The first nitrogen atom of urea is contributed by carbamoyl phosphate and second is derived from aspartate. The synthesis of urea takes place while the intermediates are covalently bound to an ornithine skeleton. When urea is released ornithine is regenerated to re-enter the urea cycle. Thus, ornithine acts catalytically in the synthesis of urea. The carbon, nitrogen, and oxygen atoms of ornithine are not exchanged in the urea cycle. Its role as a catalyst is more obvious than the role of oxaloacetate in the citric acid cycle.