It is filled with interstitial fluid, including lymph. The interstitial compartment (also called 'tissue space') surrounds tissue cells. The volume of the intravascular compartment is regulated in part by hydrostatic pressure gradients, and by reabsorption by the kidneys. The average volume of plasma in the average (70-kilogram or 150-pound) male is approximately 3.5 liters (0.77 imp gal 0.92 U.S. gal). The blood represents both the intracellular compartment (the fluid inside the blood cells) and the extracellular compartment (the blood plasma). The main intravascular fluid in mammals is blood, a complex mixture with elements of a suspension ( blood cells), colloid ( globulins), and solutes ( glucose and ions). Its extracellular fluid (ECF) contains about one-third of total body water. The interstitial, intravascular and transcellular compartments comprise the extracellular compartment.

In the cell nucleus the fluid component of the nucleoplasm is called the nucleosol. It contains moderate quantities of magnesium and sulfate ions. In humans, the intracellular compartment contains on average about 28 liters (6.2 imp gal 7.4 U.S. gal) of fluid, and under ordinary circumstances remains in osmotic equilibrium. The cell membranes are the outer barrier. The cytosol and organelles together compose the cytoplasm. The cytosol is the matrix in which cellular organelles are suspended. The intracellular fluid (ICF) is all fluids contained inside the cells, which consists of cytosol and fluid in the cell nucleus. The science of fluid balance across fluid compartments has practical application in intravenous therapy, where doctors and nurses must predict fluid shifts and decide which IV fluids to give (for example, isotonic versus hypotonic), how much to give, and how fast (volume or mass per minute or hour).

The interstitial and intravascular compartments readily exchange water and solutes, but the third extracellular compartment, the transcellular, is thought of as separate from the other two and not in dynamic equilibrium with them. When illnesses upset the balance, electrolyte imbalances can result.

The movement of these molecules is controlled and restricted by various mechanisms. Water and electrolytes are continuously moving across barriers (eg, cell membranes, vessel walls), albeit often in small amounts, to maintain this healthy balance. The normal processes by which life self-regulates its biochemistry ( homeostasis) produce fluid balance across the fluid compartments. The extracellular fluids may be divided into three types: interstitial fluid in the 'interstitial compartment' (surrounding tissue cells and bathing them in a solution of nutrients and other chemicals), blood plasma and lymph in the 'intravascular compartment' (inside the blood vessels and lymphatic vessels), and small amounts of transcellular fluid such as ocular and cerebrospinal fluids in the 'transcellular compartment'. Ībout two-thirds of the total body water of humans is held in the cells, mostly in the cytosol, and the remainder is found in the extracellular compartment. The intracellular compartment is the space within the organism's cells it is separated from the extracellular compartment by cell membranes. The two main fluid compartments are the intracellular and extracellular compartments. The human body and even its individual body fluids may be conceptually divided into various fluid compartments, which, although not literally anatomic compartments, do represent a real division in terms of how portions of the body's water, solutes, and suspended elements are segregated. For the concept of informal shared public space in community planning, see Third place. For the postcolonial term, see Third Space Theory.

Major body fluid compartments