Transport: blood, blood vessels and the heart
What this domain tests
Blood composition and function, the structure-function link for arteries, veins and capillaries, the heart's chambers and valves, the cardiac cycle and the double circulation. Labelling the heart and tracing blood flow are frequent tasks, along with explaining why the left ventricle is thickest.
Core rules
- Plasma carries dissolved glucose, amino acids, salts, hormones, urea and plasma proteins, and distributes heat. Red cells are biconcave, lack a nucleus in humans, are packed with haemoglobin and number about 5 million per cubic millimetre. Phagocytes engulf pathogens; lymphocytes make antibodies; platelets trigger clotting.
- Arteries carry blood away from the heart: thick muscular elastic walls, narrow lumen, no valves, high pressure and a pulse. Veins carry blood back: thin walls, wide lumen, valves, low pressure, helped by skeletal muscle. Capillaries are one cell thick for rapid exchange.
- The heart has two atria and two ventricles separated by a septum. The left ventricle wall is thickest because it pumps blood to the whole body.
- Circulation is double: pulmonary (right ventricle to lungs to left atrium) and systemic (left ventricle to body to right atrium). Blood passes through the heart twice per circuit.
- The tricuspid, bicuspid and semilunar valves prevent backflow. The sinoatrial node sets the rhythm.
Worked example
Trace a red blood cell from the right atrium to the kidney. Right atrium, tricuspid valve, right ventricle, pulmonary valve, pulmonary artery, lungs where it gains oxygen, pulmonary veins, left atrium, bicuspid valve, left ventricle, aortic valve, aorta, renal artery, kidney. It passes through the heart twice, which is what double circulation means. Note that the pulmonary artery is the only artery carrying deoxygenated blood.
Traps that cost marks
Assuming every artery carries oxygenated blood and every vein deoxygenated blood. Saying the right ventricle has the thickest wall. Confusing the pulmonary vein with the vena cava. Writing that valves pump blood rather than prevent backflow.