A variation of the indicator dilution method (see preceding

A variation of the indicator dilution method (see preceding problem) is used to measure total blood volume. A known amount of a tracer is injected into the bloodstream and disperses uniformly throughout the circulatory system. A blood sample is then withdrawn, the tracer concentration in the sample is measured, and the measured concentration (which equals (tracer injected) / (total blood volume) if no tracer is lost through blood vessel walls] is used to determine the total blood volume. In one such experiment, 0.60 cm3 of a solution containing 5.00 mg/L of a dye is injected into an artery of a grown man. About 10 minutes later, after the tracer has had time to distribute itself uniformly throughout the bloodstream, a blood sample is withdrawn and placed in the sample chamber of a spectrophotometer. A beam of light passes through the chamber, and the spectrophotometer measures the intensity of the transmitted beam and displays the value of the solution absorbance (a quantity that increases with the amount of light absorbed by the sample). The value displayed is 0.18. A calibration curve of absorbance A versus tracer concentration C(micrograms dye/liter blood) is a straight line through the origin and the point (A = 0.9. C = 3μg/L). Estimate the patient’s total blood volume from these data.


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