Текст the most important muscle in the body is the heart. without the heart and its circulatory system human life would not be possible. it contracts at an average rate of 72 beats per minute. these rhythmical contractions called the pulse rate can be felt in the radial artery or the wrist. the human heart consists of four chambers, two atria and two ventricles. each is made up of several layers of cardiac muscle arranged in circles and spirals. scientists have established that there are four valves in the heart: two in the left chamber and two in the right one. during the contraction phase, called the systole, oxygenated blood is pumped out of the left ventricle into the aorta. the blood carrying carbon dioxide is passed back to the right atrium through the veins during the diastole. from there it is pumped into the right ventricle and to the pulmonary artery to be sent to the lungs. the rest of the system consists of arterioles, venules and capillaries, the smallest of blood vessels.

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