blood_circulationBlood vessels, which leave as well as return to the heart, form two „circles”: the pulmonary circulatory system is the portion of blood circulation which, together with the pulmonary artery, originates from the right ventricle, passes round the initial section of the aorta then by getting behind the aorta it is divided into 2 branches and by subdividing into capillaries enters into the right and left lungs.
The name pulmonary artery is misleading as it does not carry oxygen-rich but carbon dioxidated blood to the lungs which, as a result of gas exchange, turns into oxygen-rich and carbon dioxide- depleted blood. After gas exchange the veins create a system and, by dividing into larger branches, they enter into the left atrium through the pulmonary veins. The systemic circulation leaves the left ventricle with the aorta, the branches of which are divided into smaller and smaller veins, then they lead to capillaries supplying the body with fresh blood. The „used up”, oxygen-depleted and carbon dioxide- rich blood is transported from the tissues by the venous system which, by merging into successively larger veins, coalesces into two major veins that empty into the right atrium.