Dehydration

The majority of the body is made up of water, with up to 75% of the body's weight due to H2O. Most of the water is found within the cells of the body (intracellular space). The rest is found in the so-called extracellular space, which consists of the blood vessels (intravascular space) and the spaces between cells (interstitial space).

Total body water = intracellular space + intravascular space + interstitial space

Dehydration occurs when the amount of water leaving the body is greater than the amount being taken in. The body is very dynamic and always changing. This is especially true with water in the body. We lose water routinely when we: breathe, as humidified air leaves the body, when we sweat to cool the body, and when we urinate or have a bowel movement to rid the body of waste products.
In a normal day, a person has to drink a significant amount of water to replace this routine loss. If intravascular (within the blood vessels) water is lost, the body can compensate somewhat by shifting water from cells into the blood vessels, but this is a very short-term solution. Signs and symptoms of dehydration will occur quickly if the water is not replenished. The body is able to monitor the amount of fluid it needs to function. The thirst mechanism signals the body to drink water when the body is dry. As well, hormones like anti-diuretic hormone (ADH) work with the kidney to limit the amount of water lost in the urine when the body needs to conserve water.

Dehydration occurs because there is too much water lost, not enough water taken in, or most often a combination of the two.

Diarrhea: Diarrhea is the most common reason a person loses excess water. A significant amount of water can be lost with each bowel movement. Worldwide, more than four million children die each year because of dehydration from diarrhea.

Vomiting: Vomiting can also be a cause of fluid loss; as well, it makes it difficult to replace water by drinking it.

Sweat: The body can lose significant amounts of water when it tries to cool itself by sweating. Whether the body is hot because of the environment (for example, working in a warm environment), intense exercising in a hot environment, or because a fever is present due to an infection, the body uses a significant amount of water in the form of sweat to cool itself. Depending upon weather conditions, a brisk walk will generate up to 16 ounces of sweat (a pound of water).

Diabetes: In people with diabetes, elevated blood sugar levels cause sugar to spill into the urine and water then follows. Significant dehydration can occur. For this reason, frequent urination and excessive thirst are among the symptoms of diabetes.

Burns: Burn victims become dehydrated because water seeps into the damaged skin. Other inflammatory diseases of the skin are also associated with fluid loss.

Inability to drink fluids: The inability to drink adequately is the other potential cause of dehydration. Whether it is the lack of availability of water or the lack of strength to drink adequate amounts, this, coupled with routine or extraordinary water losses, can compound the degree of dehydration.