Thermoregulation

Where it happens

Thermoregulation is coordinated by the hypothalamus, which acts as the body's thermostat, monitoring blood temperature and sending nerve impulses to the skin to trigger the correct response.

Inputs and outputs

• Input: a change in blood temperature, detected by thermoreceptors in the hypothalamus.
• Input: temperature changes at the skin surface, detected by thermoreceptors in the skin and relayed to the hypothalamus.
• Output when too hot: increased sweat secretion, vasodilation of arterioles supplying skin capillaries, and relaxed hair erector muscles.
• Output when too cold: reduced sweat secretion, vasoconstriction, contracted hair erector muscles, shivering and a raised metabolic rate.
• End result: blood temperature returns to the set point of about 37°C and the corrective response switches off.

The control loop

1. The hypothalamus detects a change in blood temperature above or below the normal set point of about 37°C.
2. If the body is too hot, the hypothalamus sends impulses that increase sweat secretion and widen the blood vessels near the skin surface (vasodilation).
3. If the body is too cold, the hypothalamus sends impulses that narrow the blood vessels near the skin surface (vasoconstriction) and trigger shivering, the rapid contraction of muscles.
4. Hair erector muscles also respond: they relax when the body is hot, and contract to raise the hairs and trap an insulating layer of air when the body is cold.
5. As body temperature returns to normal, the hypothalamus detects this and reduces the response, another example of negative feedback.
6. A cold stimulus also raises the metabolic rate: the hypothalamus signals for more heat to be released by respiration in the liver and muscles, adding to the heat generated by shivering.
7. Behaviour supports the physiological responses: a person moves into shade or removes clothing when hot, and puts on clothing or curls up when cold, reducing the load on the skin responses.

Why it matters and how it is controlled

Body temperature must stay close to 37°C because enzymes work fastest near this optimum. A rise of a few degrees above normal begins to change the shape of the active site of enzymes, so metabolic reactions slow and eventually stop.

A fall below normal slows reactions because molecules have less kinetic energy, and severe cold can stop the heart from beating properly.

Control follows the negative feedback model that appears throughout the homeostasis topic. The hypothalamus is both the detector and the control centre; the skin, blood vessels and skeletal muscles are the effectors.

The response always acts to reverse the original change, and once the set point is reached the hypothalamus reduces the signal. This is why body temperature oscillates slightly around 37°C rather than staying exactly on it.

Thermoregulation is coordinated by nerves, not hormones, so the responses are fast. The one hormonal contribution to mention at SPM level is the raised metabolic rate over longer periods of cold, which produces extra heat as a by-product of respiration.

Skin responses compared

The skin produces almost opposite responses depending on whether the body is too hot or too cold.

How it is examined

You may be asked to explain why sweating cools the body, to describe what vasodilation and vasoconstriction do to heat loss, or to explain why shivering produces heat.

Structured questions often present a diagram of a section through the skin and ask you to label the sweat gland, blood capillary and hair erector muscle, then explain each structure's response to heat or cold. A second style gives a graph of body temperature over time during exercise and asks you to explain the shape of the curve using the terms detector, control centre, effector and negative feedback.

Essay questions tend to ask you to compare the responses of the skin when the body is too hot and too cold. A full answer states the stimulus, names the hypothalamus, describes each effector response, and links each response to heat gain or heat loss.

Marks are lost when the mechanism of heat loss (evaporation, radiation) is omitted.

Common misconceptions

Worked exam-style question

Question. A student runs for 20 minutes on a hot afternoon. Her skin becomes flushed and wet with sweat.

(a) Name the part of the brain that detects the rise in her blood temperature. (b) Explain why her skin appears flushed.

(c) Explain how sweating lowers her body temperature. (d) After she rests in the shade, her skin colour returns to normal.

Explain, using the term negative feedback, why this happens.

Model answer. (a) The hypothalamus. (b) The hypothalamus sends nerve impulses to the arterioles supplying the skin, which widen (vasodilation).

More blood flows through the capillaries near the skin surface, so the skin looks red and more heat is lost by radiation. (c) The sweat glands secrete more sweat onto the skin surface.

When sweat evaporates it absorbs latent heat from the skin, so the skin and the blood flowing through it are cooled. (d) Once blood temperature falls back to the set point of about 37°C, the hypothalamus detects this and stops sending the impulses; the arterioles constrict back to normal.

The response reverses the original change, which is the definition of negative feedback.

Source:SRC-DSKP-EN

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