How is blood volume calculated and measured in the human body?

[mktsgm]

TL;DR Summary

How the volume of blood that is circulating is calculated? Is it an approximation or can one really find out the actual volume of blood in our body?

We know that we humans have roughly 5 liters of blood circulating in our body. The volume also may be varying. We also know that its volume is tightly regulated and multiple organs are involved in it.

My question is, how we calculated that we have 5 liters of blood? Is it an approximation or did they have any study/research and found out a mechanism to measure and arrived at the approximate figure of 5 liters? Can the actual volume of blood be measured?

How do the doctors diagnose the low and high volume of blood?

 

[Vanadium 50]

Can the actual volume of blood be measured?

Sure. It's just not very healthy for the subject.

But why do you need an exact number? Why would you not expect natural variations to make a one-digit value insufficient?

 

[mktsgm]

Sure. It's just not very healthy for the subject.

But why do you need an exact number? Why would you not expect natural variations to make a one-digit value insufficient?

I wanted to know how the measurement/calculation is done. My interest is to know if it is just an approximation. If it is an approximation, how can we verify it? I wanted to know how far it is closer to the truth?

Actually, I have been studying the consequences of low blood volume and high blood volume. I wanted to know from this context.

I don't want to measure by invasive methods.

 

[Vanadium 50]

I wanted to know how the measurement/calculation is done.

Measurement: You drain the organism.
Calculation: there is a formula with input height, weight, etc.

 

[mktsgm]

Measurement: You drain the organism.
Calculation: there is a formula with input height, weight, etc.

Draining the organism is out of the question.

My question revolves around the formula with input height and weight. How can we depend upon a formula to determine the blood volume? Isn't it too simplistic to rely on Nadler's equation or Lemmens-Bernstein-Brodsky equation to determine the volume?

There has to be a more scientific way of estimating the volume. Perhaps from the hematocrit.

Also, I wanted to know if we can detect the change in the volume of blood?

 

[berkeman]

Draining the organism is out of the question.

Then this sounds like a question that you should post on an embalmer's forum...

 

[gleem]

Blood volume can be obtained using the radioisotope dilution method. By injecting a given amount of radioactive labeled components of the blood (both in volume and activity), allowing this sample to circulate and mix, and then removing a mixed sample and comparing it to the administered sample one can fairly accurately determine blood volume. There is about 66 ml/kg of blood in an average adult.

 

[BillTre]

Blood volume can be obtained using the radioisotope dilution method. By injecting a given amount of radioactive labeled components of the blood (both in volume and activity), allowing this sample to circulate and mix, and then removing a mixed sample and comparing it to the administered sample one can fairly accurately determine blood volume. There is about 66 ml/kg of blood in an average adult.

My conjectures:
Depending on what was labelled and how you define blood volume, this could produce accurate or inaccurate results.
I would take blood volume as fluids of the body containing red blood cells (rbc's).
The non-blood cell components of the body (in humans) exchange with non-rbc containing fluids as some of the blood is filtered to become lymph and intercellular fluids. Tracers not limited to the blood volume would reflect these additional volumes also.

Using a similar tracing method where the rbc's are somehow labeled, would restrict their dilution to the rbc containing volume of bodily fluids only. Tracer could be radioactive, but also could something optical like a fluorescent label stably linked to the rbc's or other things restricted to the blood only. I would expect this would be a more accurate method of blood volume determination.
A labelled albumin is mentioned below, but it seems to leak into other fluids.

Clinical methods according to Wikipedia:

Semi-automated system
Blood Volume may also be measured semi-automatically. The BVA-100, a product of Daxor Corporation, consists of an automated well counter interfaced with a computer.[4] It is able to report Total Blood Volume (TBV), Plasma Volume (PV) and Red Cell Volume (RCV) using the indicator dilution principle, microhematocrit centrifugation and the Ideal Height and Weight Method.[3] The indicator or tracer, is an I-131 albumin injection. An equal amount of the tracer is injected into a known and unknown volume. Clinically, the unknown volume is the patient's blood volume, with the tracer having been injected into the patient's blood stream and tagged to the blood plasma. Once the tracer is injected a technician takes five blood samples which undergo microhematocrit centrifugation to extrapolate true blood volume at time 0. The concentration of the I-131 in the blood is determined from the blood radioactivity against the standard, which has a known I-131 dilution in a known volume. The unknown volume is inversely proportional to the concentration of the indicator in the known volume; the larger the unknown volume, the lower the tracer concentration, thus the unknown volume can be calculated. The microhematocrit data along with the I-131 indicator data provide a normalized hematocrit number, more accurate than hematocrit or peripheral hematocrit measurements.[5] Measurements are taken 5 times in 6 minute intervals so that the BVA-100 can calculate the albumin transudation time to understand the flux of liquid through capillary membranes.

Blood volumes have also been measured in humans using the non-radioactive, carbon monoxide (CO) rebreathing technique for more than 100 years. With this technique, a small volume of pure CO gas is inhaled and rebreath for a few minutes. During rebreathing CO bindes to heamoglobin present in red blood cells and based on the increase in blood CO after the rebreathing period the volume of blood can be determined through the dilution principle (i.e. similar as the case for radioactive tracer methods). While CO gas in large volumes is toxic to humans, the volume used to access blood volumes corresponds to what would be inhaled when smoking one cigarette. While researchers typically use custom made rebreathing circuits, the Detalo Performance from Detalo Health has fully automated the procedure and made the measurement available to a larger group of users.[6]

There also appear to be calculations based on human body size that give you a number. I would consider these estimates, not actual determinations.

 

[gleem]

I did not describe the whole procedure and is more involved than I may have suggested. The method I described has been the gold standard for blood volume measurements for decades. It uses two isotopes one that labels the RBCs and one that only labels the plasma volume. It is very accurate if down correctly but because of the many steps, it is prone to human error and it takes many hours for the results to be available. The BVA-100 system introduced in 1998 has basically replaced the two isotope method because it is as accurate (less prone to error) and much faster.