The human heart works like a pump, which beats about 70 times per minute in normal conditions. The heart acts as a dual pump and is comprised of two sets of chambers, two on the right side and two on the left. They are: right and left auricles and right and left ventricles. It is connected with arteries and veins that circulate the blood. These arteries and veins divide to form finer blood vessels called the capillaries.
(1) Circulation starts when the deoxygenated blood (oxygen-poor blood), returning from the body, enters into the right atrium with the help of superior and inferior venae cavae. From here, blood is pumped through the tricuspid valve into the right ventricle.
(2) The deoxygenated blood then travels from the right ventricle into the lungs through the pulmonary artery (through the pulmonary semi-lunar valve).
(3) In the lungs, the blood gets oxygenated and then the oxygenated (oxygen-rich) blood returns to the left atrium from the lungs through the pulmonary veins.
(4) The left atrium contracts and pumps the blood to the left ventricle with the help of the bicuspid (mitral) valve. Finally, the oxygenated blood is pumped by the left ventricle out through the aorta and then to the entire body.