IGCSE Biology Transport in animals notes

The circulatory system is a system of blood vessels with a pump (the heart) and valves to ensure one-way flow. Blood is pumped away from the heart in arteries and returns in veins — artery and vein names follow direction relative to the heart, not the oxygen content.

Supplement: a fish has single circulation: heart → gills → body → heart, the heart pumping once per circuit, blood losing pressure in the gills. A mammal has double circulation: the right side of the heart pumps blood to the lungs (pulmonary circulation), and the left side pumps it to the body (systemic circulation) — the blood passes through the heart twice per complete circuit.

The advantage of double circulation: each loop has its own fresh pump. Blood returning from the lungs — fully oxygenated — is boosted at full pressure around the body, so delivery to respiring tissues is fast. In single circulation the pressure drops in the gills and stays low for the whole body trip.

A labelled diagram to picture: the heart in section, four chambers — left and right atria on top, left and right ventricles below — separated down the middle by the muscular septum. One-way valves sit between each atrium and ventricle, and at the exits into the arteries. Coronary arteries branch from the aorta across the heart’s own surface, feeding the heart muscle itself. Structures to identify at Core: muscular wall, septum, both ventricles, both atria, one-way valves, coronary arteries.

Supplement: the atrioventricular valves (between atria and ventricles) and semilunar valves (in the aorta and pulmonary artery) all prevent backflow. Wall thickness matches workload: the left ventricle’s wall is the thickest because it pumps blood at high pressure around the whole body; the right ventricle only sends blood a short distance to the lungs, so its wall is thinner; atria are thinnest of all, only pushing blood a few centimetres down into the ventricles. The septum matters because it keeps the oxygenated left side fully oxygenated — if blood mixed, delivery of oxygen to tissues would fall.

Supplement — the cardiac cycle: the atria contract, squeezing blood down into the relaxed ventricles (atrioventricular valves open); then the ventricles contract, the atrioventricular valves slam shut (first heart sound) and blood is forced out into the aorta and pulmonary artery past the semilunar valves; then the heart muscle relaxes, semilunar valves close to stop backflow, and the chambers refill. One cycle = one heartbeat. Heart activity is monitored by ECG (electrical traces), pulse rate (an artery bulging with each beat) and listening to the valve sounds.

Coronary heart disease is the blockage of the coronary arteries, usually by fatty deposits, which starves the heart muscle of oxygen and glucose — causing chest pain or a heart attack. Risk factors: a fatty/high-salt diet, lack of exercise, stress, smoking, genetic predisposition, age and sex. Diet and exercise reduce the risk: eating less saturated fat and salt slows the deposits, and regular aerobic exercise strengthens the heart, lowers resting pulse and helps maintain a healthy mass.

Supplement: the structure matches the pressure. Arteries carry blood at high, pulsing pressure from the heart, so they need thick, strong, elastic walls that stretch and recoil — which is why an artery bulges with every beat (the pulse). Veins carry blood at low pressure back to the heart, so their walls are thin; blood would flow backwards without valves, which is why veins have them. Capillaries are the exchange points: a wall one cell thick (short diffusion distance), a lumen so narrow that red cells pass in single file (bringing them close to every tissue cell), and leaky walls allowing tissue fluid to form.

Vessels to know by name (Core): vena cava, aorta, pulmonary artery, pulmonary vein (heart); pulmonary artery and pulmonary vein (lungs); renal artery and renal vein (kidney). Supplement adds the liver trio: hepatic artery (oxygenated blood to the liver), hepatic veins (blood back to the vena cava) and the hepatic portal vein — unusual, because it carries blood from the gut to the liver between two organs, loaded with absorbed nutrients.

In a photomicrograph, red cells outnumber everything and show as plain pink discs with no nucleus; white cells are the scarce cells with dark-staining nuclei. Supplement: among white cells, lymphocytes have a large, round nucleus and make antibodies — proteins with complementary shapes that bind specific pathogens (Topic 9); phagocytes have lobed nuclei and engulf pathogens by phagocytosis, wrapping around and digesting them.

Clotting prevents blood loss and stops pathogens entering through wounds. Supplement: when a vessel is damaged, platelets and damaged tissue trigger a cascade that converts the soluble plasma protein fibrinogen into insoluble fibrin. Fibrin forms a mesh of fibres across the wound, trapping red cells and platelets; the mesh dries into a scab — a seal that is both a plug and a barrier to pathogens.