two complementary processes. Catabolism breaks down molecules to release energy; for example, glucose is oxidised through glycolysis, the Krebs cycle and oxidative phosphorylation to produce ATP. Anabolism uses energy and small molecules to build complex molecules like proteins or glycogen for storage. The balance between catabolism and anabolism depends on nutritional state and hormones.

Energy balance and storage
Excess energy is stored as glycogen in liver and muscle and as fat in adipose tissue. During fasting, glycogen is mobilised, and prolonged fasting leads to fat and protein breakdown. Energy balance (calories in vs out) determines weight gain or loss. Body Mass Index (BMI) is a simple population measure of weight relative to height but does not measure body composition directly.

Dietary guidelines and health
A balanced diet with appropriate portions of all nutrient groups, regular meals, adequate hydration and limited intake of added sugars and saturated fats maintains health. Malnutrition (deficiency or excess) affects growth, immunity and organ function. For adolescents, adequate proteins, calcium, iron and vitamins are especially important to support growth and development.

Overall function
The circulatory system transports oxygen, nutrients, hormones and wastes and helps maintain temperature and pH. It comprises the heart, blood vessels and blood. Efficient circulation ensures each cell receives substances required for metabolism and that wastes are removed promptly.

Structure of the heart
The heart is a muscular pump with four chambers: right atrium, right ventricle, left atrium and left ventricle. It is divided into right and left sides by a septum. Valves—atrioventricular valves (tricuspid on right, bicuspid/mitral on left) and semilunar valves (pulmonary and aortic)—prevent backflow and ensure unidirectional flow. The heart wall has three layers: the outer epicardium, muscular myocardium and inner endocardium. The coronary arteries supply the heart muscle itself with oxygen and nutrients.

Blood flow and circulation types
There are two circulation loops: pulmonary circulation carries deoxygenated blood from the right ventricle to the lungs via the pulmonary artery and returns oxygenated blood to the left atrium via pulmonary veins; systemic circulation carries oxygenated blood from the left ventricle through the aorta to body tissues and returns deoxygenated blood to the right atrium via the venae cavae. The coordinated contraction of atria and ventricles maintains efficient pumping.

Cardiac cycle and heart sounds
The cardiac cycle comprises systole (contraction) and diastole (relaxation). Atrial systole completes ventricular filling, then ventricular systole ejects blood into arteries. Heart sounds arise from valve closures: the ‘lub’ is the atrioventricular valves closing and the ‘dub’ is the semilunar valves closing. Timing of phases ensures continuous flow and adequate filling.

Blood vessels and exchange
Arteries carry blood away from the heart under higher pressure and have thick muscular, elastic walls to withstand pressure changes. Arterioles regulate flow into capillary beds. Capillaries are thin-walled sites of exchange where nutrients, gases and wastes move by diffusion and bulk flow between blood and tissues. Venules and veins return blood to the heart under lower pressure; veins have valves to prevent backflow, aided by muscle contractions in limbs.

Cardiac output and regulation
Cardiac output is the volume of blood pumped per minute and equals heart rate times stroke volume. It varies with activity, fitness and demands of organs. The autonomic nervous system and hormones (adrenaline) adjust heart rate and contractility. Maintaining healthy blood vessels through diet, exercise and avoiding smoking reduces risk of hypertension and atherosclerosis which impair circulation.

Composition of blood
Blood is a specialised fluid connective tissue made of plasma and formed elements. Plasma is the pale yellow liquid portion, mostly water, containing dissolved proteins (albumin, globulins, fibrinogen), electrolytes, nutrients, hormones and wastes. Formed elements include red blood cells (RBCs or erythrocytes), white blood cells (WBCs or leukocytes) and platelets (thrombocytes).

Red blood cells
RBCs are biconcave discs that lack nuclei in mature human cells, enabling more space for haemoglobin. Haemoglobin is an iron-containing protein that binds oxygen in the lungs and releases it in tissues; it also helps transport some carbon dioxide. RBCs are produced in bone marrow and have a lifespan of about 120 days. The number of RBCs and haemoglobin concentration determine the oxygen-carrying capacity; low levels cause anaemia, producing fatigue and breathlessness.

White blood cells and immunity
WBCs are diverse cells involved in defence. Granulocytes (neutrophils, eosinophils, basophils) participate in phagocytosis and inflammation; neutrophils are first responders to bacterial infection. Agranulocytes include lymphocytes (B cells and T cells) and monocytes; B cells produce antibodies while T cells assist or kill infected cells. Monocytes become macrophages in tissues and clear debris and pathogens. WBC counts change in infection, allergy and immune disorders.

