the heart

The heart is a fist-sized muscular organ in the chest that squeezes rhythmically to push blood around the body. It is essentially a tireless pump that beats around 100,000 times a day, every day, without ever taking a real break.

Anatomically it has four chambers: two upper atria that receive incoming blood and two lower ventricles that do the heavy pumping. One-way valves between chambers and at the exits keep blood flowing forward and stop it sliding back. The muscular wall, the myocardium, contracts when triggered by the heart's own electrical system, while the coronary arteries feed the heart muscle itself.

Although poets treat the heart as the seat of emotion, in medicine it is a mechanical and electrical organ. Its rhythmic contraction is driven by a built-in electrical pacemaker, so the heart can beat on its own without any signal from the brain — a property called automaticity.