-Extremely soluble molecular waste
-And diffuse rapidly through water
-Toxic to animal
-And cannot be stored in the body
-Mammals are very sensitive to ammonia and cannot tolerate concentrations in excess of 0.02mg/100cm-3 blood.
-Formed in the liver of vertebrates by interaction of ammonia, produced by deamination and carbon dioxide produced by respiration
-In cyclical reaction known as the ornithine cycle
-Less soluble and less toxic than ammonia and is the main nitrogenous excretory substance in cartilaginous fish, certain bony fish, adult amphibian and mammals
-The normal level or urea in mammalian blood is 2.5-6.00moldm-3 blood
-Are ideal excretory products for some terrestrial organisms, (e.g birds, insects)
-And a pre-requisite for organisms producing a cleidoic egg)
-They combine a high nitrogen content with low toxicity and low solubility they can be stored in cells, tissues and organs
-And require a minimal amount of water for their excretion.