What compound comprises the majority of the nonprotein-nitrogen fractions in serum?

What compound comprises the majority of the nonprotein-nitrogen fractions in serum?

• A. Uric acid
• B. Creatinine
• C. Ammonia
• D. Urea

Answer: (D)

The main concept here is how nitrogen is carried in the blood when it’s not bound to proteins. In serum, the largest nonprotein-nitrogen component is urea. It’s produced in the liver from ammonia through the urea cycle, converting the highly toxic ammonia into a much less toxic, water-soluble compound that travels in the blood to the kidneys to be excreted. Because this liver-to-kidneys pathway handles most of the amino acid nitrogen disposal, urea dominates the nonprotein-nitrogen pool in serum. Uric acid, creatinine, and ammonia exist in the blood as nitrogen-containing compounds, but they are present in much smaller amounts. Uric acid comes from purine breakdown; creatinine comes from muscle metabolism and is a steady, smaller contributor; ammonia is normally kept at very low levels because it’s rapidly converted to urea in the liver. Therefore, they don’t make up the majority of nonprotein-nitrogen in serum.

The main concept here is how nitrogen is carried in the blood when it’s not bound to proteins. In serum, the largest nonprotein-nitrogen component is urea. It’s produced in the liver from ammonia through the urea cycle, converting the highly toxic ammonia into a much less toxic, water-soluble compound that travels in the blood to the kidneys to be excreted. Because this liver-to-kidneys pathway handles most of the amino acid nitrogen disposal, urea dominates the nonprotein-nitrogen pool in serum.

Uric acid, creatinine, and ammonia exist in the blood as nitrogen-containing compounds, but they are present in much smaller amounts. Uric acid comes from purine breakdown; creatinine comes from muscle metabolism and is a steady, smaller contributor; ammonia is normally kept at very low levels because it’s rapidly converted to urea in the liver. Therefore, they don’t make up the majority of nonprotein-nitrogen in serum.