Platelets and clotting
Platelets are cell fragments that help prevent blood loss. On vessel injury, platelets adhere to the damaged surface, aggregate and release substances that activate clotting factors. The clotting cascade leads to conversion of fibrinogen (soluble plasma protein) into insoluble fibrin threads that form a stable mesh, trapping blood cells and forming a clot which seals the wound. Impairment of clotting results in excessive bleeding; inappropriate clotting can cause thrombosis.

Functions of blood
Blood transports oxygen, carbon dioxide, nutrients, hormones and wastes; helps regulate body temperature and pH; and protects by clotting and immune responses. Plasma proteins maintain osmotic balance and act as carriers for substances. Maintaining healthy blood depends on good nutrition (iron, vitamins), hydration and prompt treatment of infections or blood disorders.

Components and flow
The lymphatic system is a network of vessels, lymph nodes and lymphoid organs (tonsils, spleen, thymus) that returns excess tissue fluid to the bloodstream, transports absorbed lipids from the gut and participates in immune defence. Tissue fluid that bathes cells enters blind-ended lymphatic capillaries as lymph. Lymphatic vessels carry this fluid through lymph nodes, where immune cells screen for pathogens, and ultimately drain into large veins near the heart.

Structure and function of lymph nodes
Lymph nodes are small, bean-shaped organs placed at intervals along lymphatic vessels. They contain a cortex rich in B cell follicles and a deeper medulla with T cells and macrophages. As lymph passes through, pathogens and foreign particles are trapped and presented to lymphocytes, initiating adaptive immune responses. Swollen, tender lymph nodes commonly indicate local infection where immune activity is heightened.

Innate immunity
Innate immunity provides immediate, non-specific defence. It includes physical barriers (skin, mucous membranes), secretions (acidic stomach contents, lysozyme in tears), phagocytic cells (neutrophils, macrophages) and natural killer cells. Inflammation directs immune cells to sites of injury or infection by increasing blood flow and vascular permeability, allowing immune components to access tissues.

Adaptive immunity
Adaptive (specific) immunity involves lymphocytes that recognise specific antigens. B lymphocytes produce antibodies that neutralise pathogens or mark them for destruction. T lymphocytes include helper T cells that coordinate immune responses and cytotoxic T cells that kill infected cells. Adaptive immunity creates immunological memory; upon re-exposure to the same antigen, the secondary response is faster and stronger.

Vaccination and clinical relevance
Vaccination exposes the immune system to a harmless form of antigen (inactivated pathogen, attenuated organism or purified components) to stimulate memory cell formation without causing disease. This prepares the body for rapid defence on real exposure. Immune disorders include allergies (overreaction), autoimmune diseases (attack on self tissues) and immunodeficiency (reduced defence). Maintaining nutrition, hygiene and vaccination schedules strengthens lymphatic and immune health.

Anatomy of the respiratory tract
The respiratory system includes the nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles and alveoli within the lungs. Air enters through the nose or mouth where it is filtered, warmed and humidified. The trachea divides into two bronchi, which branch into progressively smaller bronchioles ending in clusters of alveoli—tiny thin-walled sacs ideal for gas exchange.

Mechanics of breathing
Breathing depends on changes in thoracic volume driven by the diaphragm and intercostal muscles. Inspiration occurs when the diaphragm contracts and flattens and external intercostal muscles lift the ribs, increasing chest cavity volume. This reduces intrapulmonary pressure below atmospheric pressure and air flows in. Expiration is usually passive: muscles relax, chest volume decreases, pressure rises and air flows out. Forced expiration involves abdominal muscles and internal intercostals.

Alveolar gas exchange
Alveoli have extremely thin walls and are surrounded by a dense network of capillaries. Oxygen diffuses from the alveolar air into blood because of the partial pressure gradient; it binds to haemoglobin in red blood cells. Carbon dioxide diffuses from blood into alveoli to be expelled. Carbon dioxide is transported in blood partly dissolved, largely as bicarbonate ions formed in red blood cells, and some bound to haemoglobin.

Transport and regulation
Oxygen carried by haemoglobin is released in tissues where partial pressure is lower, and carbon dioxide produced by metabolism is carried back to lungs. The respiratory centre in the medulla oblongata and pons controls breathing rate and depth by sensing carbon dioxide, oxygen and pH of blood. High CO2 or low pH increases ventilation; low CO2 reduces it. During exercise, increased muscular activity raises CO2 production and lowers pH, stimulating deeper and faster breathing.

