What you need to know
- 7.17homeostasisKeeping the internal environment within narrow limits, despite changes outside. keeps the body in dynamic equilibriumA state that is kept roughly constant by continual small corrections, so it fluctuates around a set point. during exercise. The hypothalamusA region of the brain that controls homeostasis, including body temperature and water balance. is the thermoregulatory centreThe region of the hypothalamus that detects blood temperature and controls responses to keep body temperature stable..
Heatstroke first aid is not the exam list. Do not say capillaries move. Adrenaline’s heart effects stay in 7B. Sweat composition calculations are not required.
By the end, you should be able to…
- Define homeostasis and dynamic equilibrium.
- Describe responses when body temperature rises or falls.
- Link exercise to several variables being corrected at once.
Build your understanding
A moving balance
homeostasisKeeping the internal environment within narrow limits, despite changes outside. is the maintenance of a stable internal environment. dynamic equilibriumA state that is kept roughly constant by continual small corrections, so it fluctuates around a set point. means the variables are not frozen. They fluctuate in a narrow band and are continually corrected by negative feedbackA change from the normal level triggers a response that reverses the change, returning conditions to the norm.. During exercise, heat production, carbon dioxide, oxygen demand and water loss all shift. heart rateThe number of heartbeats per minute., ventilation, sweating and ADHAntidiuretic hormone: released from the posterior pituitary, it makes the collecting duct more permeable to water, so less water is lost in urine. do not wait for the exercise to finish before they matter. The internal environment stays inside workable limits while the work is going on.
If you write “homeostasis keeps everything constant”, you have missed the word dynamic. A small rise that is then corrected is success, not failure.
The hypothalamus and a rise in temperature
The hypothalamusA region of the brain that controls homeostasis, including body temperature and water balance. contains the thermoregulatory centreThe region of the hypothalamus that detects blood temperature and controls responses to keep body temperature stable.. It detects the temperature of the blood and receives information from skin thermoreceptors. If core temperature rises, it coordinates heat-loss responses. Sweat glands secrete, and evaporation of that water cools the skin. Arterioles in the skin vasodilate, so more blood flows near the surface and more heat is lost by radiation. Hair erector muscles relax, so hairs lie flat and trap less air. Metabolic heat production can fall.
Say arterioles, not capillaryA tiny vessel whose wall is a single layer of endothelium, giving a short diffusion distance for exchange with tissues.. Capillaries do not have a muscle layer that “moves them closer to the skin”. vasodilationWidening of arterioles when their smooth muscle relaxes, so more blood flows through. is a widening of the arterioles that supply the skin capillaries.
A fall in temperature, and the exercise mix
If core temperature falls, the hypothalamus coordinates the opposite. Skin arterioles vasoconstrict, so less blood flows at the surface and less heat is lost. Sweating stops. Hairs are raised and trap air. Skeletal muscles shiver: those contractions respire and release heat. These are negative feedback. The response opposes the fall.
Exercise is a simultaneous test of several loops. Muscles warm the body, so thermoregulationKeeping body temperature within narrow limits. shifts toward heat loss. Ventilation and cardiac outputThe volume of blood pumped by one ventricle per minute: cardiac output = stroke volume × heart rate. rise for gases. Water lost in sweat tends to raise plasmaThe liquid part of blood: water carrying dissolved substances such as glucose, ions, hormones and proteins. concentration, so ADH may rise. The specification example of dynamic equilibrium is this whole pattern, not temperature alone. If heat loss cannot keep up, the result can become heat illness. That is a failure of the limits, described on the NHS page as enrichment and as a medical problem, not as another effector to memorise.
Follow the process
Core temperature rises → the hypothalamus detects warmer blood and input from skin thermoreceptors → sweat is secreted and its evaporation removes heat → skin arterioles vasodilate, so more blood flows near the surface and more heat is lost by radiation → hairs lie flat. Core temperature falls → the same centre coordinates the opposite → arterioles vasoconstrict, sweating stops, hairs are raised, and shivering contractions release heat. Say arterioles, not capillaries. The cooling is evaporation, not “sweat is a cold liquid”. Each response opposes the change, so the equilibrium can move during exercise and still be corrected.
NOT: capillaries do not move closer to the skin, and shivering is not positive feedback.
- The centre is the hypothalamus
- Hot: sweat, wide arterioles, flat hairs
- Cold: narrow arterioles, raised hairs, shiver
- Cooling is evaporation
Two traps this diagram blocks
- Do not say capillaries move toward the skin.
- Do not call shivering positive feedback.
From the diagram alone
Question
Explain why the skin looks flushed when core temperature rises, and name the centre that coordinates this. [3]
Show a model answer
The hypothalamus coordinates vasodilation of skin arterioles, so more warm blood flows through surface capillaries and more heat is lost by radiation. The capillaries themselves do not move.
| Effector | Too hot | Too cold |
|---|---|---|
| Skin arterioles | Vasodilate | Vasoconstrict |
| Sweat glands | Secrete; evaporation cools | Little or no sweat |
| Hair erector muscles | Relax; hairs flat | Contract; hairs raised |
| Skeletal muscle | No shiver | Shivering generates heat |
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: skin looks flushed when core temperature rises. Y: the hypothalamusA region of the brain that controls homeostasis, including body temperature and water balance. causes skin arterioles to vasodilate. Z: more blood flows through the surface capillaryA tiny vessel whose wall is a single layer of endothelium, giving a short diffusion distance for exchange with tissues. and more heat is lost by radiation. Incomplete: “Capillaries move closer to the skin.” Improved: “Capillaries do not move. The arterioles that supply them widen, so more warm blood reaches the surface.”
Name the centre
Hypothalamus, then the effector, then how heat is gained or lost.
Evaporation, not “sweat is cold”
The cooling is the evaporation of water, which takes heat from the skin.
Avoid this misconception
The claim: Capillaries move toward the skin surface when you are hot, and shivering is positive feedback.
The correction: Arterioles vasodilate. Capillaries do not migrate. Shivering when you are cold opposes the fall in temperature, so it is negative feedbackA change from the normal level triggers a response that reverses the change, returning conditions to the norm..
Why does a runner stay near 37 °C?
Exercising muscles release a lot of heat, yet core temperature only rises a little. Something in the brain is detecting the change and switching on corrections.
What the model shows
Core temperature, the hypothalamus, the skin effectors and shivering, with the heat gained and lost each second.
What to look for
Which effectors switch on when temperature rises or falls, and how far temperature moves before it is held.
What it tells us
The hypothalamus uses negative feedback: each response opposes the change, keeping temperature in a narrow band (dynamic equilibrium).
Be careful what you claim
One body compartment; illustrative gains.
Try it in the model: choose Jog and watch the thermometer.
Thermoregulation and exercise
Exercise, then change the air around a model body. Watch the hypothalamus switch effectors on and off to hold core temperature near 37 °C, then switch the control centre off or overwhelm it. Typical adult values; effector gains illustrative.
What you see
- The core temperature on a thermometer with the 37 °C set point, the hypothalamus, a section through the skin, and the heat gained and lost each second.
- A graph of core temperature over the last hour, and a graph of what the effectors are doing.
How to use it
- Choose the activity, the air temperature and humidity, and whether the hypothalamus is working. The clock runs one minute of body time per second.
What it shows
- Homeostasis as dynamic equilibrium: temperature moves a little when exercise disturbs it and is continually corrected by negative feedback (7.17).
- The hypothalamus as the