Maintaining homeostasis often requires conditions to be limited to a narrow range. When conditions exceed the upper limit of homeostasis, a specific action, usually the production of a hormone, is triggered. When conditions return to normal, hormone production is discontinued. If conditions exceed the lower limit of homeostasis, a different action, usually the production of a second hormone, is triggered. Hormones that act to return body conditions to within acceptable limits from opposite extremes are called antagonistic hormones.
The regulation of blood glucose concentration (through negative feedback) illustrates how the endocrine system maintains homeostasis by the action of antagonistic hormones. Bundles of cells in the pancreas called pancreatic islets contain two kinds of cells, alpha cells and beta cells. These cells control blood glucose concentration by producing the antagonistic hormones insulin and glucagon:
- Beta cells secrete insulin. When the concentration of blood glucose rises (after eating, for example), beta cells secrete insulin into the blood. Insulin stimulates the liver and most other body cells to absorb glucose. Liver and muscle cells convert the glucose to glycogen (for short‐term storage), and adipose cells convert the glucose to fat. In response, glucose concentration decreases in the blood, and insulin secretion discontinues (through negative feedback from declining levels of glucose).
- Alpha cells secrete glucagon. When the concentration of blood glucose drops (during exercise, for example), alpha cells secrete glucagon into the blood. Glucagon stimulates the liver to release glucose. The glucose in the liver originates from the breakdown of glycogen and the conversion of amino acids and fatty acids into glucose. When blood glucose levels return to normal, glucagon secretion discontinues (negative feedback).
Another example of antagonistic hormones occurs in the maintenance of Ca 2+ concentration in the blood. Parathyroid hormone (PTH) from the parathyroid glands increases Ca 2+ in the blood by increasing Ca 2+ absorption in the intestines and reabsorption in the kidneys and stimulating Ca 2+ release from bones. Calcitonin (CT) produces the opposite effect by inhibiting the breakdown of bone matrix and decreasing the release of calcium into the blood.