To maintain acid-base balance, it is necessary that the blood ammonia level be kept within narrow limits. This is accomplished primarily by which of the following?

• A. Synthesis of urea from ammonia
• B. Synthesis of glutamine from ammonia
• C. Excretion of ammonia in the bile
• D. Excretion of ammonia in the stools

Answer: (A)

Ammonia is a highly toxic base, so the body keeps its blood level very low. The liver does this primarily by converting ammonia into urea through the urea cycle. Urea is then carried to the kidneys and excreted in the urine, which effectively removes ammonia from the circulation and helps maintain acid-base stability. While tissues can detoxify ammonia by forming glutamine, and small amounts can be excreted via bile or in stools, these routes are not the main mechanism for keeping blood ammonia within normal limits.