What is Sodium?

Blood plasma, the liquid part of blood, is part of that fluid, so sodium sets blood volume too. A rise in sodium triggers thirst, and the kidney adjusts sodium and water together, which is why a sodium measurement is read as a statement about water balance rather than about salt intake.

The sodium-potassium pump moves the two in opposite directions across cell membranes, yet they report on different things: sodium on water balance, potassium on the electrical charge across those membranes. Measuring both keeps a shift in one from being mistaken for the other. eGFR, the estimated filtration rate, says how much filtering capacity the kidney has left for balancing sodium and water.

Fluid arriving and fluid lost through sweating, vomiting or diarrhoea is the main disturbance. Against that, the kidney varies how much sodium and water it releases, minute by minute, and thirst prompts the drinking that replaces what has gone.

Thirst and the kidney hold sodium inside a narrow band hour to hour, so it has no slow drift for repeated testing to follow. What a yearly measurement checks is whether that control is still working. Illness, persistent vomiting, heavy sweating and some medicines can each defeat it, and sodium then moves while diet has not changed. This is Do Health's monitoring schedule, not a general recommendation for everyone.