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Coordination and Response: Question 4

Syllabus 14.4

Structured Extended 6 marks

A marathon runner has been running for over three hours without eating. Partway through the race, their blood glucose concentration begins to fall below its normal set point.

(a) Explain what is meant by homeostatic control by negative feedback, using the runner's falling blood glucose concentration as an example. [3]

(b) Name the pancreatic hormone that would be released to raise the runner's blood glucose concentration back towards the set point, and describe the role of the liver in this response. [3]

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Worked solution

Part (a): Negative feedback and the set point

The body maintains many internal conditions, including blood glucose concentration, close to a particular set point. Homeostatic control by negative feedback works by constantly monitoring a factor and correcting any deviation from this set point:

  1. Blood glucose concentration is detected to have fallen below the set point (because the runner has been exercising for hours without eating).
  2. This deviation triggers a response designed to reverse the change.
  3. The response raises blood glucose concentration back towards the set point.

The key idea of negative feedback is that the correcting response acts in the opposite direction to the original change, a fall in blood glucose triggers a response that raises it back up again, rather than letting it fall further.

Part (b): The role of glucagon and the liver

The hormone responsible for raising blood glucose concentration is glucagon, released by the pancreas.

Glucagon acts on the liver, which stores excess glucose as glycogen. In response to glucagon, the liver breaks down its stored glycogen into glucose and releases this glucose into the blood. This raises the runner’s blood glucose concentration back towards the set point, providing the muscles with a fresh supply of glucose to keep respiring during the rest of the race.

Final answers

  • (a) Blood glucose falling below the set point is detected, triggering a response that reverses the change and restores the concentration towards the set point.
  • (b) Glucagon, released by the pancreas, causes the liver to break down stored glycogen into glucose and release it into the blood.