Dehydration is the excessive loss of body fluids (water, along with electrolytes). Water makes up approximately 60 to 70 percent of an adult’s body weight (and a higher percentage in infants and children). Electrolytes are minerals the body needs to help maintain the balance of fluids. The major electrolytes include sodium, potassium, magnesium, phosphates, chloride and calcium.

Water is necessary for all of the body’s functions. Among its roles, water is essential to digestion, metabolism, regulation of body temperature, as well as carrying waste out of the body, maintaining blood pressure and helping cells communicate. Unlike food (which a healthy person can live without for weeks) an adult without water will die in four to five days.  

Water and electrolytes move freely in and out of cells (osmosis) according to a delicate balance primarily regulated by the kidneys and hormones (chemical messengers that regulate the body’s activities). When cellular fluid imbalances occur, the body automatically responds to restore the balance. For example, if there is too much sodium inside the cell, the body will send water inside to restore the proper balance of water and sodium. If there is too much water inside a cell, and not enough of other electrolytes, the body will restore balance by sending water out of the cell into the area between cells.

The body continually makes quick, automatic adjustments to achieve fluid balance. This constant maintenance of the balance of body fluids is called homeostasis.

However, water is not stored in the body and must be replenished to make up for its loss. Normally, about 10 cups (2.4 liters) of fluid are lost daily through:

• Urination (60 percent)
• Breathing and perspiration (35 percent)
• Stool (5 percent)

Fluid lost through breathing and regular perspiration is known as insensible water loss because people are essentially unaware of this process as it happens. Conditions that produce additional water and electrolyte loss include diarrhea, vomiting, intense sweating and breastfeeding.

The daily amount of water needed may vary from person to person. Hydration needs may be based on factors such as a person’s health, level of physical activity and climate. A rough guideline for healthy adults is about 8 to 12 cups a day from all sources. If bodily fluids are lost at higher than normal amounts due to illness or excessive sweating, then extra fluids are needed to replace those that are lost.

Urine output is the easiest way for most people to know if the body is adequately hydrated. Normal urine from a well-hydrated person will be abundant and appear clear or pale yellow. Infants who are well-hydrated will wet a diaper approximately every three hours (about eight diapers per day). If urine output decreases or becomes dark yellow, it is a signal that the body is becoming dehydrated.

Thirst is another signal that the body needs fluid. However, by the time a person experiences thirst, mild dehydration has already begun. Therefore, it is important to take in an adequate amount of fluids and food, and not rely solely on thirst to determine when to drink.

When not enough fluids are taken in to replace fluids lost, dehydration develops. As dehydration sets in, there is not enough water to move back and forth between cells in order to restore balance of body fluids. This lack of water can affect the entire body.

The heart pumps harder to maintain adequate blood flow to the organs. Blood pressure lowers as fluid levels diminish. Reduced blood flow to the skin and reduced levels of water keep the body from perspiring and dissipating heat. The distribution of nutrients the body needs and elimination of wastes becomes impaired. A person’s weight begins to drop rapidly as the body literally dries out. Eventually, the kidneys and other major organs begin to fail. Blood clots can form as the water content of blood is reduced. A person can develop seizures, slip into a coma and will eventually die without emergency intervention.

When dehydration develops from heat-related causes, additional complications may include heat exhaustion and heat stroke (a potentially fatal condition that requires immediate emergency intervention).

Dehydration is cumulative and often increases gradually over time if fluid loss is not replaced adequately. In those at higher risk (e.g., infants, small children, the elderly), dehydration can become moderate to severe in as little as 24 hours. Worldwide, severe dehydration caused by vomiting and diarrhea is a leading killer of children. In the United States, few infants and children develop severe cases of dehydration due to easy access to replacement fluids and medical care.

In addition, the more body fluid lost, the longer it takes to make up the deficit. The dehydrated body can only absorb so much fluid at a time, and needs time to reabsorb water gradually. The kidneys may no longer work efficiently due to dehydration and may be unable to quickly process the fluid received, which can lead to edema (excessive fluid retention). This type of fluid imbalance is the opposite of dehydration, and can also be harmful. In cases of moderate to severe dehydration where intravenous (I.V.) fluids are necessary, medical personnel closely monitor patients to ensure the kidneys are working properly and that the body can handle the reintroduction of fluids. If the body shows signs of fluid overload during rehydration, the rate of replacement is slowed.