The principal nitrogenous exretory compound in humans is syntheised

In humans, the principal nitrogenous excretory compound (i.e., urea) is synthesised in liver by Ornithine cycle and is eliminated mostly through kidney as nitrogeneous excretory product. In liver, one molecule of $C{O}_2$ is activated by biotin and combines with two molecules of $N{H}_3$ in the presence of carbamyl phosphate and one molecule of of $H_{2}O$ is released.
Carbamly 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.
$2N{H}_2+C{O}_2\stackrel{\text{Arginase}}{\to }N{H}_2-CO-N{H}_2+H_{2}O$