The series of biochemical reactions that converts ammonia, which is highly toxic, and carbon dioxide to the much less toxic urea during the excretion of metabolic nitrogen derived from the deamination of excess amino acids. In ureotelic vertebrates, these reactions take place in the liver. The cycle requires energy in the form of ATP, and involves the addition of two –NH2 groups and carbon dioxide to ornithine to form arginine, via the intermediates citrulline and arginosuccinate. The enzyme arginase then catalyses the removal of urea from arginine, and ornithine is re-formed. In mammals the urea is ultimately excreted in solution in urine. For elasmobranch fishes urea is an important component of body fluids, helping to maintain their osmolarity slightly above that of seawater. It is also their main nitrogenous waste product, being excreted across the gills.