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Overview of Sodium

by James L. Lewis, III, MD

Most of the body’s sodium is located in blood and in the fluid around cells. Sodium helps the body keep fluids in a normal balance (see About Body Water). Sodium plays a key role in normal nerve and muscle function.

The body obtains sodium through food and drink and loses it primarily in sweat and urine. Healthy kidneys maintain a consistent level of sodium in the body by adjusting the amount excreted in the urine. When sodium consumption and loss are not in balance, the total amount of sodium in the body is affected. The concentration of sodium in the blood may be too low (see Hyponatremia (Low Level of Sodium in the Blood)) or too high (see Hypernatremia (High Level of Sodium in the Blood)).

Controlling blood volume

The total amount of sodium affects the amount of fluid in blood and around cells (blood volume). The body continually monitors blood volume and sodium (and other electrolyte) concentrations. When either becomes too high, sensors in the heart, blood vessels, and kidneys detect the increases and stimulate the kidneys to increase sodium excretion, thus returning blood volume to normal. When blood volume or sodium concentration becomes too low, the sensors trigger mechanisms to increase blood volume. These mechanisms include the following:

  • The kidneys stimulate the adrenal glands to secrete the hormone aldosterone. Aldosterone causes the kidneys to retain sodium and to excrete potassium. When sodium is retained, less urine is produced, eventually causing blood volume to increase.

  • The pituitary gland secretes antidiuretic hormone. Antidiuretic hormone causes the kidneys to conserve fluid. Then blood volume increases.

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