Transcellular fluid is the portion of total body water contained within epithelial lined spaces.. It is the smallest component of extracellular fluid, which also includes interstitial fluid and plasma. It is often not calculated as a fraction of the extracellular fluid, but it is about 2.5% of the total body water. Examples of this fluid are gastrointestinal fluid, cerebrospinal fluid, and ocular fluid, joint fluid, and bladder urine.
Due to the varying locations of transcellular fluid, the composition changes dramatically. Some of the electrolytes present in the transcellular fluid are sodium ions, chloride ions, and bicarbonate ions.
There are also varied functions for the transcellular fluid. In the joints, it serves a lubrication function, while the urine allows for the removal of electrolytes and molecules from the body.