The body water can be divided into two components – intracellular fluid (ICF) and extracellular fluids (ECF); the intracellular fluid is the major component and composes approximately two thirds of total body water (TBW).
The ICF volume represents the fluid content within the body's cells. This volume cannot be measured directly but is calculated as the difference between the measured TBW and the measured ECF volume. Potassium provides the osmotic skeleton for the ICF in much the same way that sodium provides the osmotic skeleton for the ECF. In general, ICF are stable and do not readily adjust rapidly to changes.
The proportion of ECF is much greater than that of ICF in preterm infants and reaches 60% of TBW at term. The membranes retaining this fluid allow the passive diffusion of water, whereas active transport mechanisms maintain an internal solute milieu different from that found outside the cells. K+, P2-, and Mg2+ are intracellular ions, and Na+ and Cl- are predominantly extracellular.
The extracellular fluids can be further divided into interstitial fluid (that between the cells) and the plasma, with the plasma volume representing approximately one-quarter of the extracellular fluid volume.
Interstitial fluid flow is the movement of fluid through the extracellular matrix of tissues, often between blood and lymphatic vessels. This flow provides a necessary mechanism for transporting large proteins through the interstitium and constitutes an important component of microcirculation.
Body fluids: Intracellular and extracellular fluids
