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How Diuretics Work in the Body

By Matthew Fox, MD

Diuretics are a type of medication that increase fluid loss through urine excretion. They most frequently do this by acting on the kidneys, but sometimes have a different mechanism. They are prescribed for a number of conditions such as high blood pressure, congestive heart failure, some kidney diseases, liver disease and the ingestion of certain toxins. It is important to consult with a physician before using diuretics and for the diagnosis and treatment of disease.

Loop Diuretics

The kidneys are made up of millions of tubes called nephrons. Fluids from the blood flow into the nephrons by passing through the walls of special blood vessels. Different segments along the length of the nephron perform various functions. One part of the nephron is called the loop of Henle. A special protein channel in the loop of Henle normally absorbs sodium, potassium and chloride out of the nephron. As a result, water follows and is reabsorbed back into the bloodstream. Loop diuretics block this protein channel. As a result, these electrolytes and fluid make their way through the nephrons and ultimately out of the kidneys and into the urinary bladder to be eliminated from the body.

Osmotic Diuretics

Osmotic diuretics such as mannitol are filtered from the blood into the nephron. Once in the nephron, they cannot be reabsorbed into the body. Water tends to follow these medicines because of their molecular characteristics. They essentially pull water into the nephron and subsequently out of the body.

Thiazide Diuretics

Thiazide diuretics act similarly to loop diuretics. Compared to loop diuretics, however, they work in a different part of the nephron and on a different protein channel. Thiazide diuretics act on part of the nephron after the loop of Henle, called the distal convoluted tubule. They bind to a protein channel and prevent the absorption of sodium and chloride. They also lower calcium excretion. Water follows sodium and chloride into the urine.

Potassium Sparing Diuretics

Potassium sparing diuretics block sodium resorption in the last part of the nephron called the collecting duct. This indirectly leads to less secretion of potassium into the nephron. Since many diuretics lead to the loss of potassium, this class of medications is often prescribed if a person is receiving another diuretic that is lowering potassium too much.

Carbonic Anhydrase Inhibitors

Carbonic anhydrase inhibitors act on an enzyme called carbonic anhydrase. They prevent the absorption of bicarbonate in the first part of the nephron, called the proximal tubule. This causes a mild diuretic effect.

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