Human Body Systems and Health
Systems overview
Examinable systems include digestion, circulation, respiration, excretion, nervous control, and the skeleton and muscles. Each system has organs with specialised structures that suit their function. The digestive system breaks food into small soluble molecules: mechanical digestion in the mouth, chemical digestion by enzymes (amylase in saliva for starch, protease in the stomach for protein, lipase in the small intestine for fat). The small intestine has villi with thin walls and a rich blood supply for absorption; the large intestine reabsorbs water.
Respiration and circulation
The lungs contain alveoli with thin walls, large surface area, and moist surfaces for gas exchange. Breathing (ventilation) moves air; respiration is a chemical reaction in cells releasing energy from glucose. Smoking damages cilia and alveoli, reducing gas exchange. The heart has four chambers; the left ventricle has the thickest muscular wall because it pumps blood around the whole body.
Health and disease
Health is physical and mental wellbeing, not just absence of disease. Pathogens (bacteria, viruses, fungi, protozoa) cause communicable disease; lifestyle factors (diet, exercise, smoking, alcohol) cause non-communicable disease. The body defends itself with skin, mucus, cilia, stomach acid, and white blood cells (phagocytes engulf, lymphocytes make antibodies). Vaccination provides immunity by stimulating antibody production. Antibiotics work on bacteria, not viruses - a common exam trap.
Worked example
A patient with a blocked coronary artery has reduced blood flow to heart muscle, causing chest pain and possible heart attack. Stents or bypass surgery restore flow; lifestyle changes reduce risk. Note that the cause is the blocked vessel, not a lack of oxygen in the air.