Cardiac Cycle: Systole and Diastole

Systole is contraction (the chamber squeezes and pumps blood out). Diastole is relaxation (the chamber dilates and fills with blood). So when you read 'atrial systole', it means the atria are contracting; 'ventricular diastole' means the ventricles are relaxing. NEET often asks this in a twisted way, so lock it in: Systole = Squeeze.

Joint diastole is the stage where all four chambers (both atria and both ventricles) are relaxed at the same time. The tricuspid and bicuspid (AV) valves are open, so blood from the pulmonary veins and vena cavae flows freely from the atria into the ventricles. The semilunar valves are closed at this stage. NCERT and NEET 2022/2023 both stress that blood moves freely from atrium to ventricle during joint diastole.

Atrial systole is when the atria contract. The SAN fires an action potential, both atria squeeze, and this pushes about 30% extra blood into the already-filling ventricles. Ventricular systole is when the ventricles contract (after the AVN relays the impulse). This is the powerful phase that raises pressure, snaps the AV valves shut, forces the semilunar valves open, and ejects blood into the aorta and pulmonary artery.

When the ventricles contract, ventricular pressure rises sharply. Blood tries to flow backward into the atria (the path of least resistance at first), and this attempted backflow pushes the flaps of the tricuspid and bicuspid (AV) valves shut. This closure prevents blood from going back into the atria and produces the first heart sound (lub).

At a normal resting heart rate of 72 beats per minute, one full cardiac cycle takes about 0.8 seconds. You get this by dividing 60 seconds by 72 beats. Each ventricle pumps about 70 mL per beat, which is the stroke volume. NEET has directly asked the 0.8 s value, so remember it.

The SAN (sino-atrial node) starts each cardiac cycle. It is the pacemaker and generates the action potential that triggers atrial systole. The AVN (atrio-ventricular node) does NOT generate the beat; it receives the impulse, delays it briefly so the atria empty fully, then passes it on to the ventricles. NEET 2022/2023 used a wrong option saying 'AVN generates the action potential' as a trap.

The main phases are joint diastole (all chambers relaxed and filling), atrial systole (atria contract), ventricular systole (ventricles contract and eject blood), and ventricular diastole (ventricles relax). Then the cycle returns to joint diastole and repeats.

Atrial systole increases the flow of blood into the ventricles by about 30 percent. Most ventricular filling already happens passively during joint diastole; the atrial contraction tops it up.

Stroke volume is the amount of blood one ventricle pumps out in a single beat, about 70 mL in a resting adult. Stroke volume multiplied by heart rate gives the cardiac output.

The semilunar valves open during ventricular systole, when high ventricular pressure forces them open to eject blood into the aorta and pulmonary artery. They close during ventricular diastole, when ventricular pressure falls and prevents backflow.

NEET repeatedly asks about valve action, joint diastole, which node starts the beat, the 0.8 second duration, and stroke volume calculations. Getting the sequence and pressure changes right lets you answer both direct and match-the-following style questions.