Homeostasis: Question 9
Syllabus 14.1
A person walks outside into very cold conditions without warm clothing, and their core body temperature begins to fall below its normal set point of about 37°C.
(a) Name the type of receptor that detects this fall in temperature, and state the location of the coordination centre that processes this information. [2]
(b) Describe two involuntary physiological responses (not behavioural responses, such as putting on more clothes) that occur in the skin and muscles to reduce heat loss and/or generate heat, explaining how each response helps to return core temperature towards the set point. [4]
(c) Explain how negative feedback prevents core temperature from continuing to rise once it has returned to the set point, so that these responses do not overcorrect. [2]
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Worked solution
Part (a): Detecting the fall in temperature
Thermoreceptors, found both in the skin and within the hypothalamus itself, detect the fall in core body temperature. The hypothalamus (in the brain) is the coordination centre that receives this information and organises the corrective response.
Part (b): Two responses that conserve or generate heat
Vasoconstriction: smooth muscle in the walls of the arterioles supplying blood to capillaries near the skin surface contracts, narrowing these vessels. Blood is diverted away from the skin surface towards deeper blood vessels, reducing heat loss by radiation and convection from the body surface, so heat is conserved within the core.
Shivering: skeletal muscles throughout the body contract and relax rapidly and repeatedly. This is an involuntary response, driven by nervous stimulation from the hypothalamus, and it substantially increases the rate of respiration in the contracting muscle cells. Because respiration is not perfectly efficient, much of the extra energy released is transferred as heat, which warms the blood flowing through the muscles and so raises core temperature.
Both responses act in the direction that opposes the original fall in temperature: vasoconstriction reduces further heat loss, while shivering actively generates additional heat.
Part (c): Switching the response off. Negative feedback
As core temperature rises back towards 37°C due to these responses, the thermoreceptors detect that temperature is no longer below the set point. The signal they send to the hypothalamus indicating “too cold” therefore weakens and stops. In response, the hypothalamus reduces the nerve impulses sent to the skin arterioles and skeletal muscles, so vasoconstriction relaxes back to normal and shivering stops, the effectors return to their resting level of activity.
Because the corrective response is removed as soon as the deviation that triggered it has been corrected, core temperature does not continue rising past the set point and overshoot into a state of overheating. This ability of a control system to switch itself off once it has restored the set point (rather than continuing to amplify a change, as in positive feedback) is the defining feature of negative feedback.
Final answers
- (a) Thermoreceptors (skin and hypothalamus) detect the fall; the hypothalamus is the coordination centre.
- (b) Vasoconstriction narrows skin arterioles, diverting blood from the surface to reduce heat loss; shivering increases respiration in skeletal muscle, releasing heat to warm the body.
- (c) Once temperature returns to the set point, thermoreceptors stop signalling a deviation, so the hypothalamus reduces stimulation of the effectors, switching off vasoconstriction and shivering before temperature can overshoot, the self-limiting property of negative feedback.