Intravascular fluids: Distribution

• Normal fluid balance: Water makes up 60% of total body weight, occupying intra- and extracellular fluid compartments. Extracellular fluid is composed of plasma and interstitial fluid, separated by a capillary membrane. Fluid movement across capillaries is dependent on the transendothelial and colloid osmotic pressure differences between the plasma and subglycocalyx space.

• Colloids (semisynthetics such as HES and plasma derivatives like albumin) have large molecular weights and do not easily cross the capillary membrane. This theoretically means that they may be more useful in resuscitation than crystalloids, but instead the effect is limited because of capillary leak that occurs in acute illness.

• The distribution of crystalloids depends on the solution’s osmolality and sodium content. Because the cell membrane is not permeable to sodium, there is no osmotic gradient between the ECF and ICF with normal saline and other balanced solutions. Because these are isosmotic with plasma, they remain in the ECF distributed proportionally between the interstitial fluid and plasma.

• 5% glucose is also isosmotic but does not contain sodium. Once infused, fluid distributes proportionally across the ICF and ECF.