Last reviewed 30 Sept 2026 · 11 min read
Overview
The human body is organised as cells → tissues → organs → organ systems. Each system does a specific job, but all work together to maintain homeostasis (a steady internal environment).
Digestive system
Digestion is the breakdown of complex food into simple, absorbable substances.
Alimentary canal: mouth → pharynx → oesophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine (colon, rectum) → anus. Associated glands: salivary glands, liver, pancreas.
| Region | Action and enzymes |
|---|---|
| Mouth | teeth grind food; saliva has the enzyme salivary amylase (ptyalin), which starts digestion of starch into maltose; pH about 6.8 |
| Oesophagus | peristalsis moves the food down |
| Stomach | gastric juice: hydrochloric acid (kills bacteria, makes the medium acidic for pepsin), pepsin (protein → peptides), rennin (curdles milk in infants), and mucus (protects the wall); food becomes chyme |
| Liver (largest gland; ~1.5 kg) | secretes bile (stored in the gall bladder) — emulsifies fats and neutralises acid; also stores glycogen, makes urea, detoxifies, makes plasma proteins |
| Pancreas | pancreatic juice: trypsin (proteins), amylase (starch), lipase (fats); also secretes insulin and glucagon (an endocrine function) |
| Small intestine (about 6–7 m long) | final digestion by intestinal juice (succus entericus); absorption by villi (finger-like projections that increase the surface area) |
| Large intestine | absorbs water and salts; forms faeces |
- Carbohydrates → glucose; proteins → amino acids; fats → fatty acids and glycerol.
- The appendix is a small vestigial organ at the junction of the small and large intestines.
- Enzymes: biological catalysts (proteins) that speed up reactions; work at specific pH and temperature (pepsin: acidic; trypsin: alkaline).
- Ptyalin — mouth; pepsin — stomach; trypsin, lipase, amylase — pancreas; lactase — small intestine (lactose intolerance).
Respiratory system
- Path of air: nose → pharynx → larynx (voice box; contains the vocal cords) → trachea (windpipe, supported by C-shaped cartilage rings) → bronchi → bronchioles → alveoli (tiny air sacs; exchange of gases takes place here; about 300–500 million in the lungs).
- The diaphragm and intercostal muscles help breathing. Inhalation: the diaphragm contracts and moves down, ribs go up and out; exhalation is the reverse.
- Breathing rate: about 12–20 times per minute at rest. Oxygen is carried by haemoglobin; most carbon dioxide is carried as bicarbonate in plasma.
- Air composition: inhaled — about 21 % O₂, 0.04 % CO₂; exhaled — about 16 % O₂ and about 4 % CO₂.
- Respiration at the cellular level (cellular respiration): occurs in mitochondria. Anaerobic respiration (without oxygen) gives lactic acid in muscles (cramps) and ethanol + CO₂ in yeast.
- Diseases: asthma (narrowing of bronchioles), pneumonia, tuberculosis (TB), bronchitis, emphysema, silicosis and asbestosis (occupational).
Circulatory system
- Heart: a muscular organ, about the size of a fist, located slightly left in the chest; weighs about 300 g; has four chambers — two atria (upper, receive blood) and two ventricles (lower, pump blood). The left ventricle has the thickest wall (pumps blood to the whole body). The right side carries deoxygenated blood to the lungs; the left side carries oxygenated blood to the body. Valves prevent backflow. Normal heart rate is about 72 beats per minute at rest; normal blood pressure is about 120/80 mm Hg (systolic/diastolic), measured with a sphygmomanometer. The natural pacemaker is the sino-atrial (SA) node. ECG records the heart's electrical activity.
- Double circulation: blood passes twice through the heart in one full circuit (pulmonary circulation: heart → lungs → heart; systemic circulation: heart → body → heart).
- Blood vessels: arteries (carry blood away from the heart; thick, elastic walls; the pulmonary artery carries deoxygenated blood), veins (carry blood to the heart; thin walls, valves; the pulmonary vein carries oxygenated blood), capillaries (thin; exchange of materials).
- Blood (about 5–6 litres in an adult; 7–8 % of body weight):
| Component | Function |
|---|---|
| Plasma (about 55 %, mostly water) | carries nutrients, hormones, wastes, proteins (albumin, fibrinogen) |
| Red blood cells (RBCs / erythrocytes) | contain haemoglobin (an iron-containing protein) to carry oxygen; no nucleus in mammals; made in the bone marrow; life about 120 days; destroyed in the spleen (the "graveyard of RBCs"); about 5 million per mm³ |
| White blood cells (WBCs / leucocytes) | fight infection (immunity); about 4,000–11,000 per mm³ |
| Platelets (thrombocytes) | blood clotting; about 1.5–4 lakh per mm³ |
- Blood groups (ABO system, Karl Landsteiner, 1901): A, B, AB, O. O is the universal donor, AB is the universal recipient. The Rh factor (Rhesus) is present (Rh⁺) or absent (Rh⁻). Rh incompatibility can affect a second pregnancy (a mother Rh⁻ and a foetus Rh⁺).
- Haemoglobin low → anaemia (iron deficiency). Normal Hb: about 13–17 g/dL in men, 12–15 g/dL in women. Haemophilia (blood does not clot; a sex-linked genetic disorder).
- Lymphatic system: lymph, lymph nodes, spleen, and thymus; helps in immunity and returns fluid to the blood.
- The spleen is the largest lymphatic organ. Heart attack (myocardial infarction) results from a blocked coronary artery; atherosclerosis is the hardening/blocking of arteries by cholesterol.