Blood glucose is one of the body's most tightly controlled variables. Too high and cells are damaged; too low and the brain runs out of fuel. The pancreas uses two hormones — insulin and glucagon — working in opposition to keep levels in a narrow safe range every minute of the day.
What is homeostasis and why is blood glucose so important?
Homeostasis is the process by which the body maintains a stable internal environment despite changes in external conditions and in the body's own activity. Factors that must be kept constant include temperature, blood pH, water content, and blood glucose concentration.
Glucose is the primary fuel for cellular respiration. Every cell in the body — particularly brain cells and red blood cells — depends on a steady supply. Normal blood glucose sits between approximately 4 and 7 mmol/L (millimoles per litre). Values persistently above this range cause damage to blood vessels and nerves; values that drop too low (hypoglycaemia) deprive the brain of fuel and can cause loss of consciousness within minutes.
The control of blood glucose is an example of a negative feedback system: a change in the variable triggers a response that opposes and reverses that change, returning conditions to the set point.
Which organ detects changes in blood glucose?
The pancreas is the key organ. Within the pancreas are clusters of hormone-secreting cells called the islets of Langerhans:
- Beta cells (β cells) — detect high blood glucose and secrete insulin
- Alpha cells (α cells) — detect low blood glucose and secrete glucagon
Both hormones travel in the blood to target organs, primarily the liver and muscle cells.
How does insulin lower blood glucose?
When blood glucose rises — for example after eating a carbohydrate-rich meal — beta cells in the pancreas detect the rise and release insulin into the bloodstream.
Insulin acts on liver cells and muscle cells, causing them to:
- Take up glucose from the blood by increasing the number of glucose transporter proteins in their cell membranes.
- Convert glucose to glycogen (a branched storage polymer) — a process called glycogenesis.
- Use more glucose in respiration — cells increase their respiration rate.
As blood glucose falls back towards the normal range, less insulin is secreted. This is the negative feedback loop in action.
Worked example — tracking glucose after a meal:
| Time | Blood glucose (mmol/L) | Response |
|---|---|---|
| Before meal | 5.0 (normal) | Low insulin secretion |
| 30 min after meal | 8.5 (high) | Beta cells release insulin |
| 60 min after meal | 7.0 (falling) | Insulin secretion decreasing |
| 90 min after meal | 5.2 (back to normal) | Minimal insulin secretion |
How does glucagon raise blood glucose?
When blood glucose falls — during exercise, prolonged fasting, or several hours after a meal — alpha cells in the pancreas detect the drop and release glucagon.
Glucagon acts primarily on the liver, causing hepatocytes to:
- Break down glycogen into glucose — a process called glycogenolysis.
- Release that glucose into the bloodstream.
- At lower blood glucose levels, synthesise new glucose from amino acids and fats (gluconeogenesis).
As blood glucose rises back to the normal range, glucagon secretion decreases. Insulin and glucagon thus act as an antagonistic pair — one goes up as the other goes down — providing very precise control of blood glucose.
What is the difference between Type 1 and Type 2 diabetes?
Diabetes mellitus is a condition in which blood glucose is chronically elevated because the control system is not working properly.
| Feature | Type 1 diabetes | Type 2 diabetes |
|---|---|---|
| Cause | Autoimmune destruction of pancreatic beta cells | Target cells become insensitive (resistant) to insulin |
| Insulin production | None (or negligible) | Initially normal or raised; may decline over time |
| Typical onset | Usually childhood or adolescence | Usually middle age or later (but increasingly younger) |
| Risk factors | Genetic; triggered by viral infection | Obesity, poor diet, family history, physical inactivity |
| Management | Insulin injections or insulin pump (essential) | Diet, exercise, weight loss, oral medication; insulin if disease progresses |
In Type 1, the immune system mistakenly attacks and destroys the beta cells, so no insulin is produced at all. Without insulin, glucose cannot enter cells efficiently and blood glucose rises dangerously after meals. Insulin must be injected (it cannot be taken orally because it is a protein and would be digested).
In Type 2, beta cells still produce insulin but the receptor proteins on liver and muscle cells fail to respond normally. Cells are said to be insulin resistant. Blood glucose remains high even though insulin is present. Lifestyle changes can often manage or even reverse early Type 2 diabetes.
How do continuous glucose monitors work?
Modern management of Type 1 diabetes often involves continuous glucose monitors (CGMs) — small sensors inserted under the skin that measure interstitial fluid glucose (which closely tracks blood glucose) every few minutes. The data feeds to a smartphone or insulin pump.
Some systems are closed-loop (sometimes called an "artificial pancreas"): the CGM sends data to a pump, which automatically adjusts the insulin dose without the patient needing to act. This mimics the function of beta cells. Understanding this technology connects your GCSE biology to real clinical practice.
How should you answer blood glucose questions in the exam?
For any question about blood glucose control, work through this sequence:
- State the stimulus — e.g. "Blood glucose concentration rises after eating."
- Name the receptor/detector — "Beta cells in the pancreas detect the rise."
- Name the hormone and effector — "Insulin is secreted; liver and muscle cells are the effectors."
- Describe the response — "Glucose is converted to glycogen (glycogenesis); blood glucose falls."
- Explain negative feedback — "As blood glucose returns to normal, less insulin is secreted."
Use the correct vocabulary throughout: glycogenesis, glycogenolysis, negative feedback, beta cells, alpha cells. Diagrams showing the two feedback loops (high glucose → insulin; low glucose → glucagon) score well in longer answers.
Frequently asked questions
Why can't people with Type 1 diabetes take insulin tablets?
Insulin is a protein (a hormone made of amino acids). If taken by mouth, the digestive enzymes in the stomach and small intestine would break it down into amino acids before it could reach the bloodstream and act on cells. It must therefore be injected directly into the subcutaneous fat, from where it is absorbed into the blood and carried to the liver and muscle cells. Research into alternative delivery methods — such as nasal sprays or swallowable capsules with special coatings — is ongoing but not yet widely available.
What is the difference between glycogenesis and glycogenolysis?
Glycogenesis is the conversion of glucose into glycogen for storage in the liver and muscles — this happens when blood glucose is high and insulin is present. Glycogenolysis is the reverse: the breakdown of stored glycogen back into glucose, releasing it into the blood — this happens when blood glucose is low and glucagon is present. A helpful memory trick: the "-lysis" in glycogenolysis means breaking apart (like hydrolysis), so glycogenolysis is the breaking-down process.
Does exercise affect blood glucose control?
Yes, in two ways. During exercise, muscle cells take up glucose rapidly for respiration, which tends to lower blood glucose. The liver releases glucose from glycogen stores and glucagon rises to compensate. In the short term, exercise helps manage blood glucose. In the long term, regular exercise improves insulin sensitivity — muscle and liver cells respond more efficiently to insulin — which is why physical activity is a cornerstone of managing and preventing Type 2 diabetes.
Can Type 2 diabetes be reversed?
Research suggests that Type 2 diabetes can be put into remission (blood glucose returning to normal without medication) through significant weight loss and dietary changes, particularly in people who have not had the condition for many years. The NHS low-calorie diet programme has shown this for some patients. However, "remission" is preferred over "cure" because the underlying tendency to insulin resistance remains — the condition can return if weight is regained or diet worsens.
Struggling with negative feedback loops or the insulin–glucagon balance? Professor Darwin at aitutors.me will walk you through it Socratically — guessing before seeing, and building the full picture step by step.