# How is the blood volume of a living organism measured without killing it?

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.

• I guess it is possible with plethysmography. If someone confirms this I'll look for a detailed answer. Commented Apr 12, 2012 at 6:12
• Could this be done with radioisotopes? I'd imagine a set of scans from different angles could give a reasonable estimate of blood volume. Commented Apr 12, 2012 at 18:57
• Polynomial, can you please illustrate a bit more? Commented Apr 12, 2012 at 20:15
• @Polynomial, This seems to be it. tech.snmjournals.org/content/35/2/55.full Commented Apr 16, 2012 at 5:53
• Yeah, that's kind of what I was thinking about. My method was a little less refined and involved a lot of "glowing area measurement", but it's essentially the same principle. Commented Apr 18, 2012 at 17:34

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.

• How it is done according to my bioengineering textbook is injection of a water-soluble tracer into the heart or aorta, then 'listening' for the tracer somewhere else in the bloodstream. If you inject into the atrium and listen at the aorta, you can get cardiac output as well. It takes a minute or so, and gets a measure of the amount of fluid circulating, not counting blood that's trapped or basically not circulating(veins from leg muscles while sleeping, compartment syndrome, etc) while counting fluid that freely exchanges with the blood. Not technically accurate, but gives extra information. Commented Aug 12, 2014 at 21:21
• The same book described measuring cardiac output using ice cold saline injected directly into the atrium as a tracer, and it was written quite some time ago(70s iirc). On the whole I trust it as far as I can throw it. Commented Aug 12, 2014 at 21:23

It is easy.

Method A, simple, is based on "counting the fish in the pond" method.

1. Make rough estimate of amount of blood in the organism.
2. Choose a component of blood that is replenished slowly [from liver or marrow etc]. That takes time to replenish. Red blood cells are probably OK. Some easily measurable and slowly replenished component of blood. Let's call this component of blood G.
3. Take small blood sample and measure level of G, denote result G1.
4. Drain N ml of blood, ands refill same amount back with infusion of normal saline.
N can be taken as 10% of estimate of amount of blood.
5. Take small blood sample, measure level of G, denote result G2.

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.

• How long would you wait between samples given that you would presumably have to wait for "G" to diffuse throughout the entire blood stream? Commented Apr 14, 2012 at 13:03
• @Rory M: I would suspect diffusion in the blood stream to be almost negiglible. If I remember correctly it takes ~1 min for a complete recirculation of blood in a human, and for smaller animals it would be even less than that.
– nico
Commented Apr 14, 2012 at 16:38
• @Rory M: I'd expect minutes. Pharmacokinetics manual shall have answer to this. Wrt drugs administered intravenously, manual shall have the answer (speaking of humans of course). Commented Apr 14, 2012 at 19:09
• Your model isn't very accurate since it depends on making a "rough estimate" of the amount of blood before making any measurements. That's a huge bias that can't be overcome later. I also think your suggestion of removing 10% of the estimated blood volume isn't commendable. What happens in a case where you over-estimated the blood volume? Your model works, but only for an estimation. I think you should provide the equation for any readers who aren't very mathy.
– CHM
Commented Apr 16, 2012 at 3:59
• @CHM, I believe that N doesn't need to any particular value. The dilution series will allow one to back-calculate what x% N consist of. Commented Apr 16, 2012 at 5:41

You can use ultrasonography (echography) to measure blood flow.

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.

• I didn't downvote your answer, I'm not really in much of a position to judge it, but it does seem to be unnecessarily complicated. Commented Apr 17, 2012 at 12:28