Section 12. Disorders of Nutrition and Metabolism

Chapter 155. Minerals and Electrolytes

Topics: Introduction | Calcium | Copper | Fluoride | Iodine | Iron | Magnesium | Phosphate | Potassium | Selenium | Sodium | Zinc

Sodium

Most of the body's sodium is located in the blood and in the fluid in the space surrounding the cells. Sodium is required by all cells in the body to maintain a normal fluid balance (see Section 12, Chapter 158). Sodium plays a key role in normal nerve and muscle function. Sodium is taken in through food and drink and lost 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 intake and loss are not in balance, the total amount of sodium in the body is affected. Changes in the total amount of sodium are closely linked to changes in the volume of water in the blood. A loss of sodium from the body does not necessarily cause the level of sodium in the blood to decrease but does cause blood volume to decrease. When blood volume decreases, blood pressure also decreases, heart rate increases, and light-headedness and sometimes shock occur.

Conversely, the blood volume increases when there is too much sodium in the body. When excess sodium accumulates in the body, extra fluid accumulates in the space surrounding the cells. As a result, the tissues, especially in the feet and ankles, swell (a condition called edema).

The body continually monitors blood volume. Sensors in the heart, blood vessels, and kidneys detect when blood volume becomes too high and stimulate the kidneys to increase sodium excretion, thus returning blood volume to normal. Sensors in the blood vessels and kidneys detect when blood volume is becoming low and trigger one of several mechanisms that result in an increase in blood volume. One such mechanism involves the adrenal glands, which secrete the hormone aldosterone. Aldosterone causes the kidneys to retain sodium and to excrete potassium (see Section 13, Chapter 164). Another mechanism involves the pituitary gland, which secretes antidiuretic hormone. Antidiuretic hormone causes the kidneys to conserve water. The retained sodium and water lead to decreased urine production, which eventually leads to an increase in blood volume.

Blood volume most closely parallels the total amount of sodium in the body, which cannot be directly measured with a laboratory test and requires assessment by a doctor. The sodium level in the blood, on the other hand, can be easily measured. Changes in the sodium level in the blood do not necessarily parallel changes in the total amount of sodium in the body or changes in blood volume and have a different set of causes.

See the sidebar When the Body Has Too Much Antidiuretic Hormone.

Hyponatremia

In hyponatremia, the level of sodium in the blood is too low. Hyponatremia occurs when sodium has been overdiluted in the body. Sodium can be overdiluted when people drink enormous amounts of water--as people with certain psychiatric disorders occasionally do--or when people who are hospitalized receive large amounts of water intravenously. In either case, the amount of fluid taken in exceeds the kidneys' capacity to eliminate the excess. Intake of smaller amounts of water--sometimes as little as 1 quart a day--can lead to hyponatremia in people whose kidneys are not functioning normally, such as people with kidney failure. Hyponatremia also often occurs in people with heart failure or cirrhosis. Excessive chronic loss of fluids, as occurs with chronic diarrhea, can also result in hyponatremia.

Another cause of hyponatremia is the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). People who have SIADH have a low sodium level because the pituitary gland secretes too much antidiuretic hormone. Hyponatremia also occurs in people who have underactive adrenal glands (Addison's disease (see Section 13, Chapter 164)) and who thus excrete too much sodium in the urine.

When the sodium level in the blood falls quickly, symptoms tend to develop rapidly and be more severe. The brain is particularly sensitive to changes in the sodium level in the blood. Therefore, symptoms such as lethargy and confusion occur first. As hyponatremia becomes more severe, muscle twitching and seizures may occur; stupor, coma, and death may follow. The diagnosis of hyponatremia is made by measuring the sodium level in the blood. Determining the cause of hyponatremia is more difficult and requires a full assessment by a doctor.

Mild hyponatremia can be treated by restricting fluid intake to less than 1 quart per day. Severe hyponatremia is an emergency. To treat it, doctors slowly increase the level of sodium in the blood with drugs, intravenous fluids, or sometimes both. Increasing the level too rapidly can result in severe and often permanent brain damage.

Hypernatremia

In hypernatremia, the level of sodium in the blood is too high. The body contains too little water relative to the amount of sodium. The sodium level in the blood becomes abnormally high when water loss exceeds sodium loss, as occurs with dehydration. In most people, hypernatremia results from dehydration (see Section 12, Chapter 158).

Hypernatremia occurs in people who drink too little water and in those who have diarrhea, vomiting, fever, excessive sweating (particularly during hot weather), or abnormal kidney function. For example, hypernatremia may occur in diabetes insipidus, in which the kidneys excrete too much water (see Section 13, Chapter 162). Other causes of hypernatremia include head trauma or neurosurgery involving the pituitary gland, disorders of other electrolytes (a high calcium level or low potassium level), sickle cell disease, and use of drugs (such as lithium, demeclocycline, or diuretics).

Hypernatremia is most common among older people, who tend to sense thirst more gradually and less intensely than younger people do. Older people who are bedridden or demented may be unable to obtain water even if they feel thirsty. In addition, the kidneys' ability to concentrate urine declines in advanced age, so older people are less able to conserve water. Older people who take diuretics, which force the kidneys to excrete more water, are at particular risk of hypernatremia--especially when the weather is hot or they become ill and do not drink enough water.

The most important symptoms of hypernatremia result from brain dysfunction. Severe hypernatremia can lead to confusion, muscle twitching, seizures, coma, and death. The diagnosis is made by determining that the sodium level in the blood is high.

Hypernatremia is treated by replacing water. In all but the mildest cases, dilute fluids (containing water and a small amount of sodium in carefully adjusted concentrations) are given intravenously. The sodium level in the blood is reduced very slowly, because correcting the condition too rapidly can cause permanent brain damage.