Lesson 2 of 10 / Homeostasis and response
Controlling body temperature
Why do sweating and skin blood flow change together?
I can explain how the hypothalamus coordinates responses to an increase or decrease in body temperature.
I can explain hypothalamic coordination of temperature responses.Syllabus
Syllabus K325, Topic 8(c). Explain hypothalamic coordination of temperature responses.
About 6 min
Make a guess. You are not marked.
You go red in the face when you are hot. What is happening in your skin?
Show the answer
Arterioles widen, so more blood flows near the surface.
Vasodilation of arterioles sends more blood through skin capillaries, so more heat is lost by radiation.
The key idea
The hypothalamus coordinates changes in heat loss and heat production to oppose a change in body temperature.
Blue drops represent sweat; upward arrows represent heat removed during evaporation.
Red: skin blood flow. Blue: sweat. Vessel widths are schematic, not measured diameters.
Too warm: the hypothalamus coordinates more sweating and dilation of skin arterioles. Evaporation removes heat; greater surface blood flow increases heat transfer to cooler surroundings.
Explanation
A thermoreceptor is a receptor that detects temperature. Thermoreceptors in the skin detect changes in the temperature of the surroundings, while thermoreceptors in the hypothalamus detect changes in the temperature of blood flowing through it. The hypothalamus coordinates appropriate responses through the nervous system.
When the body is too warm, sweat glands secrete more sweat. Water evaporating from the skin takes heat from the body. Sweat that remains liquid provides much less of this cooling effect, so humid air can reduce the effectiveness of sweating.
Arterioles supplying the skin capillaries also dilate. This widening is called vasodilation. More blood flows through the capillaries near the skin surface, so more heat is lost to cooler surroundings by radiation. The vessels change diameter; they do not move towards the surface.
When the body is too cold, these arterioles constrict. This narrowing is called vasoconstriction. Less blood flows through the skin capillaries, so less heat is lost. Sweating decreases. Rapid involuntary contractions of skeletal muscles cause shivering; increased respiration in these muscles releases more heat. These responses decrease as temperature returns towards its normal range.
Step by step
- 1
Detect
Thermoreceptors provide information; the hypothalamus coordinates the response.
- 2
Change heat exchange
Link sweat evaporation and skin blood flow to the rate of heat loss.
- 3
Change heat production
In cold conditions, shivering increases muscle activity, respiration and heat release.
Worked example
Warm skin after exercise
Explain why a person may have warm, flushed skin while losing heat after exercise.
One way to explain it
Vasodilation of skin arterioles increases blood flow through capillaries near the surface. More heat is transferred from the blood to cooler surroundings, helping temperature fall towards normal.
Why this answer works
- Warm skin does not mean the response is heating the body.
- Greater blood flow carries heat to the surface.
- State the corrective effect on internal temperature.
Watch out for this
A student says: "Capillaries move closer to the skin when the body is hot." What is wrong with this?
Show the answer
Blood vessels stay in place. Vasodilation, the widening of the arterioles that supply the skin capillaries, increases the flow of blood through capillaries near the skin.
Check your understanding
Why can high humidity reduce cooling by sweating?
Show the answer
Water evaporates less readily, so less heat is removed from the skin.
Evaporation requires heat; reduced evaporation reduces this route of heat loss.
Exam-style questions
Check your understanding
Body temperature falls below normal. Which response helps to bring it back up?
Show the answer
Arterioles in the skin constrict, so less blood flows near the surface
Vasoconstriction reduces heat loss from blood near the skin surface.
Quick recall
Why do sweating and skin blood flow change together?