Both blood and body fluid are liquid substances and function to nourish and moisten the viscera and the body. Compared with qi, both blood and body fluid pertain to yin. Physiologically blood and body fluid depend on and transform into each other. Pathologically, blood and body fluid affect each other.

Blood is made up of body fluid and the nutrient qi. After the transformation by the spleen and stomach, body fluid is transported first to the vessels and then flows with blood to the whole body. Part of body fluid extravasates from the vessels and flows outside the vessels to moisten and nourish the viscera and the body. At the same time part of body fluid flowing outside the vessels enters the vessels again to participate in the production of blood. In fact, body fluid inside and outside the vessels frequently transforms and supplements each other. 

Normally there is a dynamic balance maintained between them. Under pathological condition, if great quantity of body fluid is consumed or if great amount of body fluid comes out of the vessels, it will lead to insufficiency of blood inside the vessels. On the contrary, if too much body fluid outside the vessels enters the vessels because of massive hemorrhage, it will lead to insufficiency of body fluid. As a result, a morbid state of scanty fluid and dryness of blood is caused due to insufficiency of body fluid and blood.

Sweat, transformed from body fluid, is closely related to blood. Blood deficiency, usually followed by insufficiency of body fluid, cannot be treated simply by diaphoresis because profuse sweating consumes body fluid and further aggravates the deficiency of blood. On the other hand, profuse sweating and scanty body fluid, often accompanied by insufficiency of blood, cannot be simply treated by blood-breaking therapy because excessive bleeding exhausts the blood and further aggravates the scanty state of body fluid. That is why it is said in Ling-shu that "the patients with massive bleeding cannot be treated by diaphoresis while the patients with profuse sweating should not be treated by bloodletting therapy."

Blood and body fluid not only depend on and trans- form into each other, but also share the same origin. That is to say that both of them come from the food nutrients. Such a relationship between blood and body fluid is generalized as "body fluid and blood sharing the same origin" in TCM.