According to Human Anatomy and Physiology, the body maintains a constant blood glucose level (roughly 80 to 100 mg of glucose in every 100 milliliters of blood) because the brain needs a constant supply of glucose to use for energy.
Insulin is a short peptide hormone produced by the pancreas. It is released in response to high blood glucose levels and causes body cells to increase their intake of glucose from the bloodstream. It also stimulates the liver to produce glycogen (a form of short-term glucose storage).
Glucagon is also produced by the pancreas, but it is released in response to low blood glucose levels. This hormone stimulates the liver to break glycogen down into glucose, which is then released into the bloodstream.
Insulin and glucagon both negatively regulate the other hormone's activity. This means that the body is not producing and breaking down glycogen at the same time.
The most prominent disease related to blood glucose levels is diabetes mellitus. A person suffering from this disease either does not produce enough insulin or has developed a resistance to insulin, meaning that his body cells do not respond properly to the hormone.