Health and environment
Respiratory health is affected by smoking, air pollution and occupational exposures. Smoking damages cilia, inflames airways and destroys alveolar walls leading to chronic obstructive pulmonary disease (COPD) and increased infection risk. Protecting air quality, avoiding smoking and seeking early treatment for infections preserve respiratory function.

Function of excretion
The excretory system removes metabolic wastes, excess salts and surplus water to maintain internal chemical balance. Kidneys are the principal organs responsible for filtering blood, regulating fluid and electrolyte balance, and eliminating nitrogenous wastes like urea. Other excretory routes include lungs (carbon dioxide), skin (sweat) and liver (metabolic processing).

Kidney anatomy
Each kidney is composed of an outer cortex and inner medulla and contains about one million nephrons—the functional units that form urine. A nephron begins with a Bowman’s capsule enclosing a glomerulus (a tuft of capillaries). The renal tubule has segments: proximal convoluted tubule, loop of Henle (descending and ascending limbs), distal convoluted tubule and collecting duct. Blood enters via the renal artery and leaves via the renal vein; filtered fluid passes through ureters to the bladder for storage.

Stages of urine formation
Urine formation involves three main processes. Filtration at the glomerulus forces water and small solutes (glucose, amino acids, salts, urea) into Bowman’s capsule due to blood pressure, producing glomerular filtrate. Reabsorption returns useful substances—from filtrate back into blood—along the tubule: glucose and most salts are actively reabsorbed in the proximal tubule, water and some salts are reabsorbed in the loop of Henle and collecting duct. Secretion is the active transport of additional unwanted substances (excess H+, K+, medicines) from blood into the tubule. The final urine contains wastes, excess salts and variable water volume.

Concentration and hormonal control
The loop of Henle establishes an osmotic gradient in the medulla that allows water reabsorption from the collecting duct. Antidiuretic hormone (ADH) from the posterior pituitary increases the permeability of collecting ducts to water; when ADH is high (dehydration), more water is reabsorbed and urine becomes concentrated. Aldosterone, from the adrenal cortex, increases sodium reabsorption and indirectly promotes water retention, affecting blood volume and pressure.

Clinical aspects and care
Kidney problems include infections, kidney stones, and chronic renal failure which impair filtration and homeostasis. Symptoms may include changes in urine volume, blood in urine, swelling from fluid retention and high blood pressure. Preventive measures include adequate hydration, moderate salt intake, timely treatment of urinary infections and avoiding unnecessary nephrotoxic drugs. In advanced renal failure, dialysis or transplantation may be necessary.

Overall organisation
The nervous system integrates information from the environment and coordinates body responses. It consists of the central nervous system (CNS)—the brain and spinal cord—and the peripheral nervous system (PNS) composed of nerves and ganglia. The PNS carries sensory information to the CNS and motor commands from the CNS to muscles and glands. The autonomic nervous system, part of the PNS, controls involuntary functions and has sympathetic and parasympathetic divisions.

Neuronal structure and types
Neurons are the functional units that transmit electrical signals. A typical neuron has a cell body (soma) containing the nucleus, dendrites that receive signals, and an axon that conducts impulses away. Axons may be covered by myelin sheaths produced by glial cells which increase conduction speed. Sensory neurons carry information from receptors to the CNS; motor neurons transmit commands to effectors; interneurons connect neurons within the CNS.

Generation and conduction of nerve impulses
Nerve impulses arise from changes in membrane potential due to ion movements through channels. At rest, the membrane has a resting potential maintained by ion pumps. A stimulus that reaches a threshold produces an action potential: rapid depolarisation (inward Na+ flow) followed by repolarisation (outward K+ flow). The action potential travels along the axon as a wave. Myelinated axons conduct impulses faster by saltatory conduction, where the impulse jumps between nodes of Ranvier.

Synapses and neurotransmitters
At synapses, the electrical signal is converted into a chemical signal. Arrival of an impulse triggers release of neurotransmitters from synaptic vesicles; these diffuse across the synaptic cleft and bind receptors on the postsynaptic cell to excite or inhibit it. Examples include acetylcholine at neuromuscular junctions and dopamine and serotonin in brain circuits.

