Nitrogenous waste products : Ammonia , urea, uric acid are some nitrogenous waste products excreted by our body. 

1. Ammonia : It is formed by the deamination of amino acids during the protein metabolism. Ammonia is highly toxic. It is highly soluble in water and needs a large amount of water for its excretion. The animals which excret ammonia are called ammonotelic organisms. For expmple bony fishes , aquatic amphibians and insects.

 2. Urea : It is less toxic than ammonia. Most mammals, fishes and amphibians excrete their nitrogenous waste products in the form of urea. Process of excretion of urea is called ureotelism and such animals are called ureotelic. It needs less water for excretion, hence helps in the conservation of water. Animals like mammals, terrestrial amphibians and marine fishes excrete urea. 

3. Uric acid : Reptiles and birds excrete uric acid and are called uricotelic. The uric acid is is excreted in the foprm of pellet and paste with minimum loss of water.