Both blood and body fluid are liquid substance. They have the same functions
moistening and nourishing, and both belong to yin by comparison with qi. Therefore there is a closely relationship between them.

Blood and body fluid both are derived from the essence of drink and food. So it said "Body fluid and blood are derived from the same source". Body fluid will become the components of blood when it inffuses into the vessels.

Under the pathological conditions, blood and body fluid affect each other. For example, when blood loses heavily, the body fluid outside the vessels may seep into the vessels to make up the insufficient blood volume, thus cause the insufficiency of body fluid outside the vessels, marked by thirst, polyuria and dry skin. Conversely, when body fluid loses heavily, the fluid inside the vess is will come out of the vessels, and result in the hollow of vessels, the depletion of body fluid and dryness of blood. For this reason the diaphoresis method should not be used for the hemorrhage patients, and the method of breaking blood with drastic herbs or bleeding method should not be used for the Patients with a severe loss of body fluid, due to polyhidrosis or vomiting.

These are the practical utilization of the theory of "body fluid and blood derived from the same source."