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3.3.3Organism Level Systems

Glucagon in Blood Sugar Control

5 min read 9 parts Lesson 12 of 13 1 quiz

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Section 1 of 9The need to raise blood glucose

Control of Blood Glucose

Blood glucose falls between meals, during fasting and during exercise because cells use it in respiration.

The brain depends on a constant supply of glucose, so the level must not drop too far.
Too low
When the concentration of glucose in the blood is too low, the pancreas releases a different hormone, glucagon.
Glucagon is released from different cells of the pancreas than insulin.

Section 2 of 9How glucagon works

Glucagon is carried in the blood to the liver, where it stimulates liver cells to break down stored glycogen into glucose.

1
The glucose is released into the blood, so the blood glucose concentration rises toward normal.
2
As the level rises, the pancreas detects the change and secretes less glucagon.
3
This is negative feedback.
4
Glucagon also stimulates the conversion of other substances, such as amino acids, to glucose in more extreme cases.
5
Muscle glycogen is kept for the muscle itself and is not released to the blood.

Section 3 of 9Insulin and glucagon together

Regulation of blood glucose by insulin and glucagon
blood glucose regulation
Blood Glucose Regulation — Insulin & Glucagon
Blood Glucose Regulation — Insulin & Glucagon

Insulin and glucagon have opposite effects, which are described as antagonistic. Together they keep blood glucose within a narrow range.

FeatureInsulinGlucagon
Released whenBlood glucose is highBlood glucose is low
Target organsLiver, musclesLiver
Effect on glycogenGlucose converted to glycogenGlycogen converted to glucose
Effect on blood glucoseFallsRises

The cycle

  • high glucose, insulin released, glucose into cells and stored as glycogen, level falls;
    • low glucose, glucagon released, glycogen broken down, level rises.
  • Each response ends when the glucose level returns to normal.

Section 4 of 9Exercise, fasting and diabetes

During exercise the muscles take up glucose quickly, so the level in the blood falls; glucagon helps replace it.

1
Adrenaline also raises blood glucose by stimulating the liver to release glucose, which is useful in fight or flight.
2
In a person with diabetes, the system does not work properly: without insulin, glucose cannot be stored,
  • and the person must balance insulin dose, food and exercise.
3
If someone with diabetes takes too much insulin, blood glucose falls rapidly.
4
A sugary drink or an injection of glucagon can restore it by making the liver release glucose.

Graph questions show how the two hormones change in opposite directions: after a meal, insulin rises as glucose rises; several hours later, as glucose falls, glucagon rises. Describe the peaks with time and figures, then explain.

Section 5 of 9Evaluating the system

The use of two hormones with opposite actions gives finer control than one hormone alone, since it can push the level either way.

1
Negative feedback ensures that neither hormone continues to act once the concentration is normal.
2
If feedback fails, as when insulin-producing cells are destroyed, glucose control is lost.
3
Scientists are researching ways to replace the failed cells,
  • such as transplants of pancreas cells or stem cells that develop into insulin-producing cells,
  • but the problem of rejection and the need for immunosuppressant drugs remain.
4
A closed-loop insulin pump that measures glucose and automatically injects insulin copies the natural system.

Section 6 of 9Worked example of a 24-hour cycle

Imagine a person eating breakfast at 8 am, lunch at 1 pm and dinner at 7 pm.

Worked example of a 24-hour cycle
After each meal, absorbed glucose raises the blood concentration, insulin is released,
  • and the level returns to normal within a couple of hours as glucose is taken into liver and muscle cells and stored as glycogen.
In the long gaps between meals,
  • and overnight, the blood glucose drifts downward as the brain and other organs use it;
    • glucagon is released and the liver breaks down glycogen to top up the blood.
A graph of blood glucose through the day therefore looks like a flat line with three small bumps after meals, never moving far from the normal range,
  • while a graph of insulin has peaks that follow the meals and glucagon has peaks in the gaps.

The liver's glycogen store is limited to roughly a day's supply when fasting; after that, the body begins to use fats as fuel and to convert some amino acids to glucose.

Worked example of a 24-hour cycle
This is why prolonged fasting leads to weight loss, and why an athlete eats carbohydrate before a long event to fill the stores.
Linking the hormone effects to storage and use of energy provides a bridge between this lesson and respiration.

Section 7 of 9Exam-style questions

Blood glucose control by insulin and glucagon feedback loops
Blood glucose control loop with insulin and glucagon
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  1. Explain how glucagon raises the blood glucose concentration. [3]3 marks

  2. Describe how insulin and glucagon act as an example of negative feedback. [3]3 marks

  3. Explain why an athlete's blood glucose may stay roughly constant during a long run. [2]2 marks

Section 8 of 9Key terms

TermMeaning
GlucagonPancreatic hormone that raises blood glucose.
GlycogenStorage carbohydrate in the liver and muscles.
AntagonisticHaving opposite effects.
LiverOrgan that stores glycogen and releases glucose.
FastingGoing without food for a period.
Negative feedbackA change triggering a response that reverses it.

Section 9 of 9Exam technique

1
Writing that glucagon is made in the liver; it is made in the pancreas but acts on the liver.
2
Saying glucagon makes glycogen; it breaks glycogen down.
3
Mixing up the words glycogen and glucagon.
4
Saying that both hormones are released at the same time.
5
Forgetting that muscle glycogen is not released to the blood.
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