Define homeostasis and explain the control of blood glucose by insulin and glucagon

What this dot point is asking

SEAB wants a clear definition of homeostasis, an understanding of why a steady internal environment matters, and a description of how the level of glucose in the blood is controlled by the hormones insulin and glucagon. The control of blood glucose is the classic example of negative feedback at O-Level.

The answer

What homeostasis is

Homeostasis is the maintenance of a constant (steady) internal environment in the body, despite changes in the conditions outside. Examples of conditions kept steady are body temperature, blood glucose level, water content and pH.

Why it matters

Cells, and especially the enzymes inside them, work best within a narrow range of conditions. If the temperature, pH, water or glucose level swings too far, enzymes work poorly or are denatured, and cells cannot function. Keeping these conditions steady lets the body work efficiently.

The principle of negative feedback

Homeostasis usually works by negative feedback: when a condition moves away from its normal level, the body detects the change and acts to bring it back. If something rises too high, the response lowers it; if it falls too low, the response raises it. Blood glucose control is the key example.

Controlling blood glucose

The level of glucose in the blood is controlled by the pancreas, using two hormones:

When blood glucose rises (for example, after a meal):

• The pancreas releases insulin into the blood.
• Insulin causes the liver and muscle cells to take up glucose and convert it to glycogen for storage, and increases the use of glucose in respiration.
• Blood glucose falls back to normal.

When blood glucose falls (for example, during exercise or fasting):

• The pancreas releases glucagon into the blood.
• Glucagon causes the liver to break down stored glycogen back into glucose and release it into the blood.
• Blood glucose rises back to normal.

The two hormones work in opposite directions, keeping the blood glucose steady.

Examples in context

Example 1. Type 1 diabetes. A person whose pancreas makes too little insulin cannot lower their blood glucose properly, so it rises too high after meals. They may need insulin injections to do the job their pancreas cannot, showing how vital insulin is for homeostasis.

Example 2. Glucose during exercise. During a long run, muscles use up glucose, so blood glucose tends to fall. Glucagon then triggers the liver to release stored glucose, keeping the blood glucose steady so the muscles and brain keep working.

Try this

Q1. Define homeostasis. [2 marks]

• Cue. The maintenance of a constant (steady) internal environment in the body, despite changes in the external conditions.

Q2. Name the hormone released when blood glucose is too high and state its effect. [2 marks]

• Cue. Insulin; it causes the liver and cells to take up glucose and store it as glycogen, lowering blood glucose.

Q3. Explain how glucagon raises blood glucose when it is too low. [2 marks]

• Cue. Glucagon causes the liver to break down stored glycogen into glucose and release it into the blood, raising the level back to normal.