: What biochemical processes in human body cause it to use-up sodium and force it to be excerted in urine? An average 70 kg adult human body contains roughly 100 grams (roughly 5000 mmol) of sodium total. At least 1.5 grams

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What biochemical processes in human body cause it to use-up sodium and force it to be excerted in urine?
An average 70 kg adult human body contains roughly 100 grams (roughly 5000 mmol) of sodium total. At least 1.5 grams (65 mmol) of this sodium (may be much more if sweating) needs to be replaced each day, much of it being exerted in urine.
The body needs to maintain 140 mmol/L sodium concentration in body water, with a rough estimate of half of our 70 kg human being 35 L water, so 35 L x 140 mmol/L = 4900 mmol total body sodium.
Without sweating, would a human starting with 140 mmol/L sodium, and zero sodium intake be able to maintain the 140 mmol/L (and this is without dumping water to maintain the concentration), or are there biochemical processes which force sodium in urine, processes in which recycling sodium is not possible? If so, what are the processes?
Imagine we start with a perfectly healthy 70 kg human specimen, with no disease or metabolic abnormality of any kind, having his all biomarkers at the level of median of healthy population. On day 1, after the first morning urination, we measure his blood serum sodium concentration and its 140 mmol/L. He is then for 24 hours in an environment which causes zero sweating (even if not practically possible, this is a theoretical exercise so we are allowed to make such assumptions). He is all day mostly sedentary and consumes an elemental diet with all nutrients equal to his RDA, except it has zero sodium. He consumes just the amount of sodium-free water which produces the median amount of urine of healthy population. All his urine is collected in a container for 24 hours, and then the amount of sodium in that urine and in his blood serum is measured. This is repeated for several days afterwards.
How could I calculate the theoretical daily total body sodium loss of such a theoretical non-sweating, zero-sodium consuming 70 kg human, and his expected 24-hour urine sodium and blood serum sodium on each day of the experiment?
We could also repeat this thought-experiment using exactly 1000 mg sodium per day in his elemental diet instead of zero sodium.

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