Sec 3 Biology Transport in Humans notes

Humans run a double circulation: the right heart sends blood through the lungs to pick up oxygen, and the left heart sends it around the body. That gives you four great vessels to know cold: the aorta (left ventricle → body), the vena cava (body → right atrium), the pulmonary artery (right ventricle → lungs, deoxygenated) and the pulmonary vein (lungs → left atrium, oxygenated).

The organ circuits hang off the aorta: the hepatic artery feeds the liver (and the hepatic vein drains its processed blood to the vena cava), while the renal artery feeds the kidneys (and the renal vein returns their cleaned blood). Remember the portal-vein exception from chapter 4: gut-to-liver traffic runs in the hepatic portal vein.

Each feature is a pressure story. Arterial walls must survive the heart’s surges, so they are muscular and elastic — you feel that recoil as your pulse. Venous blood travels gently, so veins use valves and the squeeze of surrounding muscles to keep it moving forward. Capillaries abandon strength entirely: a wall of single flattened cells is the shortest possible crossing for oxygen and glucose.

At the arterial end of a capillary, blood pressure squeezes plasma — but not the blood cells or large proteins — through the leaky capillary wall. The fluid that surrounds the cells is tissue fluid. Cells draw oxygen and dissolved nutrients from it, and load their wastes (carbon dioxide, urea) into it in exchange.

At the venous end, pressure has fallen and the concentrated blood reabsorbs most of the tissue fluid. The remainder drains into lymph vessels, which return it to the bloodstream near the heart — the same lymph system that carries the lacteal’s fat cargo from the villi.

Red blood cells transport oxygen on the pigment haemoglobin, which binds oxygen in the lungs and releases it where concentrations are low (chapter 1 covered their no-nucleus, biconcave design). Plasma, the straw-coloured liquid, carries everything dissolved: digested food, carbon dioxide, urea, hormones, ions and plasma proteins — the transport arm of the blood.

White blood cells are the defence crew in three roles: some phagocytose (engulf and digest) bacteria; others make antibodies against specific pathogens; and the same machinery underlies tissue rejection, attacking cells recognised as foreign — the reason transplants need matching and immunosuppression. Platelets — cell fragments — patch wounds: damaged tissue triggers them to convert soluble fibrinogen into insoluble fibrin threads, whose mesh traps cells as a clot.

The heart is two pumps in one muscle block. Deoxygenated blood enters the right atrium, drops into the right ventricle and leaves for the lungs; oxygenated blood returns to the left atrium, drops into the left ventricle and leaves by the aorta. Valves between atrium and ventricle, and between ventricles and arteries, snap shut against any backward flow — the heartbeat’s "lub-dub" is their slamming.

The cardiac cycle runs: systole — the muscle contracts, atria first to top up the ventricles, then ventricles to fire blood into the arteries — followed by diastole — relaxation, when the chambers refill. The left ventricle’s wall is the thickest because it drives blood the whole way round the body; the right ventricle only presses blood to the nearby lungs.

The four ABO groups (A, B, AB, O) differ in the markers on red cells and the antibodies floating in plasma. Give the wrong group and the recipient’s antibodies agglutinate the donor red cells — a lethal clumping. The safe rules: O has no A or B markers, so it donates to everyone (universal donor); AB carries neither anti-A nor anti-B antibodies, so it receives from everyone (universal recipient). Same-group transfusion is always safest.

Coronary heart disease begins when fatty plaques build up inside the coronary arteries — the vessels feeding the heart muscle itself. Occluded coronaries starve the muscle of oxygen, causing chest pain or a heart attack. The causes are the modern lifestyle set: a diet high in saturated fat, a sedentary lifestyle, and smoking. Prevention mirrors them: a balanced low-fat diet, regular exercise, and not smoking — plus managing stress and weight.