Why does blood pressure increase from a lack of salt rather than an excess?

This statement is biochemically justified when considering the actual physiology of kidney function and the role of sodium in regulating blood pressure, particularly renal pressure.

What is renal pressure?

Renal pressure is a part of the overall arterial pressure that directly depends on the functioning of the kidneys, their ability to filter blood, and regulate the volume of fluid in the body. It is particularly sensitive to blood volume and the condition of the vessels passing through the kidneys.

Why can sodium deficiency cause an increase in renal pressure?

Sodium deficiency = activation of the RAAS system. When there is insufficient sodium (and thus salt) in the diet, the body perceives this as a threat to the water-salt balance. This triggers the renin-angiotensin-aldosterone system (RAAS):

• The kidneys begin to produce renin to increase pressure and retain sodium.
• This leads to vasoconstriction (narrowing of blood vessels), including in the kidneys.
• As a result, arterial and renal pressure increase to "force" more blood through the kidneys and recover lost sodium.

Fluid retention in sodium deficiency. In the case of sodium deficiency, the mechanism of sodium reabsorption in the renal tubules is activated — the body starts to "take back" not only sodium but also water. The result:

• Increased circulating blood volume (CBV).
• Increased pressure in the glomeruli of the kidneys.
• Disruption of microcirculation, especially in the deeper parts of the nephron.

Low sodium = chronic stress for the kidneys. Constant "starvation" for sodium is perceived by the kidneys as chronic stress. This:

• Increases the production of aldosterone, which retains sodium while simultaneously increasing potassium loss.
• This disrupts the electrolyte balance and exacerbates vascular resistance.
• In the long term, this can lead to hyperfiltration, fibrosis of the kidney vessels, and persistent increases in pressure.

What about excess salt?

The myth about the harm of salt is based on a simplified logic: "the more salt — the more water — the higher the pressure." However, in a healthy person with normal kidney function, excess sodium is quickly excreted if the diet contains enough potassium, magnesium, and water.

Moreover, with adequate salt intake (8–10 g per day):

• The activity of the RAAS decreases.
• The kidneys function normally.
• Correct osmotic pressure of the blood is maintained.

Conclusion

Instead of a hypotensive effect with a low-salt diet, we get:

• Hyperactivity of the vascular system,
• Fluid retention,
• And, ultimately, a sustained increase in pressure.