This nitrogen here came from the aspartate and this nitrogen came from this free ammonium. So once we formed the urea, it then moves via the bloodstream to our kidneys and the kidneys ultimately excrete that via urine. Now, let's actually look at the details of each one of these steps and let's begin with step number one. So in step number one, the enzyme that catalyzed the step is carbon phosphate synthetase. So this carboil phosphate synthetase ultimately utilizes two ATP, two ATPs in this three step process. So we begin with a carbon dioxide, but actually the carbon dioxide is in the form of Bicarbonate.