cardiac cycle
Imagine a hand-pump filling and emptying over and over. The cardiac cycle is the heart doing exactly that: one complete round of filling with blood and then squeezing it out, repeated roughly once a second for a lifetime.
Each cycle has two main phases. During diastole the heart muscle relaxes and the chambers fill; during systole the muscle contracts and pushes blood forward into the arteries. The atria and ventricles take turns: the atria give the ventricles a final top-up before the ventricles do the heavy lifting of ejection.
Pressure, volume, valve movements, and the electrical signal all stay tightly coordinated within each cycle. When that timing breaks down — from an abnormal rhythm, a stiff or leaky valve, or weakened muscle — the heart fills or empties less efficiently, which is the root of many cardiac symptoms.
Note that the same word can describe one beat or the recurring pattern. In physiology the cardiac cycle usually means the full sequence of events for a single heartbeat, measured from one point (such as the start of filling) to the same point on the next beat.
A textbook diagram of the cardiac cycle lines up the ECG, aortic and ventricular pressures, ventricular volume, and the heart sounds on one time axis, so a learner can see how the electrical signal precedes the mechanical squeeze.
The classic Wiggers diagram aligns electrical, pressure, volume, and sound events of one beat.
At a resting heart rate near 75 beats per minute, one cardiac cycle lasts about 0.8 seconds. Faster rates shorten the cycle mostly by cutting into diastole, which is why very fast heart rates can impair filling.