Mammalian heart structure

The mammalian heart is a muscular organ that pumps blood. Its muscular wall contracts to force blood out of the heart. A wall called the septum separates the heart into right and left sides, preventing blood from mixing between them.

Chambers and valves

Each side has two chambers. An upper chamber, called an atrium, receives blood entering the heart. A lower chamber, called a ventricle, pumps blood out. Blood passes from the right atrium to the right ventricle, and from the left atrium to the left ventricle.

One-way valves keep blood moving in the correct direction. Valves between the atria and ventricles, and at the exits from the ventricles, prevent blood from flowing backwards when the chambers contract.

Recognising the heart’s structures

In a heart diagram, look for the septum separating the two sides, then identify the atrium above the ventricle on each side. The labels left and right refer to the animal’s own left and right. In a front-view diagram, the heart’s left side therefore often appears on the right of the image.

The coronary arteries run over the surface of the heart and supply its muscle with oxygen and nutrients. This supply allows the heart muscle to keep contracting.