diastole
Diastole is the rest and refill. It is the part of the heartbeat when the ventricular muscle relaxes, the chambers expand, and blood pours in to get the heart ready for the next push.
It begins when the outlet valves close and the ventricular muscle relaxes; pressure inside falls until it drops below atrial pressure, which opens the inlet valves and lets filling begin. Most filling happens passively early in diastole, with a final boost from atrial contraction at the very end.
Diastole is not idle time — it is when the coronary arteries supplying the heart muscle itself receive most of their blood flow, because during systole the contracting muscle squeezes those vessels shut. So adequate diastole protects the heart's own blood supply.
Because filling needs time, very fast heart rates eat into diastole and can reduce how much blood the ventricle takes in. Stiff ventricles that relax poorly also fill inadequately, a problem called diastolic dysfunction.
The lower number in 120/80 is the diastolic pressure — the arterial pressure that lingers while the ventricles relax and the outlet valves are shut.
Diastolic blood pressure reflects arterial pressure between beats.
At rest, diastole takes up roughly two-thirds of the cardiac cycle and systole one-third. As heart rate climbs, diastole shrinks far more than systole does.