CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 18

(a) The transport (circulatory) system of human beings has these components: (i) Heart: a muscular pump with four chambers. Its right side (right atrium and right ventricle) receives deoxygenated blood from the body and pumps it to the lungs; its left side (left atrium and left ventricle) receives oxygenated blood from the lungs and pumps it to the whole body. (ii) Blood vessels: arteries carry blood away from the heart to the organs (thick, elastic walls for the high pressure); veins collect blood from the organs and bring it back to the heart (valves keep it flowing one way); capillaries, with walls one cell thick, allow materials to be exchanged between the blood and the surrounding cells. (iii) Blood: a fluid connective tissue. Plasma transports food, carbon dioxide and nitrogenous wastes in dissolved form; red blood cells carry oxygen (with haemoglobin); white blood cells fight infection; platelets help in clotting. (iv) Lymph: a fluid that drains the excess tissue fluid back into the blood and carries digested and absorbed fat from the intestine. (b) When a blood vessel develops a leak, the platelets circulating in the blood gather at the point of injury and plug the leak by helping the blood to clot there. The platelets and the injured tissue release clotting factors; the plasma protein prothrombin is changed into thrombin (calcium ions are needed), and thrombin changes the soluble plasma protein fibrinogen into insoluble fibrin threads. The fibrin threads form a mesh that traps blood cells, forming a clot that seals the leak, stops the loss of blood and helps to maintain the blood pressure.