The heart is the pump that drives blood through the blood vessels of the circulatory system. It is a muscular organ about the size of a clenched fist.

It lies to the left of centre of the chest, tucked under the left lung, and is protected by the ribs and sternum.

The heart is a hollow structure. Its right and left sides are separated by a partition known as the septum that separates the two blood flows through the heart.

Each side of the heart is further divided into an upper chamber called the atrium (plural: atria) or sometimes the auricle) and a lower chamber called the ventricle. The atria receive blood into the heart and the ventricles pump blood out of the heart.