The flow of blood through the heart takes place step by step:
(1) The oxygen-rich blood from the lungs comes to the left chamber of the heart that is the left atrium, via the pulmonary vein.
(2) When the left atrium contracts, the left ventricle expands and the blood is transferred to it.
(3) On contraction of the left ventricle, the blood is pumped out to the body through the aorta.
(4) The aorta branches into vessels which transports blood to the heart and to all the other body parts.
(5) Deoxygenated blood comes from the body to the upper right chamber of the heart, the right atrium through large veins called vena cava.
(6) When right atrium is full of blood, it contracts and the tricuspid valves open under pressure and the blood pours into the right ventricle.
(7) When the right ventricle contracts, the blood is pumped into the pulmonary trunk.
(8) The pulmonary trunk divides into the right and left pulmonary artery, each of which carries the blood to the lungs for oxygenation.