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Plasma, Erythrocyte & Total Blood Volumes:


Plasma volume is commonly measured by injection of a substance, such as a labeled plasma protein, which does not cross the capillary membrane. One technique commonly used for this purpose involves the injection of radioactive 125I-albumin. A second common technique involves injecting dyes such as Evans Blue (T-1824), that bind tightly to plasma proteins.

Equilibration of the injected protein indicator in the PV is rapid requiring only about 10 minutes. Since the permeability of capillaries to protein is very low loss of indicator from the PV need not be considered if blood samples are drawn at between 10 to 15 minutes after injection. Plasma volume is then determined by the formula presented earlier in this chapter with the indicator-dilution technique.

The hematocrit is the fraction of total blood volume made up of cells and is measured by centrifuging a blood sample in a calibrated tube. The cellular elements are packed into the bottom of the tube and the volume of packed cells relative to the total sample volume gives the hematocrit. The hematocrit may then be used to calculate total blood volume using the formula in the figure on this page.

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