Oncotic properties of proteins and other macromolecules

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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