Secretin, the first animal hormone to be identified, is a basic polypeptide consisting of 27 amino acid residues, containing 11 different amino acids, also known as secretin or pancreatin. This hormone is produced by "S" cells and is mainly found in the duodenal mucosa, with small amounts also distributed in the jejunum, ileum and gastric antrum. Its main function is to promote the pancreas to secrete pancreatic juice rich in bicarbonate, thereby assisting the digestive process.