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).