The heart is a two-stroke pump
Think of the heart as a muscular bag that does the same simple thing over and over: it tightens to push blood out, then loosens to let blood flow back in. Cardiologists call the squeeze systole and the relaxation diastole. One squeeze plus one relaxation is a single cardiac cycle — one heartbeat. At a resting heart rate of 70 beats per minute, the whole cycle takes a little under one second.
Here is the part that surprises people: at rest, the heart spends more time resting than working. Of that one-second cycle, roughly one-third is systole and two-thirds is diastole. That long diastolic pause matters — it is when the chambers fill, and, as we will see in a later guide, when the heart's own muscle gets most of its blood supply.
Following the blood through one cycle
Let's trace the left side, which feeds the body. During diastole the left ventricle is relaxed and the mitral valve is open, so blood drifts in from the left atrium and the chamber fills. When systole begins, the ventricle contracts; pressure shoots up, the mitral valve snaps shut, and once ventricular pressure exceeds aortic pressure the aortic valve opens and blood is ejected into the aorta.
- Diastole — filling. Ventricle relaxed, mitral valve open, aortic valve closed; blood pours in.
- Systole begins — building pressure. Both valves closed for a brief instant; the ventricle squeezes against trapped blood and pressure climbs fast.
- Systole — ejection. Aortic valve opens, blood is pushed into the aorta and out to the body.
- Back to diastole. The ventricle relaxes, the aortic valve closes, pressure falls, and the cycle starts over.