Figure 1

Schematic diagram of the primary forces defining transcapillary fluid movement. The opposing forces defining the steady-state net flow of fluid from the capillary into the interstitial space are defined by the hydrostatic pressure differences between the capillary lumen (P_c) and interstitial pressure (P_i) as opposed by the filtration coefficient (K_f) which itself is a function of the vascular endothelial cell integrity and the intraluminal glycocalyx. This net efflux of fluid out of the capillary into the interstitium is blunted by an opposing oncotic pressure gradient moving fluid in the opposite direction because capillary oncotic pressure (π_c) is greater than interstitial oncotic pressure (π_i). And like hydrostatic pressure-dependent flow, oncotic dependent flow is blunted by the reflection coefficient (σ) which like K_f is a function of the glycocalyx and vascular endothelial integrity. Under normal conditions (left side), both K_f and σ are high minimizing fluid flux resulting in a slight loss of plasma into the interstitium which is removed by lymphatic flow. However, if the vascular endothelium and glycocalyx are damaged (right side), oncotic pressure gradients play a minimal role because a large amount of protein-rich plasma translocated into the interstitial space minimizing the oncotic pressure gradient, whereas the constant P_c promotes massive fluid loss and interstitial edema.