Heart is the recipient and sending center of blood circulation. It pumps 2,000 gallons of blood per day and contracts and expands 100,000 times. The heart of mammals and birds has bi-circular circulation of blood. A two-circuit circulatory system where blood flows through the heart twice before completing one cycle throughout the body. Blood circulation process of heart is-
1. The two atria of the heart are dilated. Atrial volume increases and blood pressure decreases. Blood containing CO2 enters the right atrium through superior venacava-svc from upper body and inferior venacava-ivc from lower body. At the same time, O2-rich blood enters the left atrium through the four pulmonary veins.
2. Both atria contract when filled with blood. Atrial volume decreases and blood pressure increases. At the same time both ventricles dilate. The volume of the ventricles increases and the blood pressure decreases. As a result, blood containing CO2 enters from the right atrium to the right ventricle through the tricuspid valve and blood containing O2 from the left atrium to the left ventricle through the bicuspid valve.
3. When the ventricles are filled with blood, they contract and blood pressure rises. As a result, blood containing CO2 enters the pulmonary artery from the right ventricle through the pulmonary semilunar valve and blood containing O2 enters the aorta from the left ventricle through the aortic semilunar valve.
4. Blood containing CO2 passes through the pulmonary artery to the lungs, purified, enriched with O2, and enters the left atrium through the pulmonary veins.
5. O2-rich blood from the aorta flows to various parts of the body through arteries, arterioles, sub-arteries and capillaries.
In this way the blood flows through the heart periodically and this phenomenon repeats itself cyclically during the heartbeat.