Question

what is cardiac cycle ? explain how it occurs.​

Answer 1

Answer:

• The cardiac cycle refers to the repeating pattern of contraction and relaxation of the heart. The phase of  contraction is called systole, and the phase of relaxation is called diastole. The duration of a cardiac  cycle is 0.8 second under resting conditions.

• To begin with, all the four chambers of heart are in a relaxed state, i.e., they are in joint diastole. Blood  from pulmonary veins and vena cava, fills the left and right atria, respectively. The buildup pressure opens AV  valves and blood flows from atria to the ventricles. At this stage the semilunar valves are closed. The SAN  now generates an action potential which stimulates both the atria to undergo a simultaneous contraction known  as atrial systole. It results in increase of blood flow into the ventricles by about 30 per cent.

• The action potential generated by the SAN is conducted to the ventricular side. This causes contraction  of the ventricular muscles known as ventricular systole. The atria now undergo relaxation called as atrial  diastole which coincides with ventricular systole. As the ventricles begin their contraction, the  intraventricular pressure rises, causing the closure of tricuspid and bicuspid valves (AV valves) due to  attempted backflow of blood into the atria.

• When the pressure in the left and right ventricles become greater than the pressure in aorta and pulmonary  artery respectively, the semilunar valves open up. Opening of semilunar valves allows the blood in ventricles  to flow into aorta and pulmonary arteries. Now the ventricles relax and pressure falls causing the closure  of semilunar valves. It prevents the backflow of blood into the ventricles.

• The ventricular pressure declines further and the AV valves are pushed open due to the pressure in atria  exerted by the blood which was being emptied into them by the veins. Once again the blood moves freely  into the ventricles.

• The ventricles and atria are again in joint diastole (relaxed state) as earlier. Soon a new action potential  is generated by SAN and the events described above are repeated in that sequence and the process  continues.