The key enzyme in the initiation of the ornithine cycle is carbamoyl phosphate synthase I. The ornithine cycle, also known as the urea cycle, is the process of urea synthesis. The general process can be divided into four steps: synthesis of carbamoyl phosphate, synthesis of citrulline, synthesis of arginine, and hydrolysis of arginine to produce urea. The first of these steps is critical for the initiation of the ornithine cycle and is carried out in the mitochondria of hepatocytes. It is catalyzed by the key enzyme, carbamoyl phosphate synthase I, which catalyzes the synthesis of ammonia with carbon dioxide, to synthesize the energetic compound carbamoyl phosphate, which is reactive in nature, before moving on to the next step. The ornithine cycle helps the body to synthesize ammonia into urea in the liver, which is then excreted from the body via the kidneys. When hepatitis, cirrhosis, liver cancer and other diseases cause damage to liver cells, it may lead to elevated blood ammonia.