The thermodynamic principles describing how transvascular concentration differences of protein and other macromolecules (commonly known as oncotic gradients) induce fluid exchange are beyond the scope of this chapter (0pp®Dh§lm§r 1.9.90.). The effect of oncotic gradients can be readily and directly measured in hydrostatic pressure units using clinical oncometers. A simple oncometer is shown in Fig 1... Two compartments (A and B) are in communication but are separated by a semipermeable membrane which allows fluid exchange but completely prevents protein exchange. Compartment B contains pure crystalloid solution and compartment A contains plasma. The protein concentration difference (oncotic gradient) will draw fluid from B to A. This can be prevented by applying pressure to A. This applied pressure represents the oncotic effect measured in hydrostatic units ( Fig, 1 (a)). Alternatively, one could have allowed proteins to draw fluid into compartment A and measured the oncotic gradient as the difference in height between A and B ( Fig 1(b)). However, net accumulation of fluid in A dilutes the protein and decreases the oncotic effect.
Fig. 1 (a) Direct hydrostatic pressure equivalent to the oncotic effect is applied. (b) Fluid is allowed into compartment A as the result of the oncotic effect. There is autodilution of proteins. The hydrostatic gradient so developed counteracts the oncotic effect.
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