About one-third of total-body water is confined to the extracellular space. Approximately 75% of the latter is interstitial fluid, and the remainder is the plasma. The forces that regulate the disposition of fluid between these two components of the extracellular compartment frequently are referred to as the Starling forces. The hydrostatic pressure within the capillaries and the colloid oncotic pressure in the interstitial fluid tend to promote movement of fluid from the vascular to the extravascular space.