|MadSci Network: Biochemistry|
I'm a simple biochemist (not a physiologist) but here it goes anyway. If you assume that an average adult male is 60% (50% for women) water by weight and that the total fluid volume in a person is the sum total of all their intracellular fluid (ICF, 55% of total body water) and extracellular fluid (ECF, 45% of total body water) then it becomes possible to calculate how much sodium is in a person.
Sodium levels are usually reported in millimoles or milliequivalents, but for our purposes, milliequivalence is the same as millimolar (mM) because sodium has only +1 charge.
The concentration of sodium in intracellular fluid is low (10 mM) - therefore the weight of sodium in a 70 kg male is:
0.01 mol/l x 58.5g/mol Na x 70000g x 0.6 (%water) x 0.55 (%ICF) = 13.5g
A similar calculation for ECF where the concentration of sodium is about 145 mM would be:
0.145 mol/l x 58.5g/mol x 70000g x 0.6 x 0.45 (%ECF) = 160.32g
Therefore the total amount of sodium is about 173.82g
You can convert this to a number of sodium ions using the molecular weight and Avogadro's number:
173.82g / 58.5g/mole x 6.023 x 10^23 molecules/mole = 1.78 x 10^24 Na ions
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