Metabolic waste

Metabolic wastes or excretes are substances left over from respiratory processes, which cannot be used by the organism (they are surplus or have lethal effect), and must therefore be excreted. This includes nitrogen compounds, water, CO₂, phosphates, sulfates, indoles, medicals, food additives etc. Animals treat these compounds as excretes. Plants have chemical "machinery" which transforms some of them (primarily the nitrogen compounds) into useful substances, and it has been shown by Brian J. Ford that abscised leaves also carry wastes away from the parent plant. In this way, Ford argues that the shed leaf acts as an excretophore (= organ carrying away excretory products).

All the metabolic wastes are excreted in a form of water solutes through the excretory organs (nephridia, Malpighian tubules, kidneys), with the exception of CO₂, which is excreted together with the water vapor throughout the lungs. The elimination of these compounds enables the chemical homeostasis of the organism.

Nitrogen wastes

The nitrogen compounds through which the very toxic nitrogen is eliminated from the organism are ammonia, urea and uric acid. All of these substances are product from the protein metabolism.

Ammonia forms with the oxidation of amino groups (NH₂), which are removed from the proteins when they convert into carbohydrates. It is a very toxic substance and only one nitrogen atom is removed with it. A lot of water is needed for the ammonia excretion. Thus, the marine organisms excrete ammonia directly in the water (aminothelic organisms), while the terrestrial (mainly amphibians and mammals) convert ammonia into urea, a process which occurs in the liver and kidney.

Urea is a less toxic compound than ammonia; two nitrogen atoms are eliminated through it and less water is needed for its excretion. Urea is a protein metabolism product at vertebrates and part of the invertebrates. These organisms are called ureothelic.

Uric acid is a protein metabolism product of terrestrial invertebrates, birds and terrestrial reptiles. This is the reason of which these animals are called uricothelic organisms. In humans and anthropoides, this acid is purine metabolism product. Uric acid is less toxic than ammonia or urea; it contains four nitrogen atoms and a small amount of water is needed for its excretion. Out of solute, it precipitates and forms crystals.

Water and carbon dioxide

These compounds form during the catabolism of carbohydrates and lipids in condensation reactions, and in some other metabolic reactions of the amino acids.