Extracellular fluid

Extracellular fluid (ECF) or extracellular fluid volume (ECFV) usually denotes all body fluid outside the cells. The remainder is called intracellular fluid (ICF). The ECF and ICF are the two major fluid compartments, which together account for total body water (TBW).

In some animals, including mammals, the ECF can be divided into two major subcompartments, interstitial fluid and blood plasma, which make up at least 97%. The extracellular fluid also includes the transcellular fluid, which comprises about 2.5%. It also includes the acellular portion of lymph by the obligate logic of the outside-the-cells definition, although discussions of ECF usually treat lymph as negligible or implicitly lump it together with the interstitial fluid. One way of viewing the ECF is that it has "two components: plasma and lymph as a delivery system, and interstitial fluid for solute exchange."

The interstitial portion of the extracellular fluid constitutes the milieu intérieur or "internal environment" which bathes the all of the body's cells. The ECF composition is therefore crucial for their normal functions. This constancy of the "internal environment" is achieved by means of a set of homeostats or negative feedback systems, which keep the pH, and the sodium (Na⁺), potassium (K⁺), and calcium (Ca²⁺) ion concentrations within very tight limits. The osmolality and glucose concentrations are similarly rigidly regulated, each within a very narrow range of values, as are the partial pressures of oxygen and carbon dioxide.

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