Control of blood glucose: insulin, glucagon and diabetes

After a meal, glucose floods into the blood from the gut. Between meals it drains away as cells respire it. Despite that, blood glucose is normally held in a narrow range around 90 mg / 100 cm³ (≈ 5 mmol/dm³). The system that does this is one of the most testable examples of homeostasis at GCSE.

The pancreas: receptor and coordinator

Glucose-sensitive cells in the pancreas monitor the concentration of glucose in the blood. The pancreas then secretes one of two hormones:

• Insulin — secreted when blood glucose is too high.
• Glucagon — secreted when blood glucose is too low (HT only).

Both hormones travel in the blood and act on the liver as their main target.

Response to high blood glucose

After a sugary meal, blood glucose rises.

1. Pancreas detects the rise.
2. Insulin is released into the blood.
3. Insulin makes liver and muscle cells take up glucose.
4. The liver converts glucose to glycogen for storage.
5. Blood glucose falls back to the set point.

This is the negative-feedback half you are most likely to be examined on.

Response to low blood glucose (HT)

If you don't eat for hours, blood glucose falls.

1. Pancreas detects the fall.
2. Glucagon is released into the blood.
3. Glucagon causes the liver to break down glycogen back into glucose.
4. Glucose is released into the blood.
5. Blood glucose rises back to the set point.

Notice the elegant symmetry: glucose ↔ glycogen, controlled by insulin (storage) and glucagon (release).

Diabetes — when the system fails

Type 1 diabetes usually develops in childhood. The pancreas does not make enough (or any) insulin. Without treatment, blood glucose rises uncontrollably after meals — high enough to spill into the urine, damage tissues, and even be life-threatening.

Treatment: insulin injections (timed with meals), regular glucose monitoring, careful carbohydrate management.

Type 2 diabetes usually develops in adulthood. The pancreas still produces insulin but body cells no longer respond to it ("insulin resistance"). Strongly linked to obesity, lack of exercise and genetics.

Treatment: carbohydrate-controlled diet and regular exercise, often supported by medication that improves insulin sensitivity. In severe cases, insulin injections may also be needed.

How to compare blood glucose curves (a common exam graph)

Examiners often show a graph of blood glucose vs time after a sugary drink, comparing a healthy person and a person with diabetes.

• The healthy curve peaks then comes back down within ~2 hours.
• The diabetic curve rises higher and stays high for much longer because insulin is missing or ineffective.

You should be able to read off the peak value and the time to return to baseline, and link this to the absence of insulin.

Links

Builds on B5.1 (homeostasis and negative feedback) and B5.6 (endocrine system). Connects to B4.3 (metabolism — glucose, glycogen, fat) and B2.2 (effects of high blood glucose on cardiovascular health).