The heart consists of four cavities paired in two functional units, each made of an atrium and a ventricle and separated by partitions (interatrial and interventricular septa).

Right atrium and ventricle form the right pump that receives deoxygenated blood from the body and send it to the lung. The atrium, thin-walled and located posteriorly, receives blood from the periphery through the venae cavae (superior and inferior), and the thick-walled ventricle, located anteriorly, pump it into the lungs through the pulmonary artery.

Left atrium and ventricle form the left pump responsible for delivering arterial, oxygenated blood to the body. The atrium receives oxygenated blood form the lungs through the four pulmonary veins, and the ventricle pushes it into the systemic circulation through the aorta. More force is required to pump blood into the systemic circulation than into the pulmonary one, so the muscular wall of the left ventricle is thicker than that of the right.

The blood flows from atria to ventricles passing through the right and left atrioventricular orifices, that open and close at each cardiac contraction by valves
(tricuspid and mitral). Also the passage of blood from ventricles to great vessels is subordinated to the opening and closing of another pair of valves, the semilunar ones, during the cardiac cycle (pulmonary and aortic valve).

The cardiac circulation consists of the coronary arteries (left and right) that arises in the initial portion of the ascending aorta and branch out on the cardiac surface up to the apex, supplying the cardiac muscle. The cardiac veins run parallel to the arteries and merge into the coronary sinus that enters the right atrium.