Topic 14.4 · Supplement
Body Temperature Control
Keeping internal body temperature close to its normal level is another example of homeostasis, coordinated by the brain, and it depends on a small set of skin structures working in opposite ways depending on whether the body is too hot or too cold.
Required skin structures
You should be able to identify the following in a skin diagram: hairs, hair erector muscles, sweat glands, receptors, sensory neurones, blood vessels, and fatty tissue. Together, these connect temperature detection, heat loss, heat production and insulation into one coordinated system, with the brain acting as the coordinating centre for the responses below — “the hypothalamus” may be a correct supporting detail if you already know it, but it isn’t the required wording here, so the brain’s role is what an answer needs to state.
When the body is too hot
Sweatingincreases. Sweat is released onto the skin, and when the water in it evaporates, thermal energy is transferred away from the body, cooling the skin. It’s worth being precise about the mechanism here — sweating cools the body because evaporation removes thermal energy directly from the skin, not simply because the body is losing water.
Vasodilation also occurs: the arterioles supplying skin-surface capillaries widen, which increases blood flow through capillaries close to the surface and increases heat transfer from the body to the surroundings. Hair erector muscles relax and hairs lie flatter, though this effect is minor in humans compared with its role in furry mammals. The combined result of sweating and vasodilation is greater heat loss.
When the body is too cold
Shivering occurs — rapid, repeated contractions of skeletal muscle. Muscular activity requires respiration, and respiration releases energy, some of which appears as heat, increasing metabolic heat production.
Vasoconstriction occurs alongside it: the arterioles supplying skin-surface capillaries narrow, so less blood flows through capillaries near the surface, and less heat is transferred from the blood to the surroundings. Insulation plays a supporting role throughout — fatty tissue beneath the skin reduces heat transfer, and raised hairs can trap an insulating layer of air, though this is a comparatively minor effect in humans next to species with fur. Together, shivering, vasoconstriction and insulation produce reduced heat loss.
The trap: blood vessels don’t move
The single most common mistake here is describing blood vessels as physically moving towards or away from the skin surface. They don’t. The arterioles supplying skin-surface capillaries stay exactly where they are — what changes is their diameter, widening in vasodilation and narrowing in vasoconstriction. It can help to picture a fixed-depth pipe with an adjustable nozzle: the pipe never relocates, but opening or narrowing the nozzle changes how much flows through it. An answer that has vessels “coming closer to the surface” in hot conditions is describing a mechanism the syllabus doesn’t recognise, however intuitive it might sound.
This entire system is another working example of the negative feedback described on the homeostasis page: a rise in body temperature activates mechanisms that increase cooling, and a fall activates mechanisms that conserve or generate heat, both correcting the deviation back towards the same set point.