While the endocrine system is concerned with the production of hormones, the digestive system is involved with processing food. Although the two systems seem quite different, the gut is actually the largest of the body's endocrine organs, making the endocrine system and digestive system closely linked. Inside the intestines, ordinary gut cells are interspersed with individual endocrine cells which make up what is called the enteric endocrine system. More than 30 hormones are produced by this system, which regulates the complex process of food's digestion, absorption and incorporation into cells. Endocrinology is the branch of medicine concerned with the endocrine system, while the branch relating to the digestive system is called gastroenterology.
The physiology of digestion involves close cooperation between the nervous system, endocrine system, and digestive system. As well as having its own endocrine system, the gut also has a nervous system, known as the enteric nervous system, which is linked to the central nervous system. Nerves help control food movement, intestinal blood flow and movement of substances across the gut wall. The hormones of the gut endocrine system regulate secretion of substances into the intestines, gut muscle contractions and factors such as hunger and fat metabolism.
Although the digestive tract is affected by hormones from other endocrine glands, it is most strongly controlled by its own hormones, the chemical messengers secreted by cells in the enteric endocrine system. The hormone gastrin is one of the first to be released during the process of digestion, and cells which produce gastrin, known as G cells, are located in the stomach lining. When food enters the stomach, G cells release gastrin into the blood stream. Gastrin binds to receptors in what are called parietal cells in the stomach lining and stimulates them to secrete acid. As a result of this connection between the endocrine system and digestive system, many potentially harmful microorganisms which enter the stomach with food can be neutralized.
When acid is released from the stomach into the small intestine this stimulates cells in the gut lining to produce a hormone known as secretin. Secretin causes the digestive organ known as the pancreas to release an alkaline fluid, which enters the small intestine and neutralizes the stomach acid. Cells in the bile duct, which leads out of the gall bladder, are also stimulated to release bicarbonate. This interaction between the endocrine system and digestive system protects the small intestine from acid burns. As the gut becomes more alkaline, secretin production is switched off.