(also called interstitial fluid), the fluid in the intercellular and pericellular areas of tissues and organs in animals and man. Tissue fluid bathes all tissue elements and, together with the blood and lymph, constitutes the body’s internal medium. The cells absorb necessary nutrients from the tissue fluid and discharge metabolic products into it.

The chemical composition and the physical and biological properties of tissue fluid differ in the various organs and correspond to the organs’ morphological and functional characteristics. Tissue fluid is similar to blood plasma, but contains less protein (about 1.5 g per 100 ml) and different proportions of electrolytes, enzymes, and metabolites. The composition and properties of tissue fluid have a specific homeostasis, which protects organ and tissue cells from the effects of changes in blood composition.

Nutrients essential for the tissues enter the tissue fluid from the blood, and metabolites are removed from the tissue fluid through the histohematic connective tissue barrier. When the tissue fluid flows from the organs into the lymphatics, it becomes lymph. The volume of tissue fluid in a rabbit amounts to 23–25 percent of the body mass; in man it amounts to 23–29 percent, with an average of 26.5 percent. Many histologists regard as tissue fluids the cerebrospinal fluid, the pericardial fluid, the fluid of the anterior chamber of the eye, and the fluid of the pleural cavity.