The heart serves as a central pump organ. Because the heart muscles contract and relax rhythmically the heart is able to circulate the blood through the whole body. The contraction of any part of the heart is referred to as systole while the relaxation is called diastole. In a normal healthy person the heart beats at an average of 70 to 80 times per minute. The duration of one cycle is approximately 0.8 of a second. During one cycle a whole series of changes take place in the heart, which are collectively referred to as the cardiac cycle. Thus, the succession of events during each complete heartbeat is known as the heart cycle or cardiac cycle. The cardiac cycle may be represented as follows:
The two atria contract simultaneously for about 0.1 of a second. The openings from the two venae cavae, the coronary vein and from the four pulmonary veins are closed by the contraction of the atria and the blood remaining in the atria is pumped into the ventricles through the atrio-ventricular openings.
The two ventricles contract immediately thereafter for about 0.3 of a second. The tricuspid and bicuspid valves are closed which prevents the blood from flowing back into the atria. The blood is pumped from the right ventricle into the pulmonary artery and from the left ventricle into the aorta. During the ventricular systole the atria are in diastole.
Immediately hereafter both ventricles relax. Bothe atria and ventricles are now relaxed and in a of general diastole which lasts for about 0.4 of a second. Because of a general decrease in pressure in the ventricles there is a tendency for the blood to flow back from the pulmonary artery and the aorta into the ventricles. This is however prevented by the three semi-lunar valves in each of these blood vessels. During general diastole the blood flows from the two venae cavae and four pulmonary veins into the two atria and from them directly into the... [continues]
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