QS Study

Starling equilibrium for Capillary Exchange

E. H. Starling pointed out over a century ago that under average circumstances, a state of near-equilibrium exists at the capillary membrane. E. H. Starling first demonstrates the four primary forces that determine whether fluid moves out of the blood into the interstitial fluid or in the opposite direction. The slight disequilibrium that does occur accounts for the small amount of fluid that is eventually returned by way of the lymphatics.

These forces are –

(i) The capillary pressure (Pc) which tends to force fluid outward through the capillary membrane. These capillary forces are surface tension and interfacial tension.

(ii) The interstitial fluid pressure (pif) which tends to force fluid in the ward through the capillary membrane. The forces that tend to move fluid out from the capillaries are the capillary hydrostatic pressure, the interstitial-fluid osmotic pressure and the interstitial fluid hydrostatic pressure, which is normally negative.

(iii) The plasma colloid osmotic pressure (Pp) which tends to cause osmosis of fluid in the ward through the capillary membrane. Plasma colloid osmotic pressure is an important determinant in the appearance of edema.

(iv) The interstitial fluid colloid osmotic pressure (Tif) which tends to cause osmosis of fluid outward through the capillary membrane.

Starling Equilibrium for capillary Exchange: The following chart shows the principles of the starling equilibrium. For this chart, the pressures in the arterial and venous capillaries are averaged to calculate the mean functional capillary pressure for the entire length of the capillary.

Fig: Starling equilibrium for Capillary Exchange