1. PATHOPHYSIOLOGY
Starling defined the physiologic forces involved in maintaining the
fluid balance between interstitial space and plasma. They include the
gradient of hydrostatic pressures, the differences in oncotic pressures,
the hydraulic and oncotic permeabilities of the blood vessel wall, and
the surface area available for exchange (Cho & Atwood, 2002; Michel,
Woodcock, & Curry, 2020). Net pressure filtration is slightly positive
for most organs and results in fluid extravasation that is balanced by
its removal via lymph flow (Figure 1 ). Major disturbances in
this balance that favors net filtration out of the vascular space (e.g.,
increased capillary hydrostatic pressure, decreased plasma oncotic
pressure as in hypoalbuminemia) or impaired return of fluid by
lymphatics from the extravascular space, will result in edema (Cho &
Atwood, 2002). Local factors contribute to the maintenance of a normal
capillary filtration rate. Capillary pressure regulation physiologically
serves to limit profound changes in the capillary filtration rate during
variations of arterial and/or venous pressures. In all tissues, an acute
increase in blood pressure immediately increases the blood flow.
However, in less than a minute, the blood flow returns almost at its
initial level despite the blood pressure remaining high. This
maintenance of a constant blood flow in spite of an increase in upstream
pressure is called blood flow autoregulation (Cracowski & Roustit,
2020). Autoregulation aims at maintaining a vessel wall tension constant
despite increased transmural pressure. It is a metabolic consequence of
hyperoxia and of the myogenic response, the latter preventing
overstretching of the blood vessels walls due to the high pressure and
its subsequent interstitial edema (Scallan, Huxley, & Korthuis, 2010).
Arteriolar vasoconstriction occurs in a lower limb if it is dropped to
below heart level, when capillary pressure rises approximately above 25
mmHg (Cracowski & Roustit, 2020). This phenomenon, referred to as the
venoarteriolar reflex, maintains capillary pressure and the
transcapillary filtration rate within normal ranges, protecting
capillary integrity from high hydrostatic pressures and is mostly
observed in the lower limbs during venous stasis (Low, 2004).