The blood participating in the body circulation emanates from the left ventricle, passes through the aorta to reach the branch arteries at all levels and then enters the small arteries of each tissue and organ, and finally reaches the capillary network. After nutrient exchange, the blood gathers from the capillary network to the small veins of each tissue and organ, and then gathers to the branch veins of each tissue and organ, and then gathers uniformly in the upper and lower vena cava, and finally enters the right atrium to complete the body circulation. The pulmonary circulation refers to the blood issuing from the right ventricle, entering the lungs through the pulmonary artery, and the blood vessels are graded through layers, and finally distributed in the capillary network of the alveoli. After gas exchange, the blood gradually converges through layers to the pulmonary veins, and then uniformly enters the left atrium to complete the pulmonary circulation.