Essay on Main Phases through Which the Cardiac Cycle Takes Place!
The cycle of events occur in single heart beat is called cardiac cycle. It involves repeated contraction and relaxation of the heart muscle. Contraction (when blood is ejected from the heart) is called systole and relaxation (when the chambers of the heart are filled with blood) is called diastole.
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The heart is said to be in a state of joint diastole when all its four chambers are in relaxed state. The rate of heart beat varies a great deal among different animals. In general the larger the animal the slower is the heart rate.
A cardiac cycle takes about 0.8 second and is completed in the following phases –
I. Isometric relaxation:
This is the first phase when all the valves are closed and the atria and ventricles are relaxed (in diastole). Blood is flowing into the atria from the pulmonary veins and the venae cavae.
II. Atrial diastole:
Both the atria filled with blood (having deoxygenated in right and oxygenated in left) when the atria and ventricles are both relaxed.
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At first the bicuspid and tricuspid valves are closed but as the atria filled with blood, their increased pressure eventually becomes greater than that in the relaxed ventricles and the AV valves are pushed open. The SL (semilunar valve) valve remains closed.
This phase is also known as the rapid filling phase. The blood flow into the heart slows then ceases as the pressure in the heart equals to that in the veins supplying it. There is then a brief period of diastasis where there is no movement of blood at all.
At the end of this phase, the two atria contract simultaneously (stimulated by SA node) and results in the blood being pumped into ventricles. This is called atrial systole.
III. Ventricular systole:
When the contraction of ventricles occurs immediately after atrial systole, the pressure in the ventricles rises and closes the atrioventricular valves, preventing blood from returning to the atria.
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Then the pressure forces to open the semilunar valves (three half moon shaped pockets) of aorta and pulmonary artery (the great arteries) to make entry of the blood into these vessels (ejection). This lead to reduced volume of blood into the ventricles (about 40 to 50 ml). The closing of atrioventricular valves during ventricular systole produces the first heart sound lubb.
IV. Ventricular diastole:
During this the semilunar valves are closed because of the blood forced back into the ventricles due to high pressure developed in the vessels. This causes the second heart sound dupp.
Then the pressure within the ventricles continues to decrease. The biscuspid and tricuspid valves still remain closed. Blood flows from the veins into the relaxed atria. After that the tricuspid and bicuspid valves open when the pressure in the ventricles falls. Contraction of the heart does not cause this blood flow. It is due to the fact that the pressure within the relaxed ventricles is less than that in the atria and veins.