Which organ pumps blood around the human body?

The heart is a muscular pump about the size of a clenched fist, sitting between the lungs and slightly left of centre. Its walls are made of cardiac muscle, a tissue found nowhere else, which contracts rhythmically without needing a signal from the brain. The beat is set by the sinoatrial node, a patch of specialised cells in the right atrium that fires electrical impulses on its own, which is why a heart can keep beating briefly outside the body. Nerves and hormones only speed that intrinsic rhythm up or slow it down. Each beat moves blood through two circuits: a short one to the lungs to collect oxygen, and a long one to the rest of the body to deliver it and pick up waste. Over a day that comes to roughly a hundred thousand beats and several thousand litres of blood moved, and the heart muscle feeds itself through its own coronary arteries.

Veins in the limbs rely on one-way valves and the squeeze of surrounding muscles to help blood return uphill.