In humans, Urea is the major nitrogenous waste generated in human beings. Urea is formed mainly in the liver and is released into the blood stream and removed by the kidneys in the urine.

Urea is formed in the liver by a cycle called as "Urea cycle" or "Ornithine cycle" or "Kreb Henseleit cycle". Ammino acids not needed are deaminated by an enzyme called " Oxidase" and ammonia is generated. Ammonia being toxic gets converted into Urea. 3 ammino acids that participate in the cycle are,

a) Ornithine combines with NH3 and carbon di-oxide to form citrulline.

b) Citrulline combines with NH3 to form arginine.

c) Arginine then decomposes to form urea and ornithine.

Ornithine can then be sent again in urea cycle.