Transport and Circulation
What you must understand
In mammals, blood is carried in a double circulation: blood passes through the heart twice per circuit. The right side pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood to the body. Know the chambers (atria, ventricles), valves and the main vessels: aorta, vena cava, pulmonary artery and vein.
Learn the structure and function of blood: red cells (haemoglobin carries oxygen), white cells (defence), platelets (clotting) and plasma (transports dissolved substances). Relate vessel structure to function: arteries have thick elastic walls and narrow lumens and carry blood away from the heart under high pressure; veins have thin walls and valves to prevent backflow; capillaries are one cell thick for exchange.
In plants, xylem carries water and minerals upward and is dead, hollow and lignified; phloem carries sugars in either direction and is living. Transpiration is the loss of water vapour from leaves and is affected by light, temperature, humidity and wind.
Traps examiners set
- Saying arteries always carry oxygenated blood. The pulmonary artery carries deoxygenated blood.
- Confusing the functions of xylem and phloem.
- Forgetting that valves in veins prevent backflow, and that the heart's valves stop backflow within the heart.
- Describing transpiration as "breathing" or as the plant losing food.
Worked example
A student says the left ventricle has the thickest wall because it holds the most blood. Correct answer: it has the thickest wall because it pumps blood at high pressure to the whole body, not just to the lungs.
Rule of thumb: thicker wall means higher pressure, and every structure answer should name the function it serves.