Internal anatomy of the heart (overview)

Internally, the heart is divided into four chambers: right and left atria, and right and left ventricles

Functionally, the heart consists of two pumps, each consisting of an atrium and a ventricle separated by a valve: the right pump receives deoxygenated blood and pumps it into the lungs, and the left pump receives oxygenated blood from the lungs and sends it to the body. 

The atria have relatively thin walls and they receive blood, while the ventricles with relatively thick walls pump blood out of the heart.

The chambers are separated by interatrial, interventricular and atrioventricular septa.