How does the body keep blood glucose steady between meals and after eating a sugary snack?
Explain how blood glucose concentration is regulated by negative feedback, including the roles of the pancreas, insulin and glucagon and the liver
A focused answer to the WACE Year 12 Human Biology Unit 3 dot point on blood glucose regulation. The islets of Langerhans, the antagonistic action of insulin and glucagon, the liver as the main effector, and how the negative feedback loop runs in both directions.
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What this dot point is asking
WACE wants you to apply the negative-feedback model to blood glucose in full detail, naming the gland, the two cell types, the two hormones, the target organ and the storage molecule. Glucose must be regulated because cells need a steady supply for respiration, while too much glucose damages tissues and too little starves the brain.
Why blood glucose must be controlled
Glucose is the main fuel for cellular respiration. After a meal, glucose floods in from digestion; between meals and during exercise, glucose is used up. Without regulation the concentration would swing wildly. The body keeps it within a narrow tolerance range so that cells always have fuel and the blood does not become dangerously concentrated.
The pancreas: detector and effector gland
The pancreas contains clusters of endocrine cells called the islets of Langerhans. These cells both monitor blood glucose and respond to it, so the pancreas is both the receptor and the hormone-secreting effector in this loop.
- Beta cells detect a rise in blood glucose and secrete insulin.
- Alpha cells detect a fall in blood glucose and secrete glucagon.
Lowering blood glucose: insulin
When blood glucose rises above the set point (for example after eating), beta cells secrete insulin into the blood. Insulin acts mainly on the liver and muscle cells, making them take up glucose from the blood and convert it to glycogen for storage (glycogenesis). It also makes body cells take up and use more glucose. As glucose moves out of the blood and into storage, the concentration falls back toward the set point, which switches off insulin secretion. This is negative feedback because the response (lowering glucose) opposes the stimulus (raised glucose).
Raising blood glucose: glucagon
When blood glucose falls below the set point (for example between meals or during exercise), alpha cells secrete glucagon. Glucagon acts on the liver, making it break stored glycogen back down into glucose (glycogenolysis) and release it into the blood, and promoting the production of new glucose from other molecules. Blood glucose rises back toward the set point, which switches off glucagon secretion.
How this maps to the exam
Expect to be given a stimulus (a meal, fasting, exercise) and asked to write the hormonal negative-feedback loop, or to read a graph of blood glucose over time and explain the hormonal responses at the peaks and troughs. This loop is the foundation for the diabetes content in the disruption-of-homeostasis topic, where the loop fails.