Dietary advice on salt is usually framed as reducing sodium. The physiology is a balance between sodium and potassium, and modern diets have shifted both in the same unhelpful direction.
Both minerals move water
Sodium sits mainly outside cells, potassium mainly inside, and the difference across the membrane is maintained by pumps that run continuously.
Water follows dissolved minerals. Where sodium concentration rises, water is retained to dilute it, and blood volume rises with it.
Higher volume within a vessel network of fixed capacity raises the pressure inside it, which is the basic route running from salt intake to a blood pressure reading.
Potassium works in the opposite direction on the same system, relaxing vessel walls and encouraging the kidney to release sodium rather than reclaim it.
The kidneys handle both together
Kidney filtration reabsorbs sodium and excretes potassium through overlapping transport systems, so changing one alters the handling of the other.
Adequate potassium promotes sodium excretion. Low potassium intake makes the kidney hold onto more sodium than it otherwise would.
This is why blood pressure responds to the ratio between the two minerals more consistently than to sodium measured in isolation.
Industrial food shifted the ratio
Salt is added during manufacture for preservation, texture and taste, so most sodium in a modern diet arrives already inside products rather than from a shaker.
Potassium comes overwhelmingly from vegetables, fruit, legumes and tubers, which the same dietary pattern tends to displace.
The ratio therefore moves twice over: sodium rises because of what manufacturing adds, and potassium falls because of what manufactured food displaces from the plate.
Cutting the salt cellar addresses only the smaller share of the problem, since the majority of intake was never under the diner's control in the first place.
Sensitivity varies between people
Blood pressure response to sodium is not uniform. Some people show a marked change with intake, others very little, and the difference has genetic and age-related components.
Sensitivity generally increases with age as kidney function declines, which is part of why blood pressure tends to drift upward across adulthood.
Population advice is set for the average response, which will overstate the effect for some individuals and understate it for others.
Where medical supervision is required
Potassium is not universally safe to increase. Reduced kidney function impairs excretion, and certain blood pressure medications also raise potassium retention.
In those situations high-potassium foods and salt substitutes become a clinical matter rather than a general recommendation.
Anyone taking medication for blood pressure or kidney disease needs the balance set by a clinician working from their own blood results.