How is the blood-volume of an organism measured without killing it?
Note:The blood-volume of an organism is defined as the total volume of blood present inside that organism.
Polynomial gives a good hint. By adding in a known tracer of known amount that should only circulate in the blood stream, the concentration of the tracer when completely distributed will give the volume of distribution.
If the tracer only stays in the bloodstream (and that's a fairly large IF), then the VD will be equal to the VBlood. However, as many pharmacologists know, interactions with proteins and cells can skew that number. The blood volume is also reduced by the volume of the blood cells. However, using Andrei's method of dilution, one can merely swap the cell counts with the tracer concentration.
It is easy.
Method A, simple, is based on "counting the fish in the pond" method.
G2 will be smaller than than G1.
Now from G1, G2, and N it is easy to calculate organism's amount of blood. You can derive exact formula yourself.
From the blood flow measurements and vessel dimensions you can then determine blood volume!
If you want a full explanation on how blood flow is measured by Doppler ultrasonography, make sure you ask the question to the right people. They'll help you with the technicalities.