Every heartbeat is a 0.8-second relay timed by a built-in delay
At a resting rate of around 70 to 75 beats a minute, one full heartbeat lasts about 0.8 seconds. In that sliver of time four chambers fill, squeeze and seal in strict order, and the whole routine depends on a tiny cluster of cells that deliberately holds back the electrical signal for a moment.
The heart works as two pumps side by side. The right side sends blood low in oxygen through the pulmonary arteries to the lungs; the left side drives freshly oxygenated blood out through the aorta to the rest of the body. On each side, an upper chamber called an atrium receives returning blood and a lower chamber called a ventricle does the forceful ejecting. The faster the heart beats, the shorter each cycle becomes.
A cycle splits into two periods. In diastole the ventricles relax and fill as the valves between upper and lower chambers swing open. Near its end the atria contract, topping up the ventricles under pressure; though this step is named atrial systole, its real job is to finish the filling. Then systole begins. The ventricles contract, the inlet valves snap shut, and for a brief interval pressure builds with no blood moving in or out. Once ventricular pressure exceeds that in the aorta and pulmonary trunk, the outlet valves open and blood surges out. When pressure drops again those valves close, another sealed pause follows, and filling restarts.
The timing comes from the heart's own wiring. The sinoatrial node in the wall of the right atrium acts as a natural pacemaker, firing a wave that makes the atria contract. The wave then reaches the atrioventricular node, which slows it down before passing it through the bundle of His and the Purkinje fibres to the ventricles. That built-in pause gives blood time to flow into the ventricles before they squeeze. Heart muscle cells can start their contractions without instructions from outside nerves, which mainly adjust the rate to match the body's needs.
Closing of the aortic valve sends a sharp pressure blip down the arteries. As people age the aorta stiffens, which can lower the peak of the pulse felt at the wrists and ankles.
Source: Cardiac cycle
