Oncotic gradients offset hydrostatic gradients

Vascular membranes in fluid-exchanging vessels restrict, but often do not totally impede, protein escape from the vasculature to the interstitium. To the extent that they do, the resulting difference between vascular and interstitium protein concentrations results in an oncotic gradient Dp which opposes fluid filtration driven by the intravascular-interstitial hydrostatic gradient D P. The net filtration gradient is obtained by subtracting Dp, expressed in hydrostatic units as shown above, from D P. For any net gradient, the fluid filtration rate QF is directly related to the membrane surface area and its permeability to fluid. Surface area and fluid permeability are combined and expressed as the filtration coefficient KF. All the indices mentioned above are combined in an expression which has become known as the Starling equation:

Strictly speaking, every protein fraction exerts its own gradient. For simplicity, in the following discussion it will be assumed that the protein gradient reflects the transvascular albumin gradient, which is the most relevant component in humans.

The degree of restriction to protein escape varies. It is complete in the brain and is lowest in the lung (estimates of albumin retention in the vascular compartment vary between 60 and 80 per cent). Membrane permeability to protein results in a decreased oncotic effect for two reasons: protein accumulation in the interstitium reduces the protein concentration gradient across the exchanging vessels, and, for any given gradient, the oncotic effect is reduced by the degree of protein permeability (Hancock.eLal: 198.9). Therefore eq,n...(1), needs to be corrected using what has become known as the reflection coefficient s:

The pulmonary capillary hydrostatic pressure is assumed to be halfway between the mean pulmonary and left atrial pressures, at approximately 15 mmHg above atmospheric pressure. Since the interstitial pressure is close to atmospheric pressure, D P = 15 mmHg.

The oncotic pressure of normal plasma measured with a clinical oncometer is about 20 mmHg. Since about only 80 per cent of plasma albumin is retained by the vasculature, the transvascular protein gradient will be reduced accordingly. Because the reflection coefficient is 80 per cent of ideal (often expressed as s = 0.8), the oncotic effect is reduced correspondingly. Consequently Dp = 0.8 * 0.8 * 20 mmHg = 12.8 mmHg which, when subtracted from D P, leaves a small imbalance favoring continuous fluid filtration. The filtrate must be drained by the mechanisms listed previously for the lungs to stay dry.

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