Cardiac Cycle

The cardiac cycle is the name given to the complete, repeating sequence of mechanical events that make up a single heartbeat — from the start of one contraction of the heart's chambers to the start of the next. At a typical resting adult heart rate of about seventy-two beats per minute, one full cardiac cycle occupies roughly zero-point-eight (0.8) seconds, and within that short span the two atria and the two ventricles pass through an alternating sequence of relaxation, called diastole, and contraction, called systole.

The cycle can be described in three named phases. It begins with joint diastole, lasting about zero-point-four seconds, during which both atria and both ventricles are simultaneously relaxed; the atrio-ventricular valves (tricuspid and bicuspid) are open at this time, so blood returning from the veins flows passively, under its own pressure, first into the relaxed atria and then straight on through the open valves into the equally relaxed ventricles below, filling them to roughly seventy percent of their eventual volume before the atria even contract. The semilunar valves remain closed throughout this phase, since there is no pressure pushing blood towards them.

Joint diastole is followed by atrial systole, a brief phase lasting only about zero-point-one seconds, during which the two atria contract together, squeezing the remaining, comparatively small volume of blood they still hold down into the ventricles, which are still relaxed at this point; the atrio-ventricular valves remain open throughout atrial systole, since blood is still flowing in the correct direction, from atrium into ventricle. …