Cardiac Cycle

The cardiac cycle is the orderly, repeating sequence of contraction (systole) and relaxation (diastole) of the atria and ventricles that drives blood through the heart. Following it through one complete round shows how the chambers and valves cooperate.

Starting point — joint diastole

At the outset all four chambers are relaxed together, a state called joint diastole. The tricuspid and bicuspid valves are open, so blood arriving from the pulmonary veins and the vena cava flows through the left and right atria into the left and right ventricles respectively. During this phase the semilunar valves remain closed.

Atrial systole

The SAN now fires an action potential that stimulates both atria to contract at the same time — the atrial systole. This contraction pushes additional blood into the ventricles, raising the amount they receive by about thirty per cent.

Ventricular systole

The action potential is carried onward to the ventricles by the AVN and the AV bundle, from where the bundle of His spreads it through the whole ventricular muscle. The ventricles then contract — ventricular systole — while the atria relax (atrial diastole) at the same time. As the ventricles contract, pressure inside them rises. This attempted backflow of blood toward the atria shuts the tricuspid and bicuspid valves. When the ventricular pressure climbs still higher, it forces open the semilunar valves guarding the pulmonary artery (on the right) and the aorta (on the left), and blood is driven out of the ventricles into these vessels and on into the circulatory pathways.

Ventricular diastole and return to rest

The ventricles now relax — ventricular diastole — and their pressure falls. The falling pressure lets the semilunar valves close, which stops blood from flowing back into the ventricles. As ventricular pressure drops further, the blood that the veins have been emptying into the atria pushes the tricuspid and bicuspid valves open again, and blood once more passes freely into the ventricles. All four chambers are again in joint diastole, exactly as at the start. The SAN soon fires a fresh action potential and the whole sequence repeats. This cyclically repeated set of events is the cardiac cycle, consisting of the systole and diastole of both atria and ventricles.

Timing, stroke volume and cardiac output …