Blood Glucose Regulation: KS3 Biology Guide

Why must blood glucose be kept constant?

Glucose is the main fuel for cellular respiration in every cell of the body. The brain is particularly dependent on a constant glucose supply: it cannot store glycogen and has no alternative fuel source. If blood glucose falls too low (hypoglycaemia), brain function deteriorates within minutes, leading to confusion, loss of consciousness, and in extreme cases, brain damage.

Too much glucose (hyperglycaemia) damages blood vessels and nerves over time. This is why uncontrolled diabetes leads to complications including blindness, kidney failure and circulatory problems in the feet and hands.

What is the role of the pancreas?

The pancreas is a gland located beneath the stomach. It has two distinct functions:

• Exocrine function: produces digestive enzymes (amylase, lipase, protease) released into the small intestine through a duct.
• Endocrine function: contains clusters of cells called islets of Langerhans that produce hormones directly into the blood — specifically insulin (from beta cells) and glucagon (from alpha cells).

The islet cells constantly monitor blood glucose levels and respond directly to rising or falling concentrations.

How does insulin lower blood glucose?

When blood glucose concentration rises (e.g. after a meal):

1. Beta cells in the pancreas detect the rise in blood glucose.
2. They secrete insulin into the blood.
3. Insulin causes cells — especially liver and muscle cells — to take up glucose from the blood.
4. In the liver, insulin promotes conversion of glucose into glycogen (a storage molecule) — a process called glycogenesis.
5. Blood glucose concentration falls back to the normal range.
6. Insulin secretion decreases.

Insulin acts as a "key" that unlocks glucose uptake in cells by causing glucose transport proteins to move to the cell membrane.

How does glucagon raise blood glucose?

When blood glucose concentration falls (e.g. during exercise or fasting):

1. Alpha cells in the pancreas detect the drop.
2. They secrete glucagon into the blood.
3. Glucagon travels to the liver.
4. The liver breaks glycogen back into glucose — a process called glycogenolysis — and releases it into the blood.
5. Blood glucose concentration rises back to normal.
6. Glucagon secretion decreases.

What is this system an example of?

The control of blood glucose is a classic example of negative feedback. When blood glucose deviates from the set point (the normal range, approximately 4–7 mmol/L), the response is in the opposite direction — bringing it back to normal. The system "feeds back" to reduce the error:

• Rising glucose → insulin → glucose falls → insulin secretion stops.
• Falling glucose → glucagon → glucose rises → glucagon secretion stops.

This type of automatic self-correction is called homeostasis — the maintenance of a stable internal environment.

What happens when blood glucose regulation fails?

Diabetes mellitus is a condition in which blood glucose is not properly regulated:

Type 1 diabetes typically begins in childhood or adolescence; Type 2 is more common in adults and is strongly associated with a diet high in refined sugars and lack of physical activity.

Frequently asked questions

Why can't people with Type 1 diabetes simply eat less sugar?

People with Type 1 diabetes produce no insulin because their immune system has destroyed the insulin-producing beta cells in the pancreas. Without insulin, cells cannot absorb glucose even if it is present in the blood — the "locks" on cell membranes are not activated. Eating less sugar reduces the problem but does not solve it. Insulin must be supplied externally (by injection or pump) to allow cells to use glucose. Without insulin, blood glucose rises dangerously after every meal regardless of what is eaten.

Why is glycogen stored in the liver rather than in the blood?

Glycogen is an insoluble, branched polymer of glucose. If glucose were stored in the blood at high concentrations, it would raise blood osmolarity and draw water out of cells by osmosis, causing dehydration. The liver converts excess glucose into insoluble glycogen, which cannot exert osmotic pressure in the blood, allowing large amounts of glucose energy to be stored safely. When needed, glycogen is rapidly broken back down by glucagon and released as glucose.

How is physical exercise related to blood glucose regulation?

During exercise, muscle cells respire rapidly, consuming glucose for energy. This causes blood glucose to fall. The pancreas responds by secreting glucagon, which causes the liver to break down glycogen and release glucose into the blood. At the same time, exercising muscles can absorb glucose even without insulin, using transport proteins activated by contraction. This is why moderate exercise is recommended for managing Type 2 diabetes: it lowers blood glucose independently of the insulin pathway.

What is the difference between hypoglycaemia and hyperglycaemia?

Hypoglycaemia is abnormally low blood glucose (below about 4 mmol/L). Symptoms include shaking, sweating, confusion and — in severe cases — unconsciousness, because the brain is deprived of fuel. It can occur in diabetic patients who have taken too much insulin or not eaten enough. Hyperglycaemia is abnormally high blood glucose (above about 11 mmol/L after eating). It typically causes thirst, frequent urination and fatigue. Long-term hyperglycaemia damages blood vessels and nerves, causing the serious complications associated with poorly managed diabetes.

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