Blood from the veins of the whole body flows back into the right atrium through the upper and lower vena cava, and flows from the right atrium to the right ventricle, where the right ventricle is connected with the pulmonary artery, there is a pulmonary valve. The pulmonary artery is mainly from the right ventricle, blood in the pulmonary circulation process, after gas exchange, the formation of arterial blood through the pulmonary veins back to the left atrium, blood from the left atrium into the left ventricle, through the left ventricle contraction of blood, and then through the aorta into the whole body organs, the pulmonary artery is called arterial, but the flow of blood belongs to venous blood.