I keep this page for the part that diet advice tends to skip. The kidneys are the body’s main long-term control for blood pressure. They set it over days and years. They do this by changing blood volume, releasing hormones, and nudging the tone of blood vessels. In a healthy body their default job is to get rid of extra sodium and water.
The kidneys match how much sodium and water they pass to the current pressure. This loop is called pressure natriuresis. When pressure rises, they pass more sodium and water, and blood volume shrinks. When pressure falls, they hold on to both, and volume grows.
When blood flow to the kidneys drops, or sodium falls, the kidneys release renin. Renin starts the renin–angiotensin system. Angiotensin II tightens blood vessels. Aldosterone holds on to sodium and water. Vasopressin holds on to water. The result is higher pressure.
The kidneys also make local signals that adjust the width of blood vessels. Prostaglandins and nitric oxide widen them. Endothelin narrows them.
Sodium is the lever a person can pull. Eating less salt lowers blood pressure in most people who have high blood pressure. That is the sourced advice behind the blood pressure diet. For low blood pressure that lasts, the usual advice is the reverse. Drink enough water, and do not cut sodium without a reason.
Hypertensive kidney disease is the loop turning on itself. Kidney disease raises blood pressure. High pressure that goes untreated damages the kidneys. So protecting the kidneys is part of keeping pressure under control for the long run.