Insulin lowers blood glucose concentration, whereas glucagon raises it. Together, these pancreatic hormones maintain blood glucose within a narrow range through negative feedback.
The Mechanism
After a carbohydrate-containing meal, glucose is absorbed from the small intestine into the blood. The resulting increase in blood glucose is detected by beta cells in the islets of Langerhans within the pancreas, causing insulin secretion.
Insulin binds to receptors on target cells. It increases glucose uptake, especially by skeletal muscle and adipose tissue, and stimulates glycogenesis, in which glucose is converted into glycogen for storage in the liver and skeletal muscles. It also promotes glucose use in cellular respiration and conversion of excess nutrients into fat. Blood glucose therefore falls toward its normal level, reducing insulin secretion.
During fasting or prolonged exercise, falling blood glucose is detected by pancreatic alpha cells, which secrete glucagon. Glucagon mainly targets the liver and stimulates glycogenolysis, the breakdown of glycogen into glucose, and gluconeogenesis, the production of glucose from non-carbohydrate precursors such as amino acids and glycerol. The liver releases glucose into the blood, restoring its concentration and reducing glucagon secretion.
| Blood glucose condition | Pancreatic response | Main physiological effects |
|---|---|---|
| Above the normal range | Beta cells secrete insulin | Increased glucose uptake, glycogenesis and glucose use; blood glucose falls |
| Below the normal range | Alpha cells secrete glucagon | Increased liver glycogenolysis and gluconeogenesis; blood glucose rises |
This is an example of inter-system communication: the endocrine system detects a chemical change, hormones travel through the cardiovascular system, and target organs such as the liver and skeletal muscles produce coordinated responses.
Exam Technique
For an IB SEHS response, identify the stimulus, endocrine cell, hormone, target tissue, metabolic process and return toward the normal range. A common misconception is that glucagon directly causes skeletal muscle glycogen to release glucose into the blood; glucagon acts primarily on the liver, while muscle glycogen is used locally during exercise.
