heartbeat

A heartbeat is one full squeeze-and-relax of the heart that pushes a fresh slug of blood onward. It is the single beat you feel in your chest and hear as the familiar lub-dub — the basic unit of the heart's work, repeated tirelessly your whole life.

Each beat is a coordinated sequence. An electrical signal starts in the heart's natural pacemaker, spreads through the upper chambers so they squeeze first, pauses briefly, then sweeps through the lower chambers, which contract hard to eject blood. Then the muscle relaxes and refills, ready for the next beat. The two heart sounds come from valves snapping shut during this cycle.

One full sequence of contraction and relaxation is more formally called the cardiac cycle. Everyday speech uses heartbeat loosely — sometimes for one beat, sometimes for the rhythm in general — but in cardiology the mechanical event of one beat and the electrical event that triggers it are described separately.