sodium balance

Sodium balance is the bookkeeping the body keeps between the salt it takes in and the salt it gets rid of. Because sodium is the main particle that holds water in the bloodstream, controlling total body sodium is really about controlling how much fluid fills the circulation — and therefore blood pressure.

Nearly all sodium leaves through the kidney, so balance is set mainly by how much sodium the kidney reabsorbs versus excretes. Hormones tune this: aldosterone and angiotensin II tell the kidney to hold sodium when volume is low, while natriuretic peptides from the heart promote sodium loss when volume is high. Importantly, the body defends sodium content, not its concentration; concentration is guarded separately by water-handling through vasopressin and thirst.

Disturbed sodium balance shows up as fluid problems rather than as a simple high or low blood sodium reading. Holding too much sodium causes the swelling and high blood pressure seen in heart, kidney, and liver disease, whereas losing too much (as in untreated adrenal insufficiency) leads to low blood volume and low pressure. This is why dietary salt and salt-handling drugs are central to managing blood pressure.