Question

What is the duration of one cardiac cycle in man when the heart beats for 75 times per minute?
A. 0.4 sec
B. 0.8 sec
C. 0.5 sec
D. 0.3 sec

Answer 1

Hint: Cardiac cycle is defined as one heartbeat or one cycle of contraction or relaxation of the cardiac muscles which is asystole followed by diastole. During each cardiac cycle blood pumps in different chambers of the heart i.e atria and ventricles.

Complete answer:
The events of cardiac cycle :
Atrial systole: The atria contract due to a wave of contraction stimulated by SV node. The blood is forced into ventricles as the bicuspid and tricuspid valves are open. Duration – 0.1 sec.
Ventricular systole: The ventricles begin to contract due to a wave of contraction stimulated by the AV node. The bicuspid and tricuspid valves close immediately producing part of the first heart sound “lubb”. When ventricles complete their contraction the blood flows into the pulmonary trunk and aorta as semilunar valves are open. Duration- 0.3 sec.
Ventricular diastole: The ventricles relax and semilunar valves are closed. This causes the second heart sound “dup”. The tricuspid and bicuspid valves open again when the pressure in the ventricles falls and blood flows from the atria into the ventricles Duration- 0.4 sec.
Option -A- 0.4 sec
It is the time taken by ventricular diastole. In this period ventricles are relaxed and semilunar valves are closed. As explained above ventricular diastole takes 0.4 sec.
Option -B- 0.8 sec
For each event of the cardiac cycle to complete it takes a total of 0.8 sec. A man's heart beats for 70-75 times per minute. Each event of one cardiac cycle combines to a total time of 0.8 sec thus making it possible for the heart to beat 75 times in a minute.
Option -C- 0.5 sec
This time does not match any event of a cardiac cycle. Any event of the cardiac cycle including atrial systole, ventricular diastole and ventricular systole does not take this time.
Option -D- 0.3 sec
This time is for the event ventricular systole in one cardiac cycle. Because of the wave from the AV node, the ventricles begin to contract.

Hence, the correct answer is option (B).

Note: Cardiac cycle is a cycle of events which occur in living organisms in the heart signal of a healthy working heart. Cardiac cycle corresponds to a single heartbeat. It depends on the person's health physically and also depends on the age of the person. Each cardiac cycle has a diastolic phase and a systolic phase which corresponds to the state of relaxation and contraction respectively in a cardiac cycle.