Why and how do organisms excrete metabolic wastes (particularly nitrogenous wastes)?
Waste products generated in metabolic processes are often toxic, and therefore must be eliminated before they can harm the organism. The major metabolic wastes produced by animals include carbon dioxide, metabolic water, and nitrogenous wastes. Small aquatic organisms are able to get rid of wastes by simple diffusion across membranes. More complex animals with circulatory systems rely on kidneys to filter wastes out of the blood and eliminate them from the body. Carbon dioxide and metabolic water produced in respiration easily diffuse into the environment from respiratory surfaces. Nitrogenous waste excretion is more difficult, yet necessary. Elevated ammonia levels in the body can lead to convulsions, coma, and even death. This is because ammonium ions can substitute for potassium ions in ion-exchange mechanisms. Ammonia can also adversely affect metabolism and amino acid transport. Excessive amounts of ammonia in the system elevates bodily pH, which causes changes in the tertiary str