Ammonia functions as a regulatory molecule to mediate adjustments in glomerular filtration rate in response to changes in metabolic rate.

In mammals, varying in size from mouse to elephant there is a close linkage between metabolic rate (MR) and glomerular filtration rate (GFR). This linkage has evolved because of the necessity to excrete nitrogenous wastes generated predominately by amino acid catabolism. A similar linkage is observed in birds, reptiles and fish, although in the latter vertebrate group gill excretion rate and GFR need to be considered as an interdependent co-ordinated system. A change in the rate of amino acid catabolism will result in a change in the rate of production of nitrogenous wastes requiring excretion and hence GFR (or GFR plus gill excretory rate in fish) must be appropriately re-set. It is hypothesized that ammonia functions as a regulatory molecule to mediate adjustments in excretory rate (GFR or GFR plus gill excretory rate) in response to such changes in amino acid catabolism.