The process by which blood containing O2 from the heart passes through the blood vessels to different parts of the body and blood containing CO2 from different parts of the body returns to the heart is called blood circulation. Liver has the highest blood flow (1350 ml/min). Blood flows in the second volume (1100 ml/min) to the kidney and in the third volume (700 ml/min) to the brain.
The process of blood circulation in the human body is completed in four stages. Systemic circulation, pulmonary circulation, coronary circulation and portal circulation.
1. Systemic Circulation: In systemic circulation, O2 enriched blood enters the aorta or aorta from the left ventricle of the heart. From the aorta, blood flows through arteries and capillaries to various organs, tissues and cells. CO2, nutrients, hormones etc. reach the cells through the blood.
Then blood containing CO2 enters the capillary network from various organs, tissues and cells. Blood comes from the capillary network into the veins. Blood from the veins reaches the veins. Veins from the upper part of the body supply blood to the superior vena cava and veins from the lower part to the inferior vena cava. Finally the blood containing CO2 enters the right atrium of the heart through superior vena cava and inferior vena cava. In this way, it takes 25-30 seconds for the blood to flow from the heart and return to the heart.
2. Pulmonary Circulation: In pulmonary circulation, blood containing CO2 from the right ventricle of the heart enters the pulmonary artery through the semilunar ventricles. Blood comes to the lungs from the pulmonary artery. Blood is purified in the lungs. The O2-rich blood from the lungs then enters the left atrium through the pulmonary veins.
3. Coronary circulation: Coronary arteries arise from the roots of systemic arteries. O2-rich blood passes through the coronary arteries to the heart wall. The CO2-rich blood then returns to the heart through the coronary veins from the heart wall.
4. Portal Circulation: Capillaries arise from some organs. Capillary networks join together to form veins. Blood travels from one organ to another through veins. In vertebrates there are two types of portal circulation namely hepatic and renal.
