Plasma sodium is a ratio — sodium divided by the water it is dissolved in. Sweat carries away both, but sweat is more dilute than plasma, so the ratio starts moving the moment you begin. Set how much you sweat and how salty it is, then choose what you drink to replace it. Replace too little and sodium climbs. Replace the volume with plain water and it falls. Two errors, two opposite failures — watch the same body travel toward each.
Two failure modes on one map
The two ledgers underneath
Body mass change is the water ledger. Plasma sodium is set by both ledgers together — which is why the scale alone cannot tell the two failures apart.
Set a sessionthen change the drink and watch the ratio move
What you drink to replace it
The session
Plasma sodium140mmol/Lnormal
Body-mass change0.0%
This is a teaching model of a ratio, not a clinical tool and not advice for any person. It has no kidneys that adjust, no thirst that intervenes, and no hormonal feedback — it simply carries your numbers forward, which is what makes the mechanism visible and also why it overstates what a real body would do. Nothing here prescribes a fluid volume, a sodium amount, or a product for anyone. Symptomatic hyponatremia is a medical emergency handled by clinicians using a measured blood sodium, not something to manage from a chart. Body-mass change is used only as a short-term measure of fluid; it says nothing about body size, composition, or health, and no body here is better or worse than another.