The capillary acts as a selective filter. The movement of fluid across the capillary wall by filtration, between the capillary and the interstitial fluid, is governed by Starling forces. The Starling forces define water movement between the intravascular and extravascular spaces as the difference between hydrostatic forces forcing water out of the capillaries and osmotic forces drawing water into the intravascular space. The net fluid flux is directly proportional to the net driving pressure, which is outward at the arteriolar end and inward at the venous end of the capillary. This forms the basis of the plasma-interstitial fluid balance.
Qi = fluid movement across the capillary wall.
k = filtration constant for the capillary membrane (rate of filtration of fluid/ min per mm Hg per 100 grams of tissue). It is a measure of the leakiness of the capillary wall to water.
pp = plasma osmotic pressure.
pi = interstitial fluid osmotic pressure.
Pc = capillary hydrostatic pressure.
Pi = interstitial fluid hydrostatic pressure.
The mechanisms of tissue oedema can be considered in the light of the Starling forces as follows:
Increased capillary hydrostatic pressure, secondary to venous obstruction;
Reduced capillary plasma osmotic pressure, due to low plasma albumin levels as in liver disease or starvation;
Reduced interstitial hydrostatic pressure;
Increased interstitial fluid osmotic pressure, secondary to lymphatic vessel obstruction;
Alteration in the capillary filtration coefficient, i.e. increased capillary perme- S
o ability due to local inflammation, hypoxia or local toxins as in anaphylaxis. a,
Light pressure on the skin can lead to a white response, caused by pre-capillary S
s sphincter constriction. Harder pressure produces the triple response, which S
consists of a red reaction (dilatation of pre-capillary sphincters), a weal (raised capillary hydrostatic pressure and increased capillary permeability to proteins) and a flare (arteriolar dilatation).
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