| Ammonia (NH3) |
Aquatic organisms (e.g., fish) |
Gills, skin, nephridia (in some invertebrates) |
| Urea (NH2CONH2) |
Mammals, some adult amphibians |
Kidneys (producing urine) |
| Uric Acid (C5H4N4O3) |
Birds, reptiles (except some amphibians), insects |
Salt glands (in birds), reptilian kidneys (efficient water conservation) |
| Guanine (C5H5N5O) |
Certain marine birds, reptiles (e.g., marine iguanas) |
Specialized salt glands |
| Amino Acids and Urea |
Some terrestrial amphibians (e.g., frogs) |
Skin, urinary bladder, cloacal cells |
| None (Simple Diffusion) |
Lower organisms (e.g., protozoans) |
Lack specialized organs, rely on cell membrane diffusion |
