Muscle breaks down Amino Acids for energy. The Amine group is picked up and carried by Glutamine and Alanine to the liver or the kidney.
NH4 is excreted in the kidney when bicarbonate is low. Bicarbonate is needed in the urea cycle, but it is also needed in acid-base buffering.
Free Ammonia and Ammonia from Aspartate will enter the urea cycle to form Urea.