Reflexes and higher functions
Reflex arcs provide rapid, involuntary responses using a simple neural pathway: receptor -> sensory neuron -> CNS (often spinal cord) -> motor neuron -> effector. This allows immediate protection, such as withdrawal from a painful stimulus. The brain, especially the cerebral cortex, integrates sensory input for perception, voluntary movement, learning and memory. The limbic system contributes to emotions and memory formation. Damage to specific areas produces predictable deficits, underscoring localisation of function.

Function of sense organs
Sensory organs detect changes in the internal and external environment and convert stimuli into nerve impulses that the brain interprets. Major senses are vision, hearing and balance, smell, taste and somatic sensations (touch, temperature, pain). Each organ contains specialised receptor cells tuned to particular stimuli.

Eye and vision
The eye focuses light to form clear images on the retina. Light enters through the transparent cornea and pupil; the iris adjusts pupil size to control light entry. The lens fine-tunes focus by changing shape. The retina contains photoreceptors: rods (sensitive to low light) and cones (colour vision and visual acuity). Photoreceptors convert light into electrical signals transmitted through bipolar and ganglion cells; axons of ganglion cells form the optic nerve that carries information to the brain where visual perception occurs. The retinal and lens structures change with age and affect near or distant vision.

Ear: hearing and balance
The ear has outer, middle and inner parts. Sound waves enter the external ear and vibrate the tympanic membrane. Vibrations are amplified by three small bones (ossicles) in the middle ear and transmitted to the fluid-filled cochlea in the inner ear. Hair cells in the cochlea convert mechanical vibrations into nerve impulses transmitted by the auditory nerve. The vestibular apparatus in the inner ear, including semicircular canals, detects head movement and position, helping maintain balance and posture.

Taste and smell
Taste receptors in taste buds detect basic taste qualities: sweet, sour, salty, bitter and umami. Olfactory receptors in the nasal mucosa detect volatile chemicals; signals travel via the olfactory nerve to the brain where smell is identified. Taste and smell together influence appetite and food preference and are linked to memory and emotion.

Skin and somatosensation
The skin is the largest organ and provides protection, sensation, temperature regulation and vitamin D synthesis. It contains specialised receptors for touch, pressure, vibration, temperature and pain. Sweat glands and blood vessel adjustments help regulate body temperature. Skin integrity is crucial for preventing infection and conserving fluids.

Care of sense organs
Good hygiene, protective measures (sunglasses, ear protection), routine checks and avoiding harmful substances (tobacco, excessive noise) help preserve sensory function. Early detection of changes in vision, hearing or skin condition allows timely treatment and better outcomes.

Role and comparison with nervous system
The endocrine system uses hormones—chemical messengers secreted into the bloodstream by glands—to regulate growth, metabolism, reproduction and internal balance. Unlike the fast electrical signals of the nervous system, hormones act more slowly but often produce longer-lasting effects. The two systems interact: the hypothalamus and pituitary link neural and endocrine control.

Major endocrine glands and principal hormones
Important glands include the pituitary (growth hormone, TSH, ACTH, LH, FSH, ADH, oxytocin), thyroid (thyroxine regulating metabolic rate), parathyroids (parathyroid hormone regulating calcium), adrenal glands (adrenaline, noradrenaline, cortisol for stress response), pancreas (insulin and glucagon for blood glucose control), and gonads (testes produce testosterone; ovaries produce oestrogen and progesterone). Each hormone acts on specific target tissues to elicit physiological responses.

Mechanism of hormone action
Hormones reach target cells via blood and bind to specific receptors. Water-soluble hormones (peptides) bind to cell-surface receptors triggering intracellular signalling cascades, while lipid-soluble hormones (steroids) often enter cells and bind intracellular receptors to modify gene expression. The presence and number of receptors determine cell sensitivity to a hormone.

Feedback control
Hormonal systems commonly use negative feedback to maintain stable conditions. For example, the hypothalamus releases a releasing hormone that stimulates the pituitary to secrete a stimulating hormone; this prompts an endocrine gland to produce its hormone. When the blood level of the final hormone is sufficient, it inhibits upstream releases, reducing its further production. This keeps hormone levels within an optimal range. Positive feedback exists in limited contexts such as oxytocin-mediated uterine contractions during childbirth where the response amplifies until delivery.

Clinical relevance
Endocrine disorders include diabetes mellitus (insulin deficiency or resistance causing high blood sugar), hypothyroidism (low thyroid hormone causing low metabolic rate) and hyperthyroidism (excess causing weight loss and overactivity). Hormone therapies and lifestyle management help treat many endocrine conditions. Understanding how hormones control bodily functions allows students to appreciate the systemic effects of endocrine imbalances and the importance of early diagnosis.

Functions of the skeleton
The skeleton supports body shape, protects internal organs, provides anchor points for muscles to produce movement, stores minerals (especially calcium and phosphorus) and contains bone marrow that produces blood cells. The adult human skeleton has about 206 bones of various shapes adapted to their functions: long bones for weight-bearing and movement, flat bones for protection, short bones for support and irregular bones for specialised roles.

Bone structure and tissue
Bone is living connective tissue composed of cells embedded in a mineralised matrix. The organic part of the matrix is largely collagen which gives flexibility and tensile strength; the inorganic part is mainly calcium phosphate crystals (hydroxyapatite) that provide compressive strength. Key bone cells include osteoblasts (which synthesise matrix and build bone), osteocytes (mature bone cells that maintain tissue) and osteoclasts (large cells that resorb bone). Compact bone forms the dense outer layer that resists bending, while spongy (cancellous) bone inside has a trabecular network that reduces weight while providing strength along lines of stress. Bone marrow housed in cavities is of two types: red marrow produces blood cells and yellow marrow stores fat.

Microscopic organisation and growth
At the microscopic level, compact bone is organised into osteons (Haversian systems), concentric rings of lamellae around a central canal containing blood vessels and nerves. During growth, long bones lengthen at epiphyseal plates—regions of cartilage that ossify progressively. Bone remodelling continues throughout life: osteoblasts and osteoclasts work together to replace old bone, repair microdamage and adjust bone architecture in response to mechanical stresses (Wolff's law). Hormones such as growth hormone, thyroid hormone, parathyroid hormone and sex steroids regulate growth and remodelling; vitamin D and dietary calcium are essential for mineralisation.

Joints: types and features
Joints are points of contact between bones and are classified by structure and mobility. Fibrous joints (e.g., skull sutures) are immovable; cartilaginous joints (e.g., intervertebral discs) allow slight movement; synovial joints are freely movable and include hinge (elbow), ball-and-socket (hip, shoulder), pivot (neck), saddle and plane joints. Synovial joints have articular cartilage covering bone ends, a synovial cavity filled with lubricating synovial fluid, a fibrous capsule and supporting ligaments. Tendons connect muscle to bone, transmitting force to produce movement; bursae reduce friction in some joints.

Common disorders and care
Bone and joint disorders include fractures, dislocations, osteoarthritis (degeneration of joint cartilage), rheumatoid arthritis (autoimmune inflammation), osteoporosis (loss of bone density), and developmental conditions like scoliosis. Prevention and care involve adequate calcium and vitamin D intake, regular weight-bearing exercise to stimulate bone strength, good posture, protective gear for sports, and prompt treatment of injuries. In clinical settings, fractures are immobilised and sometimes surgically fixed; severe arthritis may require physiotherapy or joint replacement. Understanding bone biology explains why childhood nutrition and exercise have long-term effects on adult skeletal health.

Types of muscle and their roles
Humans have three types of muscle tissue. Skeletal muscle is attached to bones and under voluntary control to move the skeleton and maintain posture. Cardiac muscle is found only in the heart and contracts rhythmically and continuously to pump blood. Smooth muscle lines internal organs and blood vessels and works involuntarily to move substances such as food along the digestive tract and regulate vessel diameter.

Microscopic basis of contraction
Skeletal muscle fibres are long cells containing many myofibrils made of repeating sarcomeres—the basic contractile units. Sarcomeres contain thin (actin) and thick (myosin) filaments. During contraction, myosin heads form cross-bridges with actin and pull actin filaments inward in a process powered by ATP. This sliding of filaments shortens sarcomeres and thereby contracts the whole fibre. Calcium released from internal stores controls the interaction of actin and myosin by exposing binding sites on actin.

Muscle coordination and movement
Muscles work in organised groups. Antagonistic pairs (e.g., biceps and triceps) create opposite movements at a joint. Synergistic muscles assist the prime mover. Motor units—one motor neuron and the muscle fibres it innervates—allow graded strength: increasing the number of active motor units increases force. Precise movements use small motor units; powerful movements use larger ones.

Energy and fatigue
Muscles use ATP for contraction. At rest and during light activity, ATP is mainly produced by aerobic respiration in mitochondria. During high-intensity activity, anaerobic glycolysis provides faster ATP but produces lactic acid, which contributes to fatigue. Training improves muscle endurance and strength by increasing mitochondrial density, capillary supply and muscle fibre size (hypertrophy).

Care and disorders
Regular exercise, balanced protein intake and adequate rest maintain muscle health. Injuries include strains, tears and conditions like muscular dystrophy. Preventive measures include warming up, correct technique and avoiding overtraining. Rehabilitation and physiotherapy help recovery after injury.

Reproductive system structures
The male reproductive system includes testes (which produce sperm and testosterone), the epididymis (sperm storage and maturation), vas deferens (transport), accessory glands (seminal vesicles and prostate which add fluid to semen) and the penis for delivery. The female reproductive system includes ovaries (produce ova and hormones), fallopian tubes (site of fertilisation and transport), uterus (site of implantation and foetal development), cervix and vagina. Both systems have specialised ducts and glands adapted for gamete production, transport and fertilisation.

Gametogenesis and fertilisation
Gametogenesis produces haploid gametes by meiosis: spermatogenesis in testes yields many small motile sperm; oogenesis in ovaries yields usually one large ovum and polar bodies. Ovulation releases an ovum from an ovary into the fallopian tube where fertilisation by a sperm may occur. The fertilised egg (zygote) undergoes cell divisions and becomes a blastocyst that implants in the uterine lining to develop into an embryo and later a foetus.

Puberty and hormonal control
Puberty is the period when reproductive organs mature under hormonal control. The hypothalamus increases release of gonadotropin-releasing hormone (GnRH), stimulating the pituitary to secrete luteinising hormone (LH) and follicle-stimulating hormone (FSH). In boys, LH and FSH stimulate testes to produce testosterone and sperm; testosterone causes growth of facial and body hair, deepening of voice and increase in muscle mass. In girls, oestrogen and progesterone from ovaries cause breast development, widening of hips and onset of menstrual cycles. Puberty also involves psychosocial changes and rapid growth in height.

Menstrual cycle and contraception
The menstrual cycle is a monthly ovarian and uterine cycle preparing the body for pregnancy. The follicular phase culminates in ovulation, followed by the luteal phase where the corpus luteum secretes hormones to prepare the uterus. If fertilisation does not occur, the uterine lining sheds as menstruation. Contraceptive methods (barrier methods, hormonal pills, intrauterine devices, sterilisation) prevent pregnancy by inhibiting fertilisation or implantation; some also reduce transmission of sexually transmitted infections (STIs).

Sexual health and responsibilities
Reproductive health includes preventing STIs through safe practices, access to contraception, ante-natal care and education about consent and relationships. Early medical help for irregularities, infections or pain protects long-term fertility and wellbeing. Respectful education helps adolescents make informed choices during puberty.

Meaning and importance
Homeostasis is the process by which the internal environment of the body is kept within narrow limits necessary for cell function. Parameters such as temperature, blood glucose, water balance, blood pressure and pH are tightly regulated. Maintaining homeostasis ensures enzymes and cellular processes operate efficiently and the organism survives changes in the external environment.

Components of a homeostatic loop
A typical homeostatic control has a receptor that detects change, a control centre (often the brain or an endocrine gland) that processes information and issues commands, effectors (muscles, glands, organs) that enact changes, and feedback signals that inform the control centre of the result. Negative feedback is the common mechanism where the response reduces the original change, stabilising the system. Positive feedback amplifies a response in specific circumstances, such as during childbirth.

Examples of homeostatic mechanisms
Thermoregulation: when body temperature rises, thermoreceptors stimulate the hypothalamus to cause sweating and vasodilation of skin vessels to increase heat loss; when temperature falls, shivering and vasoconstriction conserve heat. Blood glucose regulation: after a meal blood glucose rises, triggering insulin release from the pancreas which promotes glucose uptake and storage; between meals, low glucose triggers glucagon release which stimulates glycogen breakdown to raise glucose. Water balance: osmoreceptors detect plasma osmolarity; high osmolarity stimulates thirst and ADH release to increase water reabsorption in kidneys, conserving water.

Failure of homeostasis and disease
When homeostatic controls fail, illnesses occur. Diabetes mellitus is an example where glucose regulation fails, causing persistently high blood sugar that damages organs. Dehydration, electrolyte imbalances, heat stroke and shock are other consequences of failed homeostasis. Early detection and interventions such as medication, fluid therapy or lifestyle changes aim to restore balance.

Practical approach
To analyse a homeostatic example, identify the variable, receptor, control centre, effectors and feedback type. Understanding these elements helps explain how the body resists disturbances and why treatments target particular steps in the loop.