Antagonistic Hormones

Maintaining homeostasis often requires body conditions to be limited to a narrow range. When conditions exceed the upper limit of homeostasis  the production of a hormone is triggered. When conditions return to normal level, hormone production is discontinued. If conditions exceed the lower limits of homeostasis 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. Insulin and glucagon produced by Islets of Langerhans of pancrease are example  antagonistic hormones.The Beta cells secrete of Islets of Langerhans secretion insulin. When the concentration of blood glucose raises such in after eating, beta cells secret insulin into the blood. Insulin stimulates the liver cells and most other body cells to absorb glucose. Liver and muscle cells convert glucose to glycogen, for short term storage, and adipose cells convert glucose to fat. In response, glucose concentration decreases in the blood, and insulin secretion discontinues through negative feedback from declining levels of glucose.When the concentration of blood glucose drops such as during exercise, 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. Glucagon also stimulates the production of ketone bodies from amino acids and fatty acids. Ketone bodies are an alternative energy source to glucose for some tissues. When blood glucose levels return to normal, glucagon secretion discontinues through negative feedback. Another example of antagonistic hormones occurs in the maintenance of Ca ²⁺ ion 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 of thyroid produces the opposite effect by inhibiting the breakdown of bone matrix and decreasing the release of calcium in the blood.