Biological Systems MCAT Practice Exam

Which organ synthesizes urea from ammonia?

1. Spleen

2. Liver

3. Kidneys

4. Skin

The correct answer is: Liver

The organ responsible for synthesizing urea from ammonia is the liver. This process occurs through the urea cycle, also known as the ornithine cycle. In this metabolic pathway, ammonia, which is a toxic byproduct of amino acid catabolism, is converted into urea, which is much less toxic and can be safely excreted by the kidneys. The urea cycle is crucial for detoxifying ammonia, and it involves several enzymatic steps that take place primarily in the liver. Once synthesized, urea enters the bloodstream and is filtered out by the kidneys for excretion via urine. This conversion is vital for maintaining nitrogen balance in the body, preventing the accumulation of ammonia that could lead to harmful effects. Other organs listed, such as the spleen, kidneys, and skin, do not have a primary role in synthesizing urea. The spleen is involved in filtering blood and managing immune response, the kidneys function to excrete waste and regulate water balance, and the skin helps in thermoregulation and water loss but does not participate directly in nitrogen metabolism.