Pulmonary circulation is also called microcirculation. Starting from the right ventricle, the contraction of the right ventricle injects venous blood into the pulmonary artery, passes through all branches of the pulmonary artery, reaches capillaries, blood vessels and alveoli in the alveolar wall, and then becomes oxygen-saturated arterial blood; Arterial blood enters venules from capillaries, flows back through various veins of the lung, finally flows into the left and right pulmonary veins, and finally reaches the left atrium. After blood enters the left ventricle from the left atrium, systemic circulation begins again.