Explain Dyes for Measurement of Plasma Volume:

A number of dyes, commonly known as 'vital dyes', have the ability to combine with proteins. When such a dye is injected into the blood, it immediately forms a slowly dissociable union with the plasma proteins. Thereafter, the dye travels wherever the proteins travel. The dye almost universally used for measuring plasma volume is T-1824, also called as evans blue. In making determinations of plasma volume, a known quantity of the dye is injected and it immediately combines with the proteins and is dispersed throughout the circulatory system within approximately 10 minutes. After that a sample of the blood is taken and the red blood cells are removed from the plasma by centrifugation. Then by spectrophotometric analysis of the plasma, one can determine the exact quantity of dye in the sample of the plasma. From the determined quantity of dye in each millilitre of plasma and the known quantity of dye injected, the plasma volume is calculated. To be even more exact in measuring the plasma volume, the rate of loss of dye from the circulatory system during the interval of mixing must also be considered. On an average, 5% of the dye is lost per hour, part of it is carried into the interstitial spaces by the leakage of plasma proteins through the capillary walls whereas a part of it is excreted out into the urine. No vital dye enters the red blood cells. Hence, this method does not measure the total blood volume. The blood volume can be measured from the plasma volume, provided the haematocrit is determined, by using the following formula:

Blood Volume = Plasma Volume * 100 / (100 - 0.87 Haematocrit)