In a typical adult female human, approximately 55% of the total body weight is composed of water. If we use 55 kg (121 lb) to represent the average adult female, the total volume of water in the body can be calculated to be about 30.25 L. Approximately 67% of total body water (or ~40% of total body weight) is in the intracellular fluid (ICF) compartment, which is the fluid that is present in the cytoplasm of all cells of the body. Therefore, the total volume of ICF is ~22 L (cytoplasmic volume of all cells combined). The intracellular fluid is separated from the extracellular fluid by the cell plasma membrane. Approximately 33% of the total body fluid (or ~20% of total body weight) is in the extracellular fluid (ECF) compartment. Therefore, the volume of ECF is ~11 L. The extracellular fluid (ECF) compartment is further subdivided into the intravascular compartment and the extravascular compartment. These two compartments are separated by the capillary endothelium (lining of the walls of the capillaries). The fluid in the intravascular compartment is the fluid that is present in all of body blood vessels. This fluid is referred to as the plasma. Plasma is the fluid portion of blood (i.e., whole blood minus the so-called formed elements; red cells, white cells, and platelets). The volume of plasma is about 2.35 L and is generally about 25% of the volume of ECF. The extravascular compartment is further subdivided into two compartments: interstitial fluid and transcellular fluid. Interstitial fluid is the fluid that directly bathes the cells and tissues in the body. It is generally about 75% of the volume of ECF (or ~8.25 L). Transcellular fluid is found in small amounts in different body regions and, in total, comprises a very small portion (though not unimportant) of the total body water. This fluid is generally separated from the plasma by an additional epithelial layer in addition to the capillary endothelium. Cerebrospinal fluid (the fluid bathing the brain and the spinal cord), intraocular fluids (aqueous and vitreous humors), inner ear fluids (endolymph and perilymph), pericardial fluid, peritoneal fluid, synovial fluids (in joints), as well as some other fluids are components of the transcellular fluid in the body. Transcellular fluid generally comprises up to about 2.5% of the total body water (or up to ~0.75 L). If we examine this figure carefully, and if we consider the functional interconnections of the fluid compartments, it becomes clear that there are three major fluid compartments: (1) intracellular fluid, (2) interstitial fluid, and (3) plasma. These fluid compartments are emphasized by the gray background color. The physical barrier between the intracellular fluid and interstitial fluid is the cell plasma membrane, and the physical barrier between the interstitial fluid and plasma is the capillary endothelium.