When a protein is eaten, it is broken down by proteases into amino acids in the alimentary canal, such as pepsin. The amino acids are then absorbed into the blood stream in the small intestine. The amino acids are then transported to the liver by the hepatic portal vein, where these amino acids are broken down even further by the removal of ammonia (NH3). The remaining part of the amino acid is recycled or oxidized in respiration. However, as ammonia is toxic, it is converted into urea (CO(NH2)2) by the addition of carbon dioxide (CO2). The urea is then excreted via the kidneys in urine.