Transcellular fluid, separated from other compartments by both endothelial and epithelial barriers, constitute about 4% of total body water
Total body water (TBW) is distributed within the intracellular and extracellular compartments. Intracellular fluid cannot be measured directly but is calculated as the difference between TBW and the measured extracellular water. Extracellular fluid can be measured directly. The extracellular fluid compartment can be further simplified into the intravascular and interstitial spaces. Intravascular space, which accounts for 20% of the extracellular fluid, contains the plasma volume which is approximately 8% of TBW or 5% of body weight. The interstitial space extends from the blood vessels to the cells themselves and includes the complex ground substance making up the acellular matrix of tissue. Although the water within the space is thought to be freely exchangeable, this water exists in two phases. The free phase contains water that is generally freely exchangeable and in a constant state of flux. The bound or gel phase is composed of water that is closely associated with glycosaminoglycans, mucopolysaccharides, and other matrix components. This water is much less freely exchangeable. An additional extracellular fluid compartment, the transcellular compartment, consists of water that is poorly exchangeable under normal circumstances. This fluid is separated from other compartments by both endothelial and epithelial barriers and includes cerebrospinal fluid, synovial fluid, water within cartilage and bone, fluids of the eye, and the lubricating fluids of the serous membranes. Together, these fluids constitute about 4% of TBW.