![]() In the previous equation, P is the hydrostatic pressure inside the capillaries (c) or the interstitial space (i) π is the oncotic pressure K f is a permeability (filtration) coefficient that includes the porosity and surface area of the capillary and σ is a reflection coefficient that relates the efficiency of the oncotic forces across this barrier. The plasma proteins, unlike the dissolved solutes, exert a force called oncotic pressure that retards the net movement of fluid out of the capillary. However, the plasma proteins (particularly albumin) do not move readily across the capillary wall. The solutes do not generate any osmotic pressure because they flow with the water across the barrier. In a capillary, water and dissolved solutes move equally well across the capillary wall. This situation is different from the movement of water across the cell membrane. The capillary wall is the barrier between these two compartments, and the movement of fluid across this barrier is described by the relationship formulated by the physiologist Starling. This fluid bathes the cells and is picked up by the lymphatic system and delivered back into the bloodstream in the common thoracic duct. Brazy, in Pathophysiology of Kidney Disease and Hypertension, 2009 B Movement of Water from Intravascular to Interstitial Space: The Starling Relationshipįluid leaves the intravascular space as blood is forced through the capillary bed.
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