The Urea Cycle and Its Significance in Excretion

1. Introduction

The excretion of nitrogenous waste is one of the most fundamental and critical processes in animal physiology. Protein catabolism and the breakdown of nucleic acids inevitably produce ammonia (NH₃), a molecule that is highly toxic even at relatively low concentrations. To manage this metabolic challenge, organisms have evolved sophisticated biochemical strategies tailored to their environment and lifestyle. In ammonotelic organisms such as most aquatic invertebrates and bony fish, ammonia is excreted directly; in uricotelic organisms such as birds and reptiles, it is converted to uric acid; and in ureotelic organisms such as adult amphibians, cartilaginous fish, and all mammals including humans, ammonia is converted into urea, a far less toxic compound, before excretion. The process responsible for this conversion is the urea cycle, sometimes called the ornithine cycle or the Krebs-Henseleit cycle, named after Hans Krebs and Kurt Henseleit who first described it in 1932 (Krebs & Henseleit, 1932).