Secretin is a hormone secreted by the duodenum, the first part of the small intestine, in response to acidic chyme arriving from the stomach. Its discovery in 1902 marked the birth of endocrinology: it was the first hormone ever identified. Secretin travels in the blood to the pancreas, stimulating the release of bicarbonate-rich pancreatic juice that neutralises stomach acid, and it also prompts the liver to produce bile and slows gastric emptying. This elegant feedback loop protects the delicate intestinal lining from acid burns and creates the right pH for digestive enzymes to work. The secretin story remains the classic illustration of chemical messengers coordinating digestion across organs.