Explain the following events of cardiac cycle with respect to type of valves and duration :(a) atrial systole(b) ventricular systole(c) joint diastole

Answer: Atrial systole 0.1 s (AV valves open); ventricular systole 0.3 s (AV valves closed, semilunar valves open); joint diastole 0.4 s (semilunar valves closed, AV valves open). Total 0.8 s.

One cardiac cycle lasts about 0.8 second (at 72 beats per minute).

• (a) Atrial systole (0.1 s): both atria contract and push the remaining blood (about 30%) into the ventricles. The atrioventricular (tricuspid and bicuspid) valves are open; the semilunar valves are closed.
• (b) Ventricular systole (0.3 s): ventricles contract; the rising pressure closes the AV valves, producing the first heart sound “lubb”. When ventricular pressure exceeds that in the aorta and pulmonary trunk, the semilunar valves open and blood is ejected into these arteries. Atria are in diastole at this time.
• (c) Joint diastole (0.4 s): atria and ventricles relax together. Ventricular pressure falls, the semilunar valves close (second heart sound “dubb”), and then the AV valves open so that blood from the veins flows through the atria into the ventricles (about 70% filling).