Biology AS · Transport in mammals. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The heart has four chambers. The right side pumps deoxygenated blood to the lungs and the left side pumps oxygenated blood to the body. The septum keeps the two sides apart. Atrioventricular valves lie between each atrium and ventricle, and semilunar valves guard the aorta and the pulmonary artery.
In one cardiac cycle the atria contract (atrial systole), then the ventricles contract (ventricular systole), then all chambers relax (diastole). Valves open and close only because of pressure differences across them.
Heart muscle is myogenic: it contracts without a nerve impulse. Each beat starts at the sinoatrial node in the wall of the right atrium.
Saying that valves open because muscles pull them or because the heart signals them. They are passive flaps moved only by the difference in blood pressure on their two sides.
Answer: 0.4 s in each cycle.
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