EXCRETION—The Excretory System

When biologists use the term “excretion”, they are almost always referring specifically to how the body rids itself of metabolic wastes that contain nitrogen. Metabolic waste (typically urine) is different from digestive waste (feces). Feces are a mix of the unabsorbed portions of your food, bacteria, some sloughed intestinal cells, etc. It does contain a small amount of metabolic waste, such as the waste pigments from the liver’s recycling of hemoglobin. In some animals such as birds and reptiles, the feces and urine are mixed into one compound waste product just before they leave the body.

You also excrete metabolic wastes every time you breath out, since the CO2 you exhale came from the metabolic breakdown of nutrients in the cells of your body. This certainly qualifies as a metabolic waste, though we don’t refer to exhaling as excretion. Water is also a significant metabolic waste, since water is also formed in the respiration of nutrient molecules. But again, the term excretion specifically refers to nitrogen-containing metabolic wastes, most of which comes from the catabolic metabolism of amino acids, each of which contains an “amino group” that must be removed before further breakdown can occur.

This removed amino group comes off as ammonia ( NH3 ), the principle nitrogenous waste of animals. A great many of the simpler aquatic invertebrate animals such as jellyfish, sea stars, sponges, corals, worms, etc. continually excrete their ammonia into the surrounding water by simple diffusion or by some slightly more complicated process.

Land animals can’t diffuse their ammonia into air (it just won’t work for a number of reasons), and they can’t store ammonia internally because it is extremely toxic. Diluting the ammonia to a safe level isn’t an option either because this would require a lot of water which would burden the animal with too much weight and require that the animal have access to large amounts of water (some animals don’t).

Many animals, including some of the larger aquatic ones, have evolved ways of detoxifying their ammonia by converting it into other compounds that are less toxic —so they can be stored in the body for longer periods of time without extreme dilution. All these compounds are more complex than the ammonia, so it costs some energy to do this conversion, but it is worth it.

** Guanine—spiders convert their ammonia into guanine, a whitish material which has essentially no toxicity at all.
           In fact some spiders store large amounts of guanine in their abdomen, which can make this part of the body
           white or light gray.
** Uric Acid—Insects, most land snails & slugs, birds, and reptiles all convert their ammonia into uric acid, once
           again a whitish substance with low toxicity (though not as low as guanine above). Since about 65% of animal
           species are insects, this is the most common excretory waste of animals in general.
** Urea—Made by mammals (including obvious humans), sharks & some other fish, amphibians, earthworms, etc.
           Urea is more toxic than either guanine or uric acid, but still much less so than ammonia.