Explain what is cardiac cycle ?
what is cardiac cycle ?
It consists of cyclical specific sequence of systole and diastole of atria and ventricles in a regular repeated manner resulting in blood flow through all the four cardiac chambers in a specific direction and at a specific rate.
The cardiac cycle is divided into four phases : atrial systole, atrial diastole, ventricular systole and ventricular diastole. Visit Biology Doubts page of Physics Wallah for more questions.
1. Atrial systole
It results in the flow of blood from the two atria to the corresponding ventricles (which are in state of diastole via the two atrioventricular openings). There is no possibility of back flow of blood from the two atria to the corresponding major veins due to the closure of their openings into the atria. The atrial systole relapses into the atrial diastole.
2. Ventricular systole
It occurs in response to the spread of electric wave of depolarization in the ventricular walls. At the beginning of ventricular systole, the rise in ventricular blood pressure results in the closure of atrioventricular valves to prevent back flow of blood from ventricles to the atria. As the semilunar valves of the two arches are already closed (since the beginning of ventricular diastole of the previous cardiac cycle), the further progression of ventricular systole results in the rapid rise in intraventricular blood pressure. It results in the opening of the semilunar valves forcing the blood to flow from the ventricles to the respective arches( i.e. deoxygenated blood from right ventricle to the pulmonary arch and oxygenated blood from left ventricle to the systemic arch)
3. Atrial diastole
The beginning of atrial diastole overlaps with the end of the ventricular systole. The atria during diastole receive blood from the respective major veins (i.e. right atrium receives deoxygenated blood from the precavals and a postcaval while left atrium receives oxygenated blood from the pulmonary veins) resulting in their progressive filling (as the two atrioventricular valves are already closed).
4. Ventricular diastole
It is due to the ventricular repolarisation. At the beginning of ventricular diastole the initial fall in the intraventricular blood pressure results in the closure of semilunar valves and hence the production of second heart sound or `dup’. The two ventricles now dilate further as closed chambers (because the two atrioventricular valves are already closed) resulting in the rapid fall in the intraventricular blood pressure to such an extent that it becomes less than that of the two atria. It is therefore followed by the opening of the two atrioventricular valves at the end of ventricular diastole. Simultaneously, the two atria also enter into the diastolic phase resulting in atrioventricular joint diastole in which there is passive blood flow from the two atria to the corresponding ventricles.