The principal nitrogenous excretory compound in humans is...

In humans, the principal nitrogenous excretory compound (ie, urea) is synthesized in liver by ornithine cycle. In liver, one molecule of $\mathrm{CO}_2$ is activated by biotin and combines with two molecules of $\mathrm{NH}_3$ in the presence of carbamyl phosphate synthetase enzyme and 2ATP to form carbamyl phosphate and one molecule of $\mathrm{H}_2 \mathrm{O}$ is released. Carbamyl phosphate reacts with ornithine and forms citrulline. Citrulline combines with another molecule of ammonia and form arginine that is broken into urea and ornithine in the presence of an enzyme arginase and water. \begin{array}{r}2 \mathrm{NH}_{3}+ \mathrm{CO}}_{2} \stackrel{\text{ Arginase } }{\longrightarrow} \mathrm{NH}_{2}- \mathrm{CO}-\mathrm{NH}}_{2} \\ +\mathrm{H} }_{2} \mathrm{O} \end{array} urea is eliminated mostly through kidney as nitrogenous excretory product.