It is white crystalline solid produced in liver from NH3 and CO2. It is comparatively less toxic. If we take more proteins, more urea is excreted out. Normal blood urea level is 18-38 mg/100 ml of blood.
It is least toxic and insoluble in water. It is produced in liver by purine catabolism (adenine and guanine) in man and protein catabolism in birds and reptiles. It can be excreted out in semisolid or crystalline form.
Excretion of uric acid in crystalline form is helpful in conservation of body water. Uric acid is commercially extracted from bird droppings (guano).
It is metabolic waste of nucleotide metabolism. It is found in spiders and penguins.
It is found in marine body fish, mollusca and crustacea. It is produced from ammonia in the following way –
Ammonia —Mediation – Trimethylamin
It is a specialised product in birds. It is formed when benzoic acid present in food combines with ornithine.
It is a specialised product in mammals. It is produced from benzoic acid when the latter combines with glycine.
It is produced from creatine phosphate present in muscles. Increased level of creatinine is indicative of kidney damage.