Describe the cardiac cycle.

One cardiac cycle, consisting of contraction (systole) and relaxation (diastole) of the heart, lasts 0.8 s (three phases). Of this time, 0.1 s is the contraction (systole) of the atria (phase I), 0.3 s is the contraction (systole) of the ventricles (phase II), and 0.4 s is the total relaxation (diastole) of the whole heart – the general pause (III phase). With each contraction of the atria, blood from them passes into the ventricles, after which the ventricles begin to contract. At the end of the atrial contraction, the leaflet valves close, and when the ventricles contract, blood cannot return to the atria. It is pushed through the open semilunar valves from the left ventricle (along the aorta) into the large circle, and from the right (along the pulmonary artery) into the pulmonary circulation. Then the ventricles relax, the semilunar valves close and prevent blood from flowing back from the aorta and pulmonary artery into the ventricles of the heart.

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