Explain how insulin and glucagon regulate blood glucose concentration by negative feedback

What this dot point is asking

SCSA wants you to name the hormones and organs involved, explain the negative feedback loop in both directions, and connect a failure of this control to diabetes. A strong answer treats the response to high and low glucose as two complementary halves of one system.

Why blood glucose must be controlled

Glucose is the main fuel for cellular respiration, so cells need a steady supply. Too little glucose starves cells, especially brain cells, of energy; too much damages tissues over time. Keeping blood glucose stable is therefore a vital example of homeostasis, controlled by the endocrine system.

The control system

The pancreas monitors blood glucose and releases hormones in response. The liver is the main effector organ, storing and releasing glucose. The two key hormones act in opposite directions:

• Insulin lowers blood glucose.
• Glucagon raises blood glucose.

Response to high blood glucose

After a meal, blood glucose rises. The pancreas detects this and releases insulin. Insulin causes:

• body cells to take up more glucose from the blood,
• the liver to convert glucose into glycogen for storage.

As glucose is removed from the blood, the level falls back toward normal, and insulin release decreases. This is negative feedback.

Response to low blood glucose

Between meals or during exercise, blood glucose falls. The pancreas detects this and releases glucagon. Glucagon causes:

• the liver to break down stored glycogen back into glucose,
• glucose to be released into the blood.

As glucose rises back toward normal, glucagon release decreases. Again, this is negative feedback.

When control fails: diabetes

Diabetes is a failure of blood glucose control. In type 1 diabetes the pancreas does not produce enough insulin, so glucose cannot be moved into cells or stored and blood glucose stays dangerously high. In type 2 diabetes cells become less responsive to insulin. Diabetes shows what happens when a negative feedback system breaks: the variable is no longer corrected and the body is harmed.

Reading a blood glucose graph

A common exam stimulus is a graph of blood glucose concentration over several hours, showing peaks after meals and troughs between them. To interpret it, link each feature to the hormone responsible. At a peak just after eating, glucose is high, so insulin secretion is high and the line then falls as glucose is taken up and stored. In a trough between meals or during exercise, glucose is low, so glucagon secretion rises and the line climbs back toward the set point. The line oscillating gently around a steady set point, rather than drifting up or down, is the visible signature of a working negative feedback system. A line that rises after a meal and fails to come back down indicates a loss of insulin action, as in diabetes.

Why this matters for survival

Stable blood glucose keeps cells supplied with fuel regardless of when an animal last ate, letting it survive periods of fasting and bursts of activity. The insulin and glucagon loop is one of the clearest examples of negative feedback and the endocrine control of homeostasis, and the consequences of its failure in diabetes make its importance vivid.