# Blood glucose regulation

> CIE A-Level Biology · Unit 15: Homeostasis
> Source: https://www.owlsprep.com/study/cie-9700-u15-blood-glucose-regulation/

This sub-topic covers homeostatic negative feedback control of blood glucose concentration, including key hormones, their mechanisms of action, and the causes and effects of Type 1 and Type 2 diabetes mellitus, a high-frequency exam topic.

**Prerequisites:** [Principles of homeostasis and negative feedback](https://www.owlsprep.com/study/cie-9700-u15-homeostasis-introduction/); [Hormone action and cell signalling](https://www.owlsprep.com/study/cie-9700-u14-cell-signalling/)

## Learning objectives

- Explain the role of negative feedback in blood glucose homeostasis
- Compare the actions of insulin and glucagon in regulating blood glucose
- Distinguish between Type 1 and Type 2 diabetes mellitus
- Explain the consequences of abnormal blood glucose regulation

## Why Regulate Blood Glucose?

Healthy blood glucose is maintained between 4–6 mmol dm⁻³. Glucose is the primary respiratory substrate for most cells, especially brain cells which cannot use alternative fuels efficiently. Hypoglycaemia (low glucose) deprives cells of energy, causing confusion, coma and death. Hyperglycaemia (high glucose) lowers blood water potential, causing osmotic cell damage and long-term harm to blood vessels and nerves.

**Normoglycaemia** — The stable normal range of blood glucose concentration maintained by homeostatic mechanisms

*Example:* A healthy person after an 8-hour fast has normoglycaemia between 4–6 mmol dm⁻³

**Worked example:** Explain why untreated hyperglycaemia leads to dehydration.

1. High blood glucose increases the solute concentration of blood plasma, which lowers its water potential.
2. Water moves out of body cells into the blood plasma by osmosis, down the water potential gradient.
3. Increased blood volume causes the kidneys to produce larger volumes of urine to excrete excess water.

> **Exam tip:** Always reference water potential changes when explaining effects of high blood glucose — this is a common CIE marking point.

## Response to High Blood Glucose

After eating a carbohydrate-rich meal, blood glucose rises above the normal range. This change is detected by β-cells (beta cells) in the islets of Langerhans, endocrine tissue in the pancreas. β-cells secrete insulin into the bloodstream.

**Insulin** — A hormone that lowers blood glucose by increasing glucose uptake, utilisation and storage in target tissues

*Notation:* Peptide hormone

1. Increases number of GLUT4 glucose transporters on cell surfaces, raising glucose uptake
2. Activates glycogenesis (glucose → glycogen) for storage in liver and muscle
3. Increases glycolysis (glucose breakdown for respiration)
4. Inhibits glycogenolysis and gluconeogenesis (glucose production)

**Worked example:** Describe how the body corrects a rise in blood glucose after a meal.

1. A rise in blood glucose above normal is detected by β-cells in the pancreatic islets of Langerhans.
2. β-cells secrete increased amounts of insulin into the bloodstream, which travels to target tissues (liver, muscle, adipose).
3. Insulin binds to cell surface receptors, triggering intracellular responses that increase glucose uptake and storage, lowering blood glucose.
4. Once blood glucose returns to normal, negative feedback reduces insulin secretion to stop the response.

## Response to Low Blood Glucose

When blood glucose falls below normal (between meals, after exercise), the change is detected by α-cells (alpha cells) in the pancreatic islets of Langerhans. α-cells secrete the hormone glucagon, whose main target tissue is the liver.

**Glucagon** — A hormone that raises blood glucose by activating glycogen breakdown and glucose synthesis in the liver

*Notation:* Peptide hormone

1. Activates glycogenolysis: breakdown of stored glycogen into glucose
2. Activates gluconeogenesis: synthesis of glucose from non-carbohydrate sources
3. Inhibits glycogenesis to prevent glucose storage

**Worked example:** How do insulin and glucagon work together to maintain stable blood glucose?

1. Both are secreted by the pancreas in response to changes in blood glucose, acting via negative feedback.
2. Insulin (from β-cells when glucose is high) lowers blood glucose, while glucagon (from α-cells when glucose is low) raises blood glucose.
3. They have antagonistic (opposing) effects that counteract each other to keep glucose within a narrow normal range.

## Diabetes Mellitus

Diabetes mellitus is a disorder where blood glucose regulation fails, causing chronic hyperglycaemia. There are two distinct main types, which CIE examiners frequently ask to compare:

| Feature | Type 1 Diabetes | Type 2 Diabetes |
| --- | --- | --- |
| Cause | Autoimmune destruction of β-cells, no insulin produced | Target cell insulin resistance, β-cell dysfunction over time |
| Typical onset | Childhood/early adolescence | Adulthood, linked to obesity |
| Treatment | Insulin injections + controlled diet | Diet/exercise, oral drugs, eventual insulin |
| Prevalence | ~10% of all diabetes cases | ~90% of all diabetes cases |

**Worked example:** Why is glucose found in the urine of people with untreated diabetes?

1. Blood glucose rises far above the maximum reabsorption threshold of kidney nephrons.
2. Excess glucose cannot be reabsorbed from the filtrate back into the blood, so it remains in urine.
3. Glucose lowers the water potential of the filtrate, so more water remains in urine, causing increased urine output.

> **Exam tip:** Practice drawing a full negative feedback diagram for blood glucose regulation, this is a common 5-6 mark question.

## Common pitfalls

- **Wrong:** Confusing glycogenesis, glycogenolysis and gluconeogenesis
  - Why it fails: Similar terms are often mixed up in exam answers
  - Correct: Remember the suffix: *genesis* = creation: glycogenesis = create glycogen; *lysis* = breakdown: glycogenolysis = break down glycogen; gluconeogenesis = create new glucose from non-carbs
- **Wrong:** Stating glucagon is produced by the liver
  - Why it fails: Confusing the site of secretion with the target tissue
  - Correct: Glucagon is secreted by α-cells in the pancreas; the liver is its main target tissue
- **Wrong:** Claiming Type 2 diabetes is caused by insufficient insulin
  - Why it fails: Early Type 2 diabetes is not caused by low insulin levels
  - Correct: Type 2 diabetes is caused by insulin resistance (target cells do not respond to insulin); insulin levels may even be elevated initially
- **Wrong:** Forgetting to mention negative feedback
  - Why it fails: Blood glucose regulation is a classic example of negative feedback, which is always a marking point
  - Correct: Always state that once blood glucose returns to normal, hormone secretion is reduced to stop the response
- **Wrong:** Only mentioning insulin when describing glucose regulation
  - Why it fails: Students often forget the role of glucagon in raising blood glucose
  - Correct: Always include both insulin (lowering) and glucagon (raising) in any full description of glucose regulation

## Cheatsheet

| Process / Condition | Key fact | Control |
| --- | --- | --- |
| Glycogenesis | Glucose → glycogen (storage) | Activated by insulin |
| Glycogenolysis | Glycogen → glucose | Activated by glucagon, adrenaline |
| Gluconeogenesis | Glucose from non-carbs | Activated by glucagon |
| Type 1 Diabetes | No insulin produced | Treated with insulin injections |
| Type 2 Diabetes | Insulin resistance | Managed with diet/exercise |
| Normal range | 4–6 mmol dm⁻³ | Negative feedback |

## What's next

Blood glucose regulation is one of the most frequently tested examples of homeostasis in CIE A-Level Biology. Mastering this topic builds a strong foundation for understanding other homeostatic control systems, which follow the same core principles of negative feedback, hormone action and cell signalling. Extended response questions often ask for comparisons between different homeostatic mechanisms, so understanding the key patterns here will help you answer those questions effectively. The next topics build on this knowledge to explore other key homeostatic systems in the human body.

- [Osmoregulation](https://www.owlsprep.com/study/cie-9700-u15-osmoregulation/)
- [Kidney Structure and Function](https://www.owlsprep.com/study/cie-9700-u15-kidney-structure-and-function/)
- [Control and Coordination](https://www.owlsprep.com/study/cie-9700-u16-overview/)

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