Describe the cardiac cycle.

(N/A) The heart beats about $72$ times in a minute. It acts as a pump, and its events show regular sequential patterns. This is called the cardiac cycle.
The contraction phase of the heart is called systole, and the relaxation phase is called diastole.
The following phases are observed during the cardiac cycle:
$(i)$ Initially, all four chambers are in a state of joint diastole.
$(ii)$ The tricuspid and bicuspid valves are open. Blood from the pulmonary veins and vena cava flows into the left and right ventricles, respectively.
$(iii)$ The semilunar valves are closed at this stage. The $SAN$ generates an action potential which stimulates both the atria to undergo simultaneous contraction (atrial systole). This increases the flow of blood into the ventricles by about $30\%$.
$(iv)$ The action potential is conducted to the ventricular side by the $AVN$ and $AV$ bundle, from where the bundle of $HIS$ transmits it through the entire ventricular musculature.
$(v)$ This causes ventricular muscles to contract (ventricular systole), while the atria undergo relaxation (diastole), coinciding with the ventricular systole. Ventricular systole causes the closure of tricuspid and bicuspid valves, which prevents the backflow of blood into the atria.
$(vi)$ As ventricular pressure increases further, the semilunar valves guarding the pulmonary artery and the aorta are forced open, allowing the blood in the ventricles to flow through these vessels into the circulatory pathways.
$(vii)$ The ventricles now relax (ventricular diastole), and the ventricular pressure falls, causing the closure of semilunar valves, which prevents the backflow of blood into the ventricles.
$(viii)$ Now, the bicuspid and tricuspid valves are pushed open by the pressure in the atria exerted by the blood which was being emptied into them by veins.
$(ix)$ The ventricles and atria are now again in a relaxed (joint diastole) state. This process repeats in this order continuously.

Time period	Atria	Ventricles
$0.10 \, s$	Systole	Diastole
$0.30 \, s$	Diastole	Systole
$0.40 \, s$	Diastole	Diastole

The heart beats $72$ times per minute. From this, it can be deduced that the duration of a cardiac cycle is $0.8 \, s$.
During a cardiac cycle, each ventricle pumps out approximately $70 \, ml$ of blood, which is called the stroke volume.
The stroke volume multiplied by the heart rate (number of beats per minute) gives the cardiac output. Therefore, the cardiac output is defined as the volume of blood pumped out by each ventricle per minute and averages $5000 \, ml$ or $5 \, L$ in a healthy person. The cardiac output of an athlete is much higher than that of an ordinary person.