What happens in the pulmonary circulation in humans?

Explanation

A vein is a blood vessel which carries blood back to the heart. The circulatory system is the body's blood distribution network, paced by the 'pump', i.e. the heart. In vertebrates there are two 'loops': the pulmonary circulation to the lungs where blood is oxygenated; and the systemic circulation, a 'loop' through the rest of the body to provide oxygenated blood.The pulmonary vein returns the now oxygen-rich blood to the heart.