Sec 3 Combined Science (Biology) Transport in Humans notes

Four organ pairings carry the marks. Heart: the aorta leaves the heart carrying oxygenated blood to the whole body; the vena cava returns blood from the body to the heart. Lungs: the pulmonary artery carries blood from the heart to the lungs (the one artery carrying deoxygenated blood); the pulmonary vein returns freshly oxygenated blood to the heart (the one vein carrying oxygen-rich blood). Liver: blood arrives by the hepatic artery (oxygen for the liver's own cells) and by the hepatic portal vein (nutrient-rich blood from the gut), and leaves by the hepatic vein. Kidneys: the renal artery brings blood to be cleaned; the renal vein carries it away.

Arteries carry blood away from the heart at high pressure, so their walls are thick, muscular and elastic — built to stretch with each pulse of pressure and recoil smoothly. Their lumen (opening) is relatively narrow. Veins carry blood back at low pressure, so their walls are thin and less muscular; because low pressure cannot always push blood uphill, veins contain valves that shut against backflow, and body-muscle movement squeezes blood along. Capillaries are the exchange points: their walls are one cell thick, so oxygen, carbon dioxide, nutrients and wastes diffuse across with almost no barrier. Capillaries form networks threading every tissue, giving every cell a trading post within a fraction of a millimetre.

The structure–function logic is the examinable core: high pressure → strong walls; low pressure → valves; exchange → minimal wall. Any "explain why" answer should connect the feature to the pressure or the exchange job.

Plasma is the pale yellow liquid carrying everything dissolved: digested food (glucose, amino acids), waste products (like urea on its way to the kidneys), hormones and carbon dioxide. Red blood cells are the oxygen fleet: tiny biconcave discs with no nucleus, packed with the red pigment haemoglobin, which binds oxygen in the lungs and releases it where respiration is running. White blood cells are the defence squad — much rarer than red cells, with nuclei, doing three named jobs: some engulf and digest microbes; some produce antibodies that target specific pathogens; and some attack tissue from transplanted organs, which is why rejection happens. Platelets are cell fragments that patch damage: when a vessel is cut they help convert the soluble protein fibrinogen into fibrin, whose threads mesh into a clot.

The heart is a fist-sized bag of cardiac muscle that contracts rhythmically and never tires. Chambers fill with blood, then the muscle contracts, squeezing blood out — atria first, ventricles after — so each beat pushes blood into the arteries. Valves sit between chambers and at the exits: they open to let blood flow forward and slam shut the instant pressure tries to push it backwards. The result is strictly one-way traffic: body → heart → lungs → heart → body.

A resting heart beats about 70 times per minute — over 4200 beats per hour, roughly 100 000 beats a day, every one powered by contraction and protected by valves. The valves' shutting is the sound of the heartbeat; a valve that leaks lets blood slop backwards and the pump loses efficiency.

The heart muscle is greedy — it needs its own supply of oxygen and nutrients, delivered by the coronary arteries branching off the aorta. Coronary heart disease occurs when these arteries are occluded — narrowed or blocked by fatty deposits building up in their walls. Starved of oxygen, the heart muscle works badly or dies in the region fed by the blocked vessel: chest pain, and in severe blockage, a heart attack.

The main causes are lifestyle-linked: a diet high in fats (especially saturated fat, which feeds the deposits), a sedentary lifestyle with little exercise, and smoking, which damages vessel walls. The same list reversed is the prevention plan: a balanced, low-fat diet, regular exercise, and not smoking. Because these arteries are narrow to begin with, small deposits matter enormously — which is why prevention starts decades before symptoms.