Sodium and potassium are usually discussed separately, one as something to limit and the other as something to obtain. Physiologically they are two halves of the same system.
The pair defines the electrical state of every cell
Cells maintain high potassium concentrations inside and high sodium concentrations outside. A membrane pump moves both ions against their gradients continuously, at considerable energy cost.
The resulting difference in charge across the membrane is what makes nerve signaling and muscle contraction possible. Every impulse is a controlled, momentary collapse and restoration of that gradient.
Because the gradient is fundamental, blood concentrations of both minerals are regulated within narrow limits, mainly by the kidneys adjusting how much is retained or excreted.
Fluid volume follows sodium
Water moves toward higher concentrations of dissolved particles. Sodium is the dominant particle outside cells, so the amount of sodium the body retains largely determines extracellular fluid volume.
Greater volume in the circulation raises the pressure the vessels must contain. This is the most direct route from dietary sodium to blood pressure.
The kidney adjusts sodium excretion in response to hormonal signals, which is why the relationship between intake on any given day and blood pressure is not immediate or uniform.
Potassium pushes in the opposite direction
Higher potassium intake promotes sodium excretion by the kidney and relaxes blood vessel walls, both of which act against the pressure-raising effect of sodium.
This opposition is why researchers often examine the ratio of the two minerals in a diet rather than sodium in isolation. The same sodium intake behaves differently against different potassium backgrounds.
Potassium is concentrated in vegetables, fruits, legumes, dairy and unprocessed meats, while sodium is concentrated in processed and restaurant foods.
The American pattern shifts both at once
A diet built on packaged and prepared foods tends to be high in sodium and low in potassium simultaneously, because processing adds one while displacing foods rich in the other.
Most sodium in American diets comes from food prepared outside the home rather than from a salt shaker, which limits how much the shaker can accomplish.
Shifting toward minimally processed foods moves both minerals at once, which is a larger change than adjusting either alone.
Where individual guidance is required
Kidney disease, heart failure and several common medication classes change how the body handles potassium, sometimes making increased intake genuinely hazardous.
Salt substitutes replace sodium with potassium, which makes them a medical question rather than a shopping preference for anyone in those categories.
The general mechanism is well established, but the correct target for a specific person depends on kidney function and medications a clinician must review.