In most organs, fluid is normally filtered across capillaries, passes through the interstitial spaces, and returns to the bloodstream via the lymphatic system. This process of fluid filtration is driven by a small imbalance in the hydrostatic and osmotic pressures that are normally exerted across the walls of capillaries. Inasmuch as 2-3 L of fluid are filtered across all capillaries in the body each day, regulation of this process is critical for the maintenance of normal plasma and interstitial volumes, with abnormalities of capillary filtration forces often resulting in abnormal fluid distribution and organ dysfunction.
Jv = fPc — Pif) — ^c — f where Jv is the rate of fluid flow, Kf is the filtration coefficient of the capillaries, Pc is the capillary hydrostatic pressure, Pf is the hydrostatic pressure of the interstitial fluid surrounding the capillaries, a is the reflection coefficient of the capillaries to plasma proteins, nc is the oncotic (colloid osmotic) pressure generated by plasma proteins within the capillaries, and ntf is the oncotic pressure generated by proteins in interstitial fluid. The term [(Pc — Pif) — a(^c — ^if)] is referred to as the net filtration pressure (NFP) when it has a positive value, and the net absorptive pressure (NAP) when it is a negative value. When an NFP exists, Jv is also positive and fluid flow across the capillary wall is directed from the plasma to the interstitial fluid (termed filtration). An NAP indicates that Jv is negative and fluid flows from the interstitial compartment to the plasma (termed absorption).