2. The right atrium receives deoxygenated blood collected from different organs of the body via large veins called venacava. The left atrium receives oxygen rich blood from the lungs simultaneously through the four pulmonary veins.
3. The atria contract and pour the blood into respective ventricles which expand to receive the blood. Left ventricle gets filled with oxygenated blood and right ventricle gets filled with deoxygenated blood.
4. Now both the thick walled ventricles contract resulting in pumping out the oxygenated blood to all the parts of the body through the aorta (the largest artery) and the deoxygenated blood from the right ventricle enters the lungs through the pulmonary artery for oxygenation.
5. The valves between the atria and ventricles ensure that the blood does not flow backwards.
6. Thus the deoxygenated blood enters the right part of the heart and again after oxygenation it enters the left part of the heart so the blood goes through the heart twice during each cycle. This is known as double circulation.
