The Extracellular fluid (ECF) denotes all the body fluid outside the cells. It is around 33% of the total fluid in the body and includes blood plasma and interstitial fluid, which make up around 97% of the ECF. The remaining portions are the transcellular fluid (around 2.5%) and lymph fluid. The ECF is the medium for the exchange of substances between the ECF and cells.
The ECF contains dissolved gases, nutrients, and ions. The composition is maintained by using ion channels, such as the sodium-potassium pump and voltage-gated ion channels, and by other mechanisms, such as the 'stirring' of the ECF by the circulatory system. This ensures that the body fluid compartments are in a state of chemical disequilibrium.
- Sodium (Na+) 145.0-153.0 mM
- Potassium (K+) ~5.0 mM
- Calcium (Ca2+) 1.0-2.0 mM
- Magnesium (Mg2+) 0.7-1.0 mM
- Chloride (Cl-) 99.0-113.0 mM
- Bicarbonate (HCO3-) 22.0-28.0 mM
- Phosphate (HPO4-) 0.8-1.5 mM