The pulmonary circulation begins in the right ventricle and ends in the left atrium. The pulmonary circulation refers to the blood circulation from the right ventricle to the left atrium, and its main circulatory pathway is the venous blood returning from the body circulation to the right atrium into the right ventricle, after which it shoots from the right ventricle into the pulmonary artery, and then flows from the branches of the pulmonary artery at all levels to the capillary network of the alveolar wall, where after gas exchange, the blood again becomes arterial blood with higher oxygen content and lower carbon dioxide content, and then flows through the veins at all levels in the lung, and finally at the The blood flows back to the left atrium through the pulmonary veins at the pulmonary hilum. After that, the blood enters the left ventricle for the body circulation. The pulmonary circulation is also known as the lesser circulation, and its main physiological function is to complete gas exchange. The pathway of the pulmonary circulation is shorter than that of the body circulation, and the walls of the pulmonary arteries are thin and have fewer elastic fibers, which are easily dilated, so the resistance to pulmonary blood flow is low and the blood pressure is relatively low. In addition, because of the large expansibility of lung tissue and pulmonary vessels, the blood volume of the lungs is larger, and when the body loses blood, the pulmonary circulation can transfer part of the blood to the body circulation and play a compensatory role.