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The heart has an increasing rhythmic activity. It pumps blood by its contraction and relaxation. The contraction of the heart is called systole and the relaxation is called diastole. The contraction and relaxation together constitute the heart beat. The heart beats at the rate of 72 beats per minute. The changes that occur in the heart during the beat one is repeated in the same order in the next beat. This cyclical repetition is called cardiac cycle. Throughout the cardiac cycle, blood pressure increases and decreases. The cardiac cycle is coordinated by a series of electrical impulses that are produced by specialized heart cells found within the sinoatrial node and the atrioventricular node. The cardiac muscle is composed of myocytes which initiate their own contraction without the help of external nerves (with the exception of modifying the heart rate due to metabolic demand). Under normal circumstances, each cycle takes 0.8 seconds.