Introduction

Water is the main component of blood and cells. It fills most of the spaces around cells. To function normally, the body must keep the amount of water in these areas in balance and relatively constant. Too little water (dehydration) or too much water (overhydration) in the body can cause problems.

The water in the body contains dissolved minerals called electrolytes. They include sodium, potassium, and calcium. The body must also keep levels of electrolytes in balance and relatively constant. The balance of electrolytes is closely tied to the balance of water in the body: If one changes, the other usually also changes.

To maintain water and electrolyte balance, the body must replace water and electrolytes that are lost as the body performs its necessary functions. The body loses water and electrolytes primarily in urine, produced by the kidneys. Water and electrolytes are also lost in sweat, feces, and air that is breathed out. The body obtains water and electrolytes primarily from beverages and foods consumed. A healthy body can adjust the amount of water and electrolytes lost and consumed. Thirst, hunger, and the kidneys help with these adjustments. For example, a person who feels thirsty usually drinks more fluids. When a person becomes dehydrated, the brain releases a hormone called antidiuretic hormone. This hormone signals the kidneys to retain more water by making and excreting less urine.

As the body ages, it changes in ways that make older people more likely to have problems with water and electrolyte balance. The older body contains less water. Water accounts for 60% of body weight in healthy young people but for only 45% in healthy older people. In older people, the kidneys are less able to regulate the excretion of water and to concentrate urine as needed. Therefore, more water may be lost in urine. Also, older people often do not drink enough water, especially on hot days, partly because they tend to be less thirsty. If older people have problems with walking, they may not be able to get themselves enough water to drink. Older people who have urinary incontinence may drink less because they are worried about getting to a bathroom in time.

Many disorders, especially those that cause fever, vomiting, or diarrhea, can result in problems with water and electrolyte balance. These disorders may be short-lived (for example, pneumonia) or chronic (for example, kidney failure). Many drugs, especially diuretics, can also cause problems.