Human Physiology
Transport and exchange
You must understand the circulatory system: the heart as a double pump, arteries carrying blood away from the heart, veins returning it, and capillaries allowing exchange with tissues. Blood transports oxygen, nutrients, carbon dioxide, hormones and heat. Learn the components of blood and their roles, including red cells for oxygen transport and white cells for defence.
Digestion and absorption
Digestion breaks large insoluble molecules into small soluble ones. Know the enzymes and their substrates: amylase acts on starch, protease on protein, lipase on fat. Absorption occurs in the small intestine, whose villi increase surface area. A classic trap is mixing up the sites of digestion or forgetting that bile emulsifies fat but is not an enzyme.
Gas exchange and breathing
The alveoli provide a large, moist, thin surface with a good blood supply for gas exchange. Breathing moves air in and out; gas exchange diffuses oxygen into the blood and carbon dioxide out. Do not say the lungs "make" oxygen or that breathing is respiration.
Excretion and control
Kidneys filter blood, reabsorbing useful substances such as glucose and water while excreting urea. The skin and lungs are also excretory organs. Homeostasis keeps internal conditions steady, for example body temperature and blood glucose.
Worked example
A question asks why an athlete's heart rate rises during exercise. Chain the reasoning: muscles respire faster, needing more oxygen and producing more carbon dioxide; the heart pumps faster to deliver oxygen and remove carbon dioxide. Always connect the change to its cause and effect.