The body normally keeps the blood glucose concentration between about 70 and 140 mg/dl. Two hormones (chemical messengers) play important roles in keeping the glucose concentration within this normal range. They are released from glands (specialized cells) into the bloodstream. The blood carries the hormones to other cells where they cause a specific response. The two hormones that regulate glucose in the body are insulin and glucagon, which are both secreted by special cells in the pancreas. The cells of the pancreas can sense small changes in blood glucose concentration. Because they are so sensitive, the cells of the pancreas can respond to changes before the blood glucose concentration can increase or decrease much. This process is occurring constantly to maintain stability within the system. 

The pancreas is constantly producing and releasing small amounts of insulin and glucagon, as they have opposite effects to control blood glucose levels. After a person eats a meal high in carbohydrates (which readily breaks down into glucose), the body detects this increase in blood glucose and triggers specific cells in the pancreas to release insulin.  Insulin acts on many other cells in the body so they can take up the glucose, lowering the overall concentration of glucose in the blood. Glucagon is released from the pancreas when the concentration of glucose in the blood is low. Glucagon stimulates cells of the liver to release stored glucose (called glycogen) into the blood, increasing the overall blood glucose concentration. It is the controlled release of both of these hormones that keeps the blood glucose concentration within the normal range.

This coordinated release of insulin and glucagon and is an example of a feedback system, in particular a negative feedback system.  Negative feedback systems work much like the thermostat in your house.  Your body has a set of internal conditions within a narrow range, called a set point.  When external conditions cause a change to those conditions (a stimulus) your body determines the appropriate action (response) to bring the conditions back to the set point.  Negative feedback loops are reminiscent of a "figure-8" diagram, because the stimulus can either be an increase or a decrease in the internal condition, so the response may be an increase or decrease to bring the system back to the set point.  Use the diagram below to help you understand this concept. 

Adapted from Diabetes Education in Tribal Schools “Health is Life Balance” curriculum.