# Total osmolarity of blood? Curious about water weight vs. biological

## Main Question or Discussion Point

Total "osmolarity" of blood? Curious about water weight vs. biological

I've seen plasma osmolality described on wikipedia:

http://en.wikipedia.org/wiki/Plasma_osmolality

This looks at the concentration of electrolytes in the "plasma solvent". However, plasma is the liquid of the blood, with cells removed.

I'm curious about the concentration of blood cells AND electrolytes in the "plasma solvent". Has anyone seen information on this?

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Borek
Mentor

In what units do you want blood cells concentration? Electrolytes are listed as molarity oir equivalents, these don't make much sense for erythrocytes or white blood cells.

In what units do you want blood cells concentration? Electrolytes are listed as molarity oir equivalents, these don't make much sense for erythrocytes or white blood cells.
I suppose units of cells/liter, or gram/liter.

I gave blood recently and I'm curious how that would affect my whole body's blood concentrations, the rate of recovery, and the amount of food energy required to make that recovery. I suspected that my cardiovascular performance was affected 24 hours later, but I want to see if there's a basis for that suspicion.

Borek
Mentor

Back of the envelope approach would be that you have around 5L of blood, assuming you donated 0.4 L and blood was immediately diluted back to 5L, new concentrations would be $\frac{5-0.4}{5}=0.92$ or 92% of the original. But it doesn't work this way - I suppose electrolytes are replaced almost instantly, as their concentration (just like pH) must be kept in a very narrow range. However, erythrocytes need weeks to be replenished and their amount is variable, so the 92% number can be reasonably accurate.

Back of the envelope approach would be that you have around 5L of blood, assuming you donated 0.4 L and blood was immediately diluted back to 5L, new concentrations would be $\frac{5-0.4}{5}=0.92$ or 92% of the original. But it doesn't work this way - I suppose electrolytes are replaced almost instantly, as their concentration (just like pH) must be kept in a very narrow range. However, erythrocytes need weeks to be replenished and their amount is variable, so the 92% number can be reasonably accurate.
Do you know about the energy required to replenish? I assume we'd calculate the concentration of erythrocytes and other molecules the body produces, then calculate the enthalpy of formation for the total cell growth, then figure out the conversion factor to attain that amount of energy from chemical energy of food.

I'm not sure how to calculate these things, though.

Borek
Mentor

You don't need concentrations, you can easily calculate number of erythrocytes. And I doubt anyone will calculate "enthalpy of formation" for a cell. They are not identical.