In each organ the internal environment consists of two phases, or compartments; an interstitial fluid (ISF) compartment which bathes the cells, and has limited communication with the intracellular fluid (ICF) and the plasma compartment of the blood. These two compartments are separated by the capillaries of the circulatory system. With the exception of proteins most solutes are readily and rapidly exchanged across the capillaries between the ISF and blood plasma compartments and their compositions are essentially similar. The plasma volume (PV) and the ISF together comprise the extracellular fluid (ECF) compartment of the organ in which the cells exist and function. These three fluid compartments (ISF, PV and ICF) together comprise the Total Body Water (TBW) and account for about 60 % of the body weight of the average lean adult male. The remaining 40 % is composed of fat and solids as illustrated in the figure.

Functional efficiency depends on the maintenance of homeostasis in the ECF. Because of the mixing of these fluids from the individual organs by the circulation of the blood throughout the body, all organs share an essentially common extracellular environment. The kidney is the primary regulator of that environment.