Kidneys and Kidney Function

What do the kidneys do?
The kidneys regulate the body's fluid volume, mineral composition and acidity by excreting and reabsorbing water and inorganic electrolytes. This helps balance these substances (which include sodium, potassium, chloride, calcium, magnesium, sulfate, phosphate and hydrogen) in the body and keep their normal concentrations in the extracellular fluid. Body fluid volumes, which are regulated by the kidneys, are related to blood volume and the blood pressure in your arteries.

How does the kidney help regulate blood pressure?

Regulating blood pressure is linked to the kidneys' ability to excrete enough sodium chloride (salt) to maintain normal sodium balance, extracellular fluid volume and blood volume. Kidney disease is the most common cause of secondary hypertension (high blood pressure). Even minor disruptions in kidney function play a role in most (if not all) cases of high blood pressure and increased injury to the kidneys. This injury can eventually cause malignant hypertension, stroke or even death.

In normal people, when there's a higher intake of sodium chloride (salt), the body adjusts. It excretes more sodium without raising arterial pressure. However, many outside influences can reduce the kidneys' ability to excrete sodium. If the kidneys are less able to excrete salt with normal or higher salt intake, chronic increases in extracellular fluid volume and blood volume result. This leads to high blood pressure. When there is an increase in hormones and neurotransmitters that cause blood vessels to narrow, even small increases in blood volume are compounded. (This is due to the smaller area of blood vessel through which the blood is forced to flow.) Although the increases in arterial pressure lead the kidneys to excrete more sodium (which restores the sodium balance), higher pressure in the arteries may persist. This shows the important link between kidney disease and high blood pressure.