Sodium is one of the body's electrolytes, which are minerals that the body needs in relatively large amounts. Electrolytes carry an electric charge when dissolved in body fluids such as blood. (See also Overview of Electrolytes.)
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 amount (concentration) of sodium in the blood may be
The total amount of sodium in the body affects the amount of fluid in blood (blood volume) and around cells. The body continually monitors blood volume and sodium concentration. 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 vasopressin (sometimes called antidiuretic hormone). Vasopressin causes the kidneys to conserve water.
As people age, the body is less able to maintain fluid and sodium balance for several reasons:
Decreased thirst: As people age, they sense thirst less quickly or less intensely and thus may not drink fluids when needed.
Changes in the kidneys: Aging kidneys may become less able to reclaim water and electrolytes from the urine (concentrate urine), and, as a result, more water may be excreted in urine.
Less fluid in the body: In older people, the body contains less fluid. Only 45% of body weight is fluid in older people, compared with 60% in younger people. This change means that a slight loss of fluid and sodium, as can result from a fever or from not eating and drinking enough (sometimes for only a day or two), can have more serious consequences in older people.
Inability to obtain water: Some older people have physical problems that prevent them from getting something to drink when they are thirsty. Others may have dementia, which may prevent them from realizing they are thirsty or from saying so. These people may have to depend on other people to provide them with water.
The above situations can result in losing fluid or not consuming enough fluid and thus can cause a high sodium level in blood (hypernatremia) and/or dehydration. Because these situations are more common among older people, hypernatremia is also more common among them. Hypernatremia is poorly tolerated by older people and can result in confusion, coma, and death if severe.
Excess fluid and sodium also occur more commonly in older people because disorders that usually result in excess fluid (fluid overload)—heart failure, liver disorders, and kidney disease—are also more common in older people.
A low sodium level in blood (hyponatremia) is more common among older people. Hyponatremia usually results when the body retains too much fluid, as occurs in heart failure or liver disease. Hyponatremia also occurs in older people who take certain types of diuretics (thiazide diuretics such as hydrochlorothiazide), particularly if the kidneys are not functioning normally. Diuretics are sometimes called water pills. Using liquid nutritional supplements or receiving intravenous fluids that are low in sodium while in the hospital also may cause hyponatremia in older people.