The heart pumps blood through movements of contraction (systole) and relaxation (diastole). The right atrium sends the blood, with its movement of systole, to the lungs, and the left, to the whole organism.
The movements of the heart are:
The veins are blood vessels that carry blood to the heart. The vena cavae carry blood from all cells to the right atrium, while the pulmonary veins carry freshly oxygenated blood from the lungs to the left atrium.
The atria fill with blood from the veins, until they fill and contract at the same time (atrial systole), and the blood passes from each atrium to its corresponding ventricle through the atrioventricular valves.
Afterwards, the ventricles contract (ventricular systole) and blood flows from the right ventricle to the pulmonary artery and blood from the left to the aorta artery, as the sigmoid valves open and the atrioventricular valves close. The sound emitted when these valves close corresponds to the first “boom” of the heartbeat .
The heart relaxes (general diastole) and blood from the vena cavae and pulmonary re-enters the atria, starting the cycle again.
Blood that has left the ventricles cannot return because the sigmoid valves close. The sound of this closure corresponds to the second “bang” of the heart.