Regulation of Fluid Exchange and Osmotic Equilibrium Between Intracellular and Extracellular Fluid
A frequent problem in treating seriously ill patients is maintaining adequate fluids in one or both of the intra-cellular and extracellular compartments. As discussed, the relative amounts of extracellular fluid distributed between the plasma and interstitial spaces are determined mainly by the balance of hydrostatic and colloid osmotic forces across the capillary membranes.
The distribution of fluid between intracellular and extracellular compartments, in contrast, is determined mainly by the osmotic effect of the smaller solutes— especially sodium, chloride, and other electrolytes— acting across the cell membrane. The reason for this is that the cell membranes are highly permeable to water but relatively impermeable to even small ions such as sodium and chloride. Therefore, water moves across the cell membrane rapidly, so that the intracellular fluid remains isotonic with the extracellular fluid.
In the next section, we discuss the interrelations between intracellular and extracellular fluid volumes and the osmotic factors that can cause shifts of fluid between these two compartments.
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