Homeostasis

Why a stable internal environment matters

Cells and the enzymes inside them work efficiently only within narrow limits of temperature, pH, and the concentration of substances such as water and glucose. If conditions inside the body vary too much, enzyme activity and cell function can be seriously disrupted.

Homeostasis is the process by which the body keeps these internal conditions relatively constant, even when the external environment changes.

Negative feedback: the control mechanism

1. A receptor detects a change (a deviation) away from the normal set point of a variable, such as body temperature or blood glucose concentration.
2. This information is sent, usually by nervous or hormonal signals, to a control centre that compares the change to the normal set point.
3. The control centre triggers an effector to produce a response that opposes the original change, pushing the variable back toward the set point.
4. As the variable returns to normal, the receptor detects this and the response is reduced or switched off.
5. This self-correcting loop, where the response always counteracts the original change, is called negative feedback.

Two examples of homeostasis

How it is examined

Exam questions on this standard commonly describe a change, such as a rise in body temperature after exercise or a fall in blood glucose after fasting, and ask you to explain the negative feedback response step by step. You may also be asked to interpret a graph showing a variable fluctuating slightly above and below a set point over time, and to identify the effector responsible at each stage.

Worked exam-style question

Question. After a person runs a long distance in hot weather, their skin becomes flushed and covered in sweat, and their body temperature is measured slightly above normal before gradually returning to normal over the next hour. (a) Name the receptor most likely responsible for detecting this change in body temperature.

(b) Describe two effector responses shown in this scenario, and explain how each helps to lower body temperature. (c) Explain what would happen to the sweating response once body temperature returns to its normal set point.

Model answer. (a) The thermoreceptors (in the skin and the hypothalamus of the brain) detect the rise in body temperature. (b) Sweating: sweat is produced onto the skin surface, and its evaporation removes heat energy from the body, cooling it down.

Vasodilation: blood vessels near the skin surface widen, increasing blood flow to the skin so more heat can be lost to the surroundings by radiation. (c) As body temperature returns to the normal set point, the thermoreceptors detect this, and the sweating response is reduced or switched off, since the original change (the deviation) has been corrected, an example of negative feedback.

Practice question

Try this. A person has not eaten for several hours, and their blood glucose concentration falls below the normal set point. Describe, step by step, the negative feedback response that would restore normal blood glucose concentration.

Exam tip

Key terms

These terms from Chapter 13 support this topic:

• Hormone, a chemical messenger, such as insulin or glucagon, that travels in the blood to act on a target organ.
• Stimulus, a detectable change in the internal or external environment that triggers a response.
• Osmoregulation, the homeostatic control of water and salt balance in the body, carried out mainly by the kidneys.

Source:SRC-DSKP-EN

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