Digestive glands

Digestion depends on exocrine glands that manufacture and release the enzymes and other secretions the canal needs. There are three pairs of salivary glands around the mouth — the large parotid glands in the cheeks (draining via Stenson's duct), the sub-maxillary/sub-mandibular glands in the lower jaw (Wharton's duct) and the sublingual glands beneath the tongue (Bartholin's duct) — which together produce roughly a litre to a litre-and-a-half of saliva a day. The stomach wall is lined with gastric glands containing three cell types: chief (peptic/zymogen) cells that secrete gastric enzymes such as pepsinogen, parietal (oxyntic) cells that secrete both hydrochloric acid and Castle's intrinsic factor (needed for absorbing vitamin B12), and goblet cells that secrete protective mucus. The liver, the largest gland in the body, sits in the upper right abdomen just below the diaphragm; it is organised into two major and two minor lobes made up of functional units called hepatic lobules, wrapped in a connective-tissue sheath called Glisson's capsule, and it has a remarkable capacity for regeneration, with its cells replaced roughly every three to four weeks. Liver cells secrete bile, which is stored and concentrated in the gall bladder; bile then travels via the cystic and hepatic ducts into the common bile duct, which joins the pancreatic duct to form the hepato-pancreatic duct, guarded at the duodenum by the sphincter of Oddi. Beyond bile production, the liver destroys old and defective blood cells, stores and releases glucose as glycogen, stores fat-soluble vitamins and iron, detoxifies harmful substances, and synthesises non-essential amino acids and urea. The pancreas, the second-largest digestive gland, sits within …