The circulation in which the blood passes through the heart twice to circulate once throughout the body is called double circuit circulation. Bi-circular circulation occurs in the human heart. Two-circuit circulation consists of pulmonary circulation and systemic circulation.
1. 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.
2. 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. O2, nutrients, hormones etc reach the cells through blood.