A marathon runner finishes a long race on a hot day. During the race, their body temperature rose and they lost a large amount of water through sweating. Explain how the body detects and responds to maintain a stable internal environment after the race.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Maintaining a stable internal environment is essential for the body's cells to function correctly. This is known as homeostasis, and it relies on communication between different parts of the body.
Model answer (5 marks)
1. Receptors in the skin and hypothalamus detect the rise in core temperature and the loss of water.
2. The signal is transmitted via afferent nerves to the hypothalamus, the brain’s homeostatic centre.
3. The hypothalamus sends efferent signals through the autonomic nervous system to effectors.
4. The effectors cause sweating and cutaneous vasodilation, increasing heat loss and fluid replacement.
5. These responses lower body temperature and restore fluid balance, returning the internal environment to normal and maintaining homeostasis.
2. The signal is transmitted via afferent nerves to the hypothalamus, the brain’s homeostatic centre.
3. The hypothalamus sends efferent signals through the autonomic nervous system to effectors.
4. The effectors cause sweating and cutaneous vasodilation, increasing heat loss and fluid replacement.
5. These responses lower body temperature and restore fluid balance, returning the internal environment to normal and maintaining homeostasis.
Examiner tips
- Use the word "receptors" and "hypothalamus" – key terms. Show the signal flow: receptor → brain → effector. Mention the specific responses: sweating and vasodilation. Finish by linking the response to the return to normal conditions.
Common mistakes
- Mixing up the direction of signals (afferent vs efferent). Failing to name the hypothalamus as the processing centre. Omitting the corrective responses or the final return to normal.
Mark scheme (5 marks)
- The body monitors internal conditions / receptors detect changes in temperature or water content
- A message / signal is sent to a processing centre (the brain / hypothalamus)
- The processing centre sends a signal to effectors to bring about a response
- A named corrective response to high body temperature, e.g. sweating increases / vasodilation occurs to lose more heat
- The response returns conditions to the normal level / maintains a stable internal environment (homeostasis)
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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