Finally, terrestrial life presents the problem of nitrogen. Nitrogen is a problem because proteins contain nitrogen. When proteins are used as metabolic fuels, one of the waste products is ammonia, NH3, which is highly toxic.

All animals, whether aquatic or terrestrial, have this problem, but it is rather more of a problem for terrestrial animals. The reason is that ammonia, like carbon dioxide, also reacts with water, but this reaction forms ammonium hydroxide:

In water, ammonia can simply leave the body as ammonium hydroxide. In air, however, the ammonia must leave either as ammonia gas (which is hard to do) or in the urine (which requires a lot of water loss).

Many terrestrial animals have solved this problem by incorporating the ammonia into less toxic substances. Urea, for example, is a common component of the urine of mammals, while the less soluble uric acid is the favored substance for birds and reptiles.

Table 7.1 summarizes some of the physiological adaptations made by animals for solving the water balance and body waste problems in different environments. Earthworms seem to have more in common physiologically with freshwater animals than with "typical" terrestrial animals.

animal living in sea water faces an environment that is typically saltier than the body fluids. Osmotic loss of water and diffusional influx of salts is the problem faced here, and animals living in sea water must produce small quantities of relatively concentrated urine.

These different hydric environments leave their mark physiologically: each environment presents a particular water balance challenge that is reflected in the structures of the kidneys of animals living there. Compare, for example, the nephrons of freshwater versus marine fishes. The nephron of a freshwater fish has a large and well-perfused apparatus for filtration, which produces large quantities of filtrate. The tubules are long, indicating a high capacity for reabsorption, and the tubule walls are relatively impermeable to water, so that salt reabsorption is favored over water reabsorption. These structural features underlie the production of the voluminous and dilute urine we expect for creatures living in fresh water. The nephrons of marine fishes, on the other hand, frequently have small and poorly perfused filtration structures, which produce filtrate sparingly. The tubules are also relatively short, indicating lower capacities for reabsorption, and permeable to both salts and water. Consequently, marine fishes' kidneys are designed to keep filtration rates low and to reabsorb as much water and salts from the filtrate as possible. The result is the small quantities of relatively concentrated urine we expect for creatures living in marine waters.

Terrestrial animals, like marine animals, live in a desiccating environment. The major difference between the two involves the physical forces that draw water from the body. In the marine environment, water is drawn from the body by osmosis. In a terrestrial environment, it is lost by evaporation. Since both must conserve water, the kidneys of terrestrial animals are constructed very much like those found in animals inhabiting marine environments.1

1. The kidneys of terrestrial vertebrates—like frogs and, to a lesser extent, reptiles—are similar in design to those of marine fishes, that is, they produce small quantities of relatively con-

Table 7.1 Physiological characteristics of animals living in marine, freshwater, and terrestrial habitats compared with those of the earthworm.

Animals' habitat

Physiological activity Freshwater Marine Terrestrial Earthworm

Salt flux Diffusion flux (TFF) Filtration flux (PF) Reabsorption flux (PF)

Water flux Osmotic flux (TFF) Evaporative flux (TFF) Filtration flux (PF) Reabsorption flux (PF)

Excretion Of ammonia Of carbon dioxide

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