After glucose is absorbed by the small intestine, it is transported to the liver, where many “decisions” are made regarding its fate. These decisions are hormonally effected, and include release of some glucose into the blood, storage of some as glycogen, and conversion of some to fat.
The hormones insulin and glucagon are produced by the pancreas; insulin is produced in response to high blood sugar, as after a meal, and tells body cells to take up glucose from the blood, while notifying the liver to store or convert any excess. Glucagon targets the liver when blood sugar is low, telling it to break down some glycogen and release it into the blood as glucose. The interaction of these hormones acts like a thermostat to maintain a relatively constant blood glucose concentration.
In individuals with diabetes mellitus type I, insulin is no longer produced by the pancreas, and must be supplemented by intravenous injection, or the afflicted individual will eventually die.
Why must insulin be injected intravenously, while other enzymes such as lactase can be taken orally, by pill.