The heart is the key organ in the circulatory system. As a hollow, muscular pump, its main function is to propel blood throughout the body.

The heart has four chambers: left atrium, right atrium, left ventricle and right ventricle. The inter-atrial septum divides the atria while inter-ventricular septum divides the ventricles. Cardiac valves connect the atrium and ventricle.

In the circulation system, the waste-rich blood flows into the right atrium and ventricle to enter the lungs for gaseous exchange by the pulmonary artery. Then oxygen-rich blood then flows back to the left atrium and ventricle, and is finally is pumped from the heart into the aorta to the whole body to sustain the function of organs.