In this circulation the deoxygenated blood goes out of the heart, and reaches the lungs and then again enters the heart. Then the deoxygenated blood leaves via right ventricle through the pulmonary artery to the capillaries, there by diffusing the carbon dioxide out of blood cell in the alveoli. And in return diffusing out the oxygen of the alveoli in to the blood. Then the blood enters the heart there by leaving the capillaries and re-entering the left atrium.
