Ammonia is formed in the liver by the process of oxidative de-amination of amino acids. Among all the modes of excretion, formation of ammonia requires least expenditure of energy. Ammonia is highly toxic to the surrounding tissues and hence it cannot stored be stored in the body for a long time. It is eliminated as soon as it is formed. Ammonia is highly soluble in water and requires a large quantity of water for its removal. 1 gram of ammonia requires 300-500 ml of water for its elimination.

Urea is formed in the liver by the addition of carbon dioxide to ammonia. This process is called ornithine cycle or urea cycle. Among all the modes of excretion, formation of urea requires moderate expenditure of energy. Urea is less toxic than ammonia. Hence it can be stored in the body for some time and can be excreted at a lower rate. Urea is soluble in water and requires a moderate quantity of water for its removal. 1 gram of urea requires 30 ml of water for its elimination.

Uric acid is formed in the liver from ammonia by inosinic acid pathway. In man and other mammals, small quantity of uric acid is formed in the body by breakdown of purine and pyrimidine nitrogen bases of nucleic acid. Among all the modes of excretion, formation of uric acid requires maximum expenditure of energy. Uric acid is least toxic. Hence it can be stored in the body for a long time. Uric acid is insoluble in water and requires very little water for its removal. 1 gram of uric acid requires 10 ml of water for its elimination.