As all blood vessels allow a degree of protein leak , true equilibrium across the membrane cannot occur and there is a continuous flow of water with small solutes.
The molecular sieving properties of the capillary wall reside in a recently-discovered endocapillary layer rather than in the dimensions of pores through or between the endothelial cells.
[1] This fibre matrix endocapillary layer is called the endothelial glycocalyx.The Starling equation describes that relationship in mathematical form and can be applied to many biological and non-biological semipermeable membranes.
The rate at which fluid is filtered across vascular endothelium (transendothelial filtration) is determined by the sum of two outward forces, capillary pressure (
The colloid osmotic pressure difference that opposes filtration is now known to be π'p minus the subglycocalyx
causes unopposed diffusion of interstitial proteins to the subglycocalyx space, reducing the colloid osmotic pressure difference that was driving absorption of fluid to the capillary.
The Michel-Weinbaum Model explains how most continuous capillaries are in a steady state of filtration along their entire length most of the time.
Transient disturbances of the Starling forces return rapidly to steady state filtration.
In some texts the product of hydraulic conductivity and surface area is called the filtration co-efficient Kfc.
[citation needed] Staverman's reflection coefficient, σ, is a unitless constant that is specific to the permeability of a membrane to a given solute.
[5] σn corrects for the partial permeability of a semipermeable membrane to a solute n.[5] Where σ is close to 1, the plasma membrane is less permeable to the denotated species (for example, larger molecules such as albumin and other plasma proteins), which may flow across the endothelial lining, from higher to lower concentrations, more slowly, while allowing water and smaller solutes through the glycocalyx filter to the extravascular space.
Glomerular capillaries have a continuous glycocalyx layer in health and the total transendothelial filtration rate of solvent (
The Starling equation can describe the movement of fluid from pulmonary capillaries to the alveolar air space.
Infusing intravenous fluids that raise plasma colloid osmotic pressure (colloid therapy) has much less effect on plasma volume than originally expected, in part because the initially reduced filtration rate allows the concentration of proteins in the subglycocalx spaces to rise, returning the colloid osmotic pressure difference and trans endothelial solvent filtration rate to their steady state levels within an hour.
Prevention and treatment of oedema (excess interstitial fluid) depends on normalisation of
[10] Starling can be credited with identifying that the "absorption of isotonic salt solutions (from the extravascular space) by the blood vessels is determined by this osmotic pressure of the serum proteins" in 1896.