Calculating An Animal’s Blood-Volume Without Killing It

There are many reasons why camels can survive the desert’s arid conditions:

1. Instead of wasting water while excreting urea, their bodies recycle part of it. The nitrogen can be used to make amino acids, the building blocks of protein.
2. Although they are warm-blooded, they still adjust their body temperature to the environment from about 37 to 40^oC.
3. Their fat is concentrated in their hump(s), so they have less insulation throughout the rest of the body. A camel does not store water in its hump(s) or in its stomach.
4. Whereas the blood of most water-deprived mammals becomes thicker, leading to poor circulation and dangerously high body temperatures, a camel’s blood vessels retain most of their water. How was this discovered?

In the 1950’s, Knut Schmidt-Nielsen and his wife injected a harmless dye in to a camel’s bloodstream. They waited a while for the dye to distribute itself evenly. Then they took a blood sample and measured the concentration of the dye. Then the camel went 8 days without drinking in the desert heat. Although it lost a lot of weight (over 40 litres of water), the concentration of the dye in the blood revealed that the blood had only lost about 1 litre of water. In other words the rest of the water had been lost from tissues.

This is the kind of calculation that the Nielsens used:

Suppose that the original concentration of the dye had been 4.95 mg/L in 100 L* of blood. If the concentration of the dye had then increased to 5.00 mg/L, using C₁V₁ = C₂V₂,

4.95(100) = 5.00V₂, would reveal V₂ to be 99 L, a change of only 1 L.

But how did they know that the camel had a 100L of blood without killing it ? Let’s say they had originally injected 8.0 mL of 6.19 g/L of dye. After even distribution of the dye(before the camel went 8 days without drinking), the concentration became diluted to 0.000495 g/L, then
C₁V₁ = C₂V₂,
(0.0080)(6.19) = 0.000495(0.008+V₂), would reveal V₂ to be about 100 L.

where

V₂  is the volume of the camel’s blood.Knut Schmidt-Nielsen who passed away three years ago(2008) wrote at the beginning of his memoirs:

 It has been said that schools impart enough facts to make children stop asking questions. Those with whom the schools don’t succeed become scientists.

Reference:

Scientific American. The Physiology of the Camel. December, 1959.

Picture from George Holton, The National Audubon Society Collection/Photo Researchers