Internally, the heart is divided into four chambers:

• Two upper chambers known as atria: right atrium (RA) and left atrium (LA);
• Two lower chambers called ventricles: right vetricle (RV) and left ventricle (LV).

Functionally, the heart consists of two pumps (right and left), each consisting of an atrium and a ventricle separated by an atrioventricular valve.

• The right pump (consisting of RA and RV) receives deoxygenated blood and pumps it into the lungs (via the pulmonary circulation).
• The left pump (LA and LV) receives oxygenated blood from the lungs and sends it throughout the whole body (via the systemic circulation).

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

The heart chambers are separated from each other by muscular walls known as septa (singular: septum). They include the following:

• Interatrial septum - between atria;
• Interventricular septum - between ventricles;
• Atrioventricular septum - between atria and ventricles.