The blood in the human body is in a closed tube for continuous circulation. When human blood flows through capillaries, some water, small molecules of protein and other substances can enter into tissue fluid through capillaries, and some tissue fluid can return to capillaries again, but a small portion of tissue fluid does not enter into capillaries, but enters into capillary lymphatic vessels. In this way, lymph nodes are formed in the capillary lymphatic vessels, and the lymphatic fluid passes through the lymphatic circulation, passes through all levels of lymphatic vessels, and finally converges into the general lymphatic vessels, and finally is injected into the upper and lower vena cava of the body, so that it returns to the blood circulation again. Therefore, the main relationship between lymph and blood is that lymph is also a supplementary part of blood circulation, that is, some of the extravasated fluid in blood can be collected into lymphatic fluid through lymphatic vessels and finally returned to the blood circulation, so that lymph circulation and blood circulation are connected.