How Terrestrial Animals Maintain Salt and Water Balance
How Terrestrial Animals
Maintain Salt and Water
Balance
The problems of living in an aquatic environment seem small indeed compared with the problems of life on land. Since animal bodies are mostly water, all metabolic activities proceed in water, and life itself was conceived in water, it might seem that animals were meant to stay in water. Yet many animals, like the plants preceding them, moved onto land, carrying their watery composition with them. Once on land, terrestrial animals continued their adaptive radiation, solving the threat of desiccation, until they became abundant even in some of the most arid parts of the earth.
Terrestrial animals lose water by evaporation from respiratory and body surfaces, excretion in urine, and elimination in the feces. They replace such losses by water in the food, drinking water when available, and retaining metabolic water formed in cells by oxidation of foods, especially carbohydrates. Certain insects—for example, desert roaches, certain ticks and mites, and the mealworm—are able to absorb water vapor directly from atmospheric air. In some desert rodents, metabolic water gain may constitute most of the animals’ water intake.
Particularly revealing is a comparison of water balance in human beings, nondesert mammals that drink water, with that of kangaroo rats, desert rodents that may drink no water at all (Table 32-1). Kangaroo rats acquire all their water from their food: 90% is metabolic water derived from oxidation of foods (see Figure 4-14, and accompanying discussion of water yield during oxidative phosphorylation) and 10% as free moisture in food. Even though we eat foods with a much higher water content than the dry seeds that make up much of a kangaroo rat’s diet, we still must drink half our total water requirement.
Given ample water to drink, humans can tolerate extremely high temperatures while preventing a rise in body temperature. Our ability to keep cool by evaporation was impressively demonstrated more than 200 years ago by a British scientist who remained for 45 minutes in a room heated to 260° F (126° C). A steak he carried in with him was thoroughly cooked, but he remained uninjured and his body temperature did not rise.Sweating rates may exceed 3 liters of water per hour under such conditions and cannot be tolerated unless the lost water is replaced by drinking. Without water, a human continues to sweat unabatedly until the water deficit exceeds 10% of the body weight, when collapse occurs. With a water deficit of 12% a human is unable to swallow even if offered water, and death occurs when the water deficit reaches about 15% to 20%. Few people can survive more than a day or two in a desert without water. Thus people are not physiologically well adapted for desert climates but prosper there nonetheless by virtue of their technological culture.
The excretion of wastes presents a special problem in water conservation. The primary end product of protein breakdown is ammonia, a highly toxic material. Fishes easily excrete ammonia by diffusion across their gills, since there is an abundance of water to wash it away. Terrestrial insects, reptiles, and birds have no convenient way to rid themselves of toxic ammonia; instead, they convert it into uric acid, a nontoxic, almost insoluble compound. This conversion enables them to excrete a semisolid urine with little water loss. The use of uric acid has another important benefit. Reptiles and birds lay amniotic eggs enclosing their embryos (Figure 28-4,), together with their stores of food and water, and wastes that accumulate during development. By converting ammonia to uric acid, a developing embryo’s waste can be precipitated into solid crystals, which are stored harmlessly within the egg until hatching.
Marine birds and turtles have evolved an effective solution for excreting large loads of salt eaten with their food. Located above each eye is a special salt gland capable of excreting a highly concentrated solution of sodium chloride—up to twice the concentration of seawater. In birds the salt solution runs out the nares (see Figure 29-13). Marine lizards and turtles, like Alice in Wonderland’s Mock Turtle, shed their salt gland secretion as salty tears. Salt glands are important accessory organs of salt excretion in these animals because their kidneys cannot produce a concentrated urine, as can mammalian kidneys.
The problems of living in an aquatic environment seem small indeed compared with the problems of life on land. Since animal bodies are mostly water, all metabolic activities proceed in water, and life itself was conceived in water, it might seem that animals were meant to stay in water. Yet many animals, like the plants preceding them, moved onto land, carrying their watery composition with them. Once on land, terrestrial animals continued their adaptive radiation, solving the threat of desiccation, until they became abundant even in some of the most arid parts of the earth.
Terrestrial animals lose water by evaporation from respiratory and body surfaces, excretion in urine, and elimination in the feces. They replace such losses by water in the food, drinking water when available, and retaining metabolic water formed in cells by oxidation of foods, especially carbohydrates. Certain insects—for example, desert roaches, certain ticks and mites, and the mealworm—are able to absorb water vapor directly from atmospheric air. In some desert rodents, metabolic water gain may constitute most of the animals’ water intake.
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Particularly revealing is a comparison of water balance in human beings, nondesert mammals that drink water, with that of kangaroo rats, desert rodents that may drink no water at all (Table 32-1). Kangaroo rats acquire all their water from their food: 90% is metabolic water derived from oxidation of foods (see Figure 4-14, and accompanying discussion of water yield during oxidative phosphorylation) and 10% as free moisture in food. Even though we eat foods with a much higher water content than the dry seeds that make up much of a kangaroo rat’s diet, we still must drink half our total water requirement.
Given ample water to drink, humans can tolerate extremely high temperatures while preventing a rise in body temperature. Our ability to keep cool by evaporation was impressively demonstrated more than 200 years ago by a British scientist who remained for 45 minutes in a room heated to 260° F (126° C). A steak he carried in with him was thoroughly cooked, but he remained uninjured and his body temperature did not rise.Sweating rates may exceed 3 liters of water per hour under such conditions and cannot be tolerated unless the lost water is replaced by drinking. Without water, a human continues to sweat unabatedly until the water deficit exceeds 10% of the body weight, when collapse occurs. With a water deficit of 12% a human is unable to swallow even if offered water, and death occurs when the water deficit reaches about 15% to 20%. Few people can survive more than a day or two in a desert without water. Thus people are not physiologically well adapted for desert climates but prosper there nonetheless by virtue of their technological culture.
The excretion of wastes presents a special problem in water conservation. The primary end product of protein breakdown is ammonia, a highly toxic material. Fishes easily excrete ammonia by diffusion across their gills, since there is an abundance of water to wash it away. Terrestrial insects, reptiles, and birds have no convenient way to rid themselves of toxic ammonia; instead, they convert it into uric acid, a nontoxic, almost insoluble compound. This conversion enables them to excrete a semisolid urine with little water loss. The use of uric acid has another important benefit. Reptiles and birds lay amniotic eggs enclosing their embryos (Figure 28-4,), together with their stores of food and water, and wastes that accumulate during development. By converting ammonia to uric acid, a developing embryo’s waste can be precipitated into solid crystals, which are stored harmlessly within the egg until hatching.
Marine birds and turtles have evolved an effective solution for excreting large loads of salt eaten with their food. Located above each eye is a special salt gland capable of excreting a highly concentrated solution of sodium chloride—up to twice the concentration of seawater. In birds the salt solution runs out the nares (see Figure 29-13). Marine lizards and turtles, like Alice in Wonderland’s Mock Turtle, shed their salt gland secretion as salty tears. Salt glands are important accessory organs of salt excretion in these animals because their kidneys cannot produce a concentrated urine, as can mammalian kidneys.
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