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# Define the cardiac cycle. Describe various events of the cardiac cycle.

Last updated date: 20th Jun 2024
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Hint: Cardiac cycle is the rhythmic and cyclic series of electrical and mechanical events, taking place during a heartbeat.

It consists of two major sets of events, namely auricular events and ventricular events. These are completed in four stages, namely auricular systole, auricular diastole, ventricular systole, and ventricular diastole. The short interval between the end of one cycle and the beginning of the next is called the refractory period. In a healthy individual, the average rate of heartbeat is 72 times per minute. Then, the duration of the cardiac cycle is $60/72= 0.83 s$. The duration of the cardiac cycle is inversely proportional to the rate of heartbeat. The stages of the cardiac cycle can be roughly divided into four stages: The filling phase, Isovolumetric contraction, building up pressure ready to pump blood into the aorta or pulmonary trunk.

During the filling phase, the ventricles are filled with blood in two stages – diastole (heart relaxation) and atrial systole. The stage of Isovolumetric contraction lasts for approximately 50ms while the pressure builds. During the outflow phase the pressure in the ventricles exceeds the pressure in the aorta or pulmonary trunk, the outflow valves open. Blood is pumped from the heart into the great arteries. At the end of the outflow phase, both sets of valves are closed once again. The ventricles begin to relax, reducing the pressure in the ventricles so that the atrioventricular valves open. The ventricles then begin to fill with blood and the cycle begins once again