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Cardiac Cycle

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Cardiac Cycle - Lesson Summary

Heart is a muscular organ in humans that helps in circulation of blood. It carries out double circulation, which involves both pulmonary and systemic circulation. Nutrients are added to the blood through the hepatic portal system, which connect the intestines and the liver to the systemic circulation. Similarly, blood circulation to the heart is carried out by the coronary arteries and cardiac veins, which together comprise the coronary circulation.

When the heart pumps blood, a heartbeat is generated by the systole and diastole of the chambers of the heart. On average, our heart beats 72 times per minute which means 72 cardiac cycles per minute.

Thus, one cardiac cycle takes 0.8 seconds. The cardiac cycle starts with the four chambers in a joint diastole. Atrial pressure causes the bicuspid and tricuspid valves to open. This allows blood to flow from the vena cava and pulmonary vein into the right and left ventricles through the right and left atria, respectively. The semilunar valves are closed at this point.

This is followed by the SAN producing an action potential that induces both the atria to contract at the same time, forcing more blood into the ventricles. Ventricular pressure increases due to the increased volume of blood, which forces the closure of the tricuspid and bicuspid valves and thereby prevents a backflow of blood into the atria. The action potential from the SAN then travels down through the AVN and passes onto the ventricles through the Bundle of His.

This results in a ventricular systole as well as atrial diastole. The ventricular systole causes blood to pump from the right and left ventricles to the pulmonary artery and aorta respectively. The semilunar valves of the pulmonary artery and aorta are opened at this point. This is followed by the ventricular diastole, causing the closure of the semilunar valves to prevent any backflow of blood from the pulmonary artery and aorta to the right and left ventricles, respectively.

At the same time, atrial pressure causes the tricuspid and bicuspid valves to open, to let the blood flow from the atria to the ventricles. Now, the four chambers are once again in a joint diastole. This sequence of events is repeated again as the SAN generates another action potential, resulting in the cardiac cycle. But our body has the ability to increase cardiac output by four to five times during exercise and decrease it to return to normal at rest.


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