THE PULMONARY CIRCUIT
Deoxygenated blood is oxygenated in the pulmonary circuit. Note that in the true cardiac cycle, both the left and right ventricles contract simultaneously.

Contraction of the right ventricle pumps deoxygenated blood out through the pulmonary semilunar valve and down the paired pulmonary arteries to the lungs. The pulmonary arteries are the only arteries in the body that carry deoxygenated blood.

In the lungs, the arteries eventually branch into millions of pulmonary capillaries. In these thin-walled capillaries, CO₂ diffuses from the surface of red blood cells to the air in the alveoli and O₂ diffuses onto the cells — thus oxygenating the blood.

Having passed through the capillaries, the now oxygen-rich blood flows back toward the heart through the only veins in the body that carry oxygenated blood. These veins eventually converge into the two pulmonary veins, which empty directly into the left atrium.