Biology
Circulation
3,945 words, 10 sections
Circulation is unit Biology 11 of the official uniform curriculum: the heart, the phases of the heartbeat, the differences between arteries, veins and capillaries, and the lymphatic system that recovers what the capillaries leak. It has been asked 249 times across 79 official papers between 2001 and 2026, which makes it one of the most sampled stretches of the 81 marks Biology carries. The load inside it is uneven: heart anatomy and the cardiac cycle 94 distinct questions, blood composition 67, blood vessels and haemodynamics 61, lymphatic 27. Mean item difficulty is 1.34, a warning rather than a comfort, since these are recall and one step items lost to the plausible chamber.
The heart is a cone between the lungs, enclosed in a pericardium whose two serous layers are separated by a lubricating fluid. Its wall has three layers: the epicardium outside, the muscular myocardium, and the endocardium, innermost, which forms the valves. Sindh alone calls the right ventricle's artery the pulmonary aorta.
Inside are four chambers, thin walled atria above thick walled ventricles. The venae cavae enter the right atrium, four pulmonary veins open into the left atrium, and one artery leaves each ventricle. Four valves keep the flow one way: tricuspid right, bicuspid or mitral left, pulmonary and aortic semilunar at the exits. Chordae tendinae anchor the atrioventricular valves to papillary muscles inside the ventricles, so they are ventricular structures, and atria are thin walled because they push blood only into chambers lying just below.
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