Kidneys and the Regulation of Blood Pressure

Along with the ureters, bladder, and urethra, the kidneys are the functioning organ of the urinary system. The two kidneys are located in the middle of the back, below the rib cages and on either side of the spine. Their function directly impacts blood pressure and the reverse is true; changes in blood pressure affect the kidneys ability to work properly.

Definition

• The kidneys serve the important function of taking wastes out of the blood while maintaining its volume. Every day the kidneys filter approximately 200 quarts of blood. As the blood flows through structures in the kidneys called nephrons, waste products and extra water are filtered out. They become urine, which flows through the ureters into the bladder, where it is stored until it is released. The kidneys regulate blood pressure through the release of renin and by maintaining the volume of blood.

Nephrons

• The nephrons are the functional units of the kidney. Each kidney has about a million nephrons and each nephron contains a structure called a glomerulus. The glomerulus is responsible for filtering the blood, keeping proteins and cells in the blood while removing extra fluid and wastes. Critical substances such as sodium, phosphorus and potassium are released back into the blood in the amount necessary to maintain the balance required for the body to function. Nephrons impact blood pressure by controlling the volume of blood in the body.

Blood Volume

• The total amount of blood circulating through the body has a direct relationship to blood pressure. If the volume drops, then blood pressure goes down. Likewise, an increase in blood volume results in a higher blood pressure. The volume of blood is determined by the kidneys in two ways: through the amount of water and salt they allow to be reabsorbed into the body and by the release of renin.

Renin

• The kidneys produce and secrete an enzyme called renin to regulate blood pressure. If blood pressure drops, renin is released, triggering the formation of a substance called angiotensin. Angiotensin influences blood pressure two ways. First, it constricts the blood vessels, causing an increase in blood pressure. Second, it stimulates the release of another substance that causes the kidneys to send more water, salt, and potassium back into the blood and that raises blood pressure.

Changes in Blood Pressure

• Either high or low blood pressure hurts the kidney's ability to function. Chronic high blood pressure damages the small blood vessels in the nephrons. The result is that they can't continue to filter wastes from the blood. Also, the blood pressure in the glomerulus is higher than the rest of the body. This difference in pressure helps force fluids across the membranes that filter the blood. Any change in blood pressure affects the difference in pressure and then the glomerulus can't filter the blood properly.

Kidney Disease

• If the kidneys are diseased or damaged, especially if glomerular disease is present, they will not return the proper amount of water and solutes back to the blood stream. Additionally, disease may affect the kidneys ability to produce and release the right amount of renin, resulting in either high or low blood pressure. It is important for those who have kidney disease to carefully maintain a normal blood pressure in order to assist better functioning of the kidneys.