Loss of water through the skin and respiratory tract is obligatory. All respiratory surfaces must be moist so that gases can pass through them. Amphibians are limited in their geographical distribution primarily because their skin is a respiratory surface and, therefore, water loss from it cannot be curtailed. The water losses vary with environmental factors such as temperature, humidity, and wind. Because of this, amphibians must remain near a source of water which their skin can imbibe.
Loss of body water through "insensible perspiration" is not controllable; it is obligatory. The sweating that helps regulate body temperature is facultative, and it varies with weather and exercise. If sweating is prevented when the ambient temperature is high, the body temperature can rise explosively. This will cause death as surely as the dehydration which was prevented by not sweating would have.
Mammals and birds can minimize water loss by modifying the depth and rate of breathing. On exertion, the rate and depth increases, and corre spondingly, the loss of water through the airways and across the skin surface increases. Unless the organism intends to quit breathing and allows its body temperature to rise, some water must be lost in this way.
Losses through the digestive tract are often involuntary. Diarrhea and vomiting accompany many illnesses, and since these uncontrolled losses are from the digestive system, which secretes a volume twice that of the plasma each day, their unreclaimed losses are massive. Dehydration and electrolyte imbalance follow quickly if these losses are not made up. This is particularly crucial for infants, for whom the daily diet makes up 25 percent of the total body water. With dehydration, the volume of the circulatory system decreases and circulatory failure results. In addition, since the extracellular fluid compartments are continuous with the intracellular compartments, the fluid inside the cells becomes hyperosmotic, and metabolic reactions cannot take place.
The retention of water depends upon several conditions. The most important is the sources of water available. If fresh water is not available, a human will die after eleven to twenty days, depending upon the rapidity of onset of dehydration. By that time, the person will have lost 15 to 20 percent of initial body weight. The excretion of wastes, the act of breathing, and insensible perspiration (even in moderate temperatures with shade available) are accompanied by obligatory water losses that cannot be reduced.
For other organisms, water is present in excess and becomes a problem. Freshwater fish take in water through their respiratory surface, the gills. They must release great quantities of urine without losing the salts necessary to maintain proper internal osmotic conditions. The hormone aldo-sterone promotes that salt absorption from the nephrons.
For marine creatures, on the other hand, the entry of salt is a problem. They must eliminate the excess salt without losing too much of their precious body water. Because kidney function always involves water loss as well as the loss of ions, and because fish and reptiles do not concentrate urine efficiently, many of these vertebrates have evolved salt glands. The salt glands use metabolic energy to excrete sodium chloride with very little water. Each environment presents its own waterbalance problems to an organism. Yet even in the world's harshest, driest conditions—in the Antarctic—tiny mites and spiders, penguins, and predatory birds have found ways to live and obtain all the water they need in a land where water is solid most of the time.
See also: Kidneys and other excretory structures; Osmoregulation; pH Maintenance; Thermoregulation.
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