Dehydration is a deficiency of water in the body.

Dehydration is particularly common in older people, because their thirst center does not function as well as a younger person's. Therefore, an older person may not recognize that he is becoming dehydrated. Certain disorders such as diabetes mellitus (see Diabetes Mellitus (DM)), diabetes insipidus (see Pituitary Gland Disorders: Central Diabetes Insipidus), and Addison's disease (see Adrenal Gland Disorders: Addison's Disease) can increase the excretion of urine and thereby lead to dehydration.

At first, dehydration stimulates the thirst center of the brain, causing a person to drink more fluids. If water intake cannot keep up with water loss, dehydration becomes more severe. Sweating decreases, and less urine is excreted. Water moves from inside the cells to the bloodstream to maintain the needed amount of blood (blood volume) and blood pressure. If dehydration continues, tissues of the body begin to dry out, and cells begin to shrivel and malfunction. Symptoms of mild to moderate dehydration include thirst, reduced sweating, reduced skin elasticity, reduced urine production, and dry mouth. Brain cells are particularly susceptible to more severe levels of dehydration. Consequently, confusion is one of the best indicators that dehydration has become severe. Very severe dehydration can lead to coma.

Dehydration causes the sodium level in the bloodstream to increase (see When the Body Has Too Much Antidiuretic Hormone). However, the common causes of dehydration (such as profuse sweating, vomiting, and diarrhea) usually result in a loss of electrolytes (especially sodium and potassium). Thus, dehydration is often accompanied by a deficiency of electrolytes—sodium is lost, but because even more water is lost, the level of sodium rises. When electrolytes in the bloodstream are also deficient, water moves less readily from inside the cells to the bloodstream. As a result, the amount of water in the bloodstream is not replenished as it would normally be. Blood pressure can fall, causing light-headedness or faintness, particularly upon standing (a condition called orthostatic hypotension). If water and electrolytes continue to be lost, blood pressure can fall dangerously low, resulting in shock and severe damage to many internal organs, such as the kidneys, liver, and brain.

Treatment

Prevention is better than cure. Adults should drink at least 6 glasses of fluids daily. Fluid intake should be increased on hot days. Exercise, fever, and hot weather increase the body's need for water. For mild dehydration, drinking plenty of water may be all that is needed. If electrolytes (especially sodium and potassium) are also lost, they must be replaced. Flavored sports drinks have been formulated to replace electrolytes lost during vigorous exercise. These drinks can be used to prevent dehydration or treat mild dehydration. Drinking plenty of fluids and consuming a small amount of salt (for example, by taking salt tablets or drinking a sports drink) during or after exercise works as well. Before exercising, people with heart or kidney disorders should consult their doctor about how to safely replace fluids.

More severe dehydration requires treatment by a doctor. If blood pressure becomes very low, a solution containing sodium chloride is usually given intravenously. The intravenous solution is given rapidly at first and then more slowly as the person's physical condition improves.

Treatment is also directed at the cause of dehydration. For example, if the person has diarrhea, drugs to control or stop the diarrhea may be necessary in addition to replacement of fluids.

After treatment, people who are recovering from dehydration are monitored to make sure that they are drinking enough fluids to prevent dehydration.

Courtesy of: The Merck Manuals