In the syllabus model, blood glucose is held steady by two hormones with opposite effects: insulin lowers it and glucagon raises it. A glucose diagram is the negative-feedback loop from interpreting a negative-feedback loop with a pancreas as the detector and the liver as the main effector.
This lesson stays academic. A diagram shows how a control model works, and it cannot tell you anything about a real person’s health.
How does the model work?
After a meal, glucose absorbed from the gut raises the blood glucose concentration. The pancreas detects the rise and releases insulin. Insulin causes the liver to convert glucose into glycogen for storage, and it also helps body cells take up glucose.
Blood glucose falls back towards the set point.
Between meals, or during exercise, glucose falls. The pancreas detects the fall and releases glucagon. Glucagon causes the liver to break glycogen down to glucose, which enters the blood.
Blood glucose rises back towards the set point.
How do I read the diagram?
Follow the same five steps for every arrow.
- Find the change in glucose and whether it is a rise or a fall.
- Identify the detector (the pancreas).
- Identify the hormone released for that direction of change.
- Identify the target (the liver) and what it does with glucose.
- Check the return: the glucose curve moves back towards the set point.
Worked example
The table shows invented glucose readings in a model patient after a meal, in arbitrary units for practice. These are not reference ranges and do not describe any real person.
| Time after meal (min) | Glucose (units) |
|---|---|
| 0 | 5.0 |
| 30 | 7.5 |
| 60 | 6.0 |
| 120 | 5.1 |
Step 1, describe the change. Glucose rises from 5.0 to 7.5 in 30 minutes, then falls. The rise is 7.5 − 5.0 = 2.5 units.
Step 2, explain the rise. Glucose from digested food is absorbed into the blood.
Step 3, explain the fall. The rise is detected, insulin is released and the liver stores glucose as glycogen. Glucose falls from 7.5 to 5.1, a drop of 2.4 units.
Step 4, calculate a rate. From 60 to 120 minutes, glucose falls by 6.0 − 5.1 = 0.9 units in 60 minutes. The rate is 0.9 ÷ 60 = 0.015 units per minute.
Step 5, name the feedback. The response opposes the rise, so this is negative feedback and the glucose is returning towards its set point.
The mistake to watch for
The common error is to attach the wrong hormone to the direction of change, or to say insulin “makes glucose”, as if it were a sugar.
Mistaken answer: “When glucose is high, the pancreas releases glucagon to use it up.”
The hormones are swapped. Glucagon raises blood glucose.
The correction is a rule: high glucose leads to insulin, which lowers it. Low glucose leads to glucagon, which raises it. Insulin and glucagon are hormones. They are not sugars.
Check yourself
Try these, then open each answer.
1. Name the organ that detects a change in blood glucose concentration.
Show answer
The pancreas.
2. After a meal, what does insulin cause the liver to do with glucose?
Show answer
It causes the liver to convert glucose to glycogen for storage, which lowers blood glucose.
3. In the table above, calculate the rate of rise in glucose between 0 and 30 minutes.
Show answer
Rise = 7.5 − 5.0 = 2.5 units. Rate = 2.5 ÷ 30 = 0.0833, so about 0.083 units per minute.
Where this leads next
This lesson completes the route, so try the excretion and homeostasis practice set next. Students who find hormone diagrams confusing can see how our teachers approach them in online one-to-one Biology tuition.