21.4: Feedback Loops
In most cases, excessive hormone production is prevented by negative feedback—a loop that starts with a stimulus inducing the release of a particular substance, like a hormone, to maintain a certain level before triggering a signal that results in a decrease in further release of the hormone.
For example, an increase in blood glucose levels releases the hormone insulin from beta cells of the pancreas into the bloodstream, delivering insulin to cells throughout the body. Insulin stimulates cells to take up glucose and use it for energy production. Insulin also converts and stores excess glucose as glycogen in the liver. Collectively, these actions lower blood glucose levels, and in turn, signals the pancreas to stop producing insulin.
When blood glucose levels fall below normal, for example during exercise, alpha cells of the pancreas release the hormone glucagon. Glucagon converts glycogen stored in the liver to glucose, which can then be used by other cells in the body for energy production. Glucagon also stimulates the liver to absorb amino acids from blood and convert them to glucose. An increase in blood glucose levels then signals the pancreas to stop releasing glucagon via negative feedback regulation.