Ornithine (amino acid) combines with one molecule of NH3 and CO2 to produce citrulline (amino acid) and water. Citrulline combines with another molecule of ammonia and forms arginine (essential amino acid) and water. Arginine is broken into urea and ornithine in the presence of an enzyme arginase and water. Thus the cycle is repeated. This is called urea or ornithine or Krebs-Henseleit cycle after the names of its discoverers.
Most of the urea is produced in the liver. The liver cells continuously release urea into the blood and kidneys withdraw it from the blood to excrete it in urine.
Therefore, in the ornithine cycle, ammonia and CO2 from the blood are converted into urea and is excreted out of the body in the form of urine.