There are 3 major fluid compartments; intravascular, interstitial, and intracellular. Fluid movement from the intravascular to interstitial and intracellular compartments occurs in the capillaries. A capillary “membrane,” consisting of capillary endothelial cells and the subendothelial cell matrix, separates the capillary intravascular space from the interstitial fluid compartment. This capillary “membrane” is freely permeable to water and small-molecular-weight particles such as ions, glucose, acetate, lactate, gluconate, and bicarbonate. Gases such as oxygen and carbon dioxide diffuse freely through the capillary endothelial cell to enter or exit the intravascular compartment.

The interstitial compartment is the space between the capillaries and the cell. Fluids support the matrix and cells within the interstitial space. The intracellular compartment is separated from the interstitial space by a cell membrane. This membrane is freely permeable to water but not to small- or large-molecular-weight particles. Any particle movement between the interstitium and the cell must occur through some transport mechanism (eg, channel, ion pump, carrier mechanism).

Fluids are in a constant state of flux across the capillary endothelial membrane, through the interstitium, and into and out of the cell. The amount of fluid that moves across the capillary “membrane” depends on a number of factors, including capillary colloid oncotic pressure (COP), hydrostatic pressure, and permeability. The natural particles in blood that create COP are proteins—globulins, fibrinogen, and albumin. The hydrostatic pressure within the capillary is the pressure forcing outward on the capillary “membrane” generated by the blood pressure and cardiac output. Fluid moves into the interstitial space when intravascular hydrostatic pressure is increased over COP, membrane pore size increases, or intravascular COP becomes lower than interstitial COP.

Last full review/revision March 2012 by Rebecca Kirby, DVM, DACVIM, DACVECC; Andrew Linklater, DVM, DACVECC