Animals produce carbon dioxide as a metabolic waste gas. The peculiar chemistry of carbon dioxide in water, discussed in Chapter 2, imposes a unique challenge to terrestrial animals. You will recall that carbon dioxide dissolves in water to form carbonic acid. Carbonic acid, in turn, dissociates to bicarbonate and hydrogen ions.
If an animal lives in water, getting rid of carbon dioxide is no problem: it simply leaves the body as bicarbonate ion. In fact, there are many physiological mechanisms for transporting bicarbonate across cell membranes or epithelial membranes. In air, though, bicarbonate must be converted back to carbon dioxide before the carbon can leave the body, and, as we saw in Chapter 2, this is difficult. Terrestrial animals, therefore, tend to retain respiratory carbon dioxide as carbonic acid, forcing the blood to be somewhat more acidic than the blood of aquatic animals. This has necessitated the evolution of new ways to cope with the greater acid load.
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