The pancreas lies just behind the lower part of the stomach. It is the second largest gland in the body and is both an endocrine and an exocrine gland. Its exocrine function is to produce digestive pancreatic juices and release them through the pancreatic duct to the intestine. The endocrine function of the pancreas is to control the levels of blood sugar. The cells that control blood sugar levels are called islets of Langerhans. These cells are scattered throughout the pancreatic tissue among the other pancreatic cells but are concentrated somewhat in the tail of the pancreas.
There are two kinds of cells in the islets: alpha and beta. The alpha cells secrete a hormone called glucagon and the beta cells secrete insulin. Insulin and glucagon work together to regulate the body's blood sugar level.
Glucagon is a catabolic hormone that accelerates the process of liver glycogenesis (a chemical process by which the glucose stored in the liver cells in the form of glycogen is converted to glucose. This glucose then leaves the liver cells and enters the blood). This process tends to increase the concentration of glucose in the blood.
Type I (adult-onset) diabetes mellitus occurs when the pancreas fails to secrete enough insulin and so fails to regulate the glucose concentration in the blood. It is also known as insulin dependent diabetes mellitus (IDDM). The normal glucose level for an average adult is about 80 to 120 milligrams of glucose in every 100 millilitres of blood. If the islets of Langerhans secrete too little insulin an excess of glucose develops. This occurs suddenly and is due to the autoimmune destruction of the beta cells. Although people who develop this form of diabetes have certain genes that make them more susceptible, a triggering factor (such as a viral infection) is required. With low insulin, there is breakdown of the body's reserve of fat, resulting in the production of organic acids called ketone bodies. This ketosis lowers the pH of the blood (acidosis) and can result in death.