Among the more common disorders involving an imbalance in the water of body fluids are dehydration, water intoxication, and edema.

Dehydration

In 1994, the world watched in horror as thousands of starving people died in the African nation of Rwanda. It wasn't the lack of food that killed most of these people, but cholera, a bacterial infection that cripples the ability of intestinal lining cells to reabsorb water. The severe diarrhea that develops can kill in days, sometimes even hours. Dehydration is deadly.

Dehydration is a deficiency condition that occurs when the output of water exceeds the intake. This condition may develop following excessive sweating, or as a result of prolonged water deprivation accompanied by continued water output. In either case, as water is lost, the extracellular fluid becomes increasingly more concentrated, and water tends to leave cells by osmosis. Dehydration may also accompany illnesses in which prolonged vomiting or diarrhea depletes body fluids. During dehydration, the skin and mucous membranes of the mouth feel dry, and body weight drops. Also, severe hyperthermia may develop as the body temperature-regulating mechanism becomes less effective due to a lack of water needed for sweating. In severe cases, as waste products accumulate in the extracellular fluid, symptoms of cerebral disturbances, including mental confusion, delirium, and coma, may develop.

Because the kidneys of infants are less able to conserve water than are those of adults, infants are more likely to become dehydrated. Elderly people are also especially susceptible to developing water imbalances because the sensitivity of their thirst mechanisms decreases with age, and physical disabilities may make it difficult for them to obtain adequate fluids.

The treatment for dehydration is to replace the lost water and electrolytes. But if only water is replaced, the extracellular fluid will become more dilute than normal. This may produce a condition called water intoxication.

Water Intoxication

Babies rushed to emergency rooms because they are having seizures sometimes are suffering from drinking too much water, a rare condition called water intoxication. This can occur when a baby under six months of age is given several bottles of water a day, or very dilute infant formula. The hungry infant gobbles down the water, and soon its tissues swell with the excess fluid. When the serum sodium level drops, the eyes begin to flutter and a seizure occurs. As extracellular fluid becomes hypotonic, water enters the cells rapidly by osmosis. Coma resulting from swelling brain tissues may follow unless water intake is restricted and hypertonic salt solutions given. Usually, recovery is complete within a few days.

Water intoxication in infancy sometimes occurs among poverty-stricken families who dilute formula to make it last longer. Another source of water intoxication in infants is bottled water products that are sold alongside infant formula. Their placement on grocery shelves leads some parents to believe that these products are adequate nutritional supplements. Used this way, they can be dangerous.

Edema

Edema is an abnormal accumulation of extracellular fluid within the interstitial spaces. A variety of factors can cause it, including decrease in the plasma protein concentration (hypoproteinemia), obstructions in lymphatic vessels, increased venous pressure, and increased capillary permeability.

Hypoproteinemia may result from liver disease causing failure to synthesize plasma proteins: kidney disease (glomerulonephritis) that damages glomerular capillaries, allowing proteins to escape into the urine; or starvation, in which the intake of amino acids is insufficient to support synthesis of plasma proteins.

In each of these instances, the plasma protein concentration is decreased, which decreases plasma osmotic pressure, reducing the normal return of tissue fluid to the venule ends of capillaries. Tissue fluid consequently accumulates in the interstitial spaces.

Lymphatic obstructions may result from surgery or from parasitic infections of lymphatic vessels. Back pressure develops in the lymphatic vessels, interfering with the normal movement of tissue fluid into them. At the same time, proteins that the lymphatic circulation ordinarily removes accumulate in the interstitial spaces, raising osmotic pressure of the interstitial fluid. This effect attracts still more fluid into the interstitial spaces.

If the outflow of blood from the liver into the inferior vena cava is blocked, the venous pressure within the liver and portal blood vessels increases greatly. As a result, fluid with a high concentration of protein is exuded from the surfaces of the liver and intestine into the peritoneal cavity. This causes a rise in the osmotic pressure of the abdominal fluid, which, in turn, attracts more water into the peritoneal cavity by osmosis. This condition, called ascites, distends the abdomen. It is quite painful.

Edema may also result from increased capillary permeability accompanying inflammation. Recall that inflammation is a response to tissue damage and usually releases chemicals such as histamine from damaged cells. Histamine causes vasodilation and increased capillary permeability, so that excess fluid leaks out of the capillary and enters the interstitial spaces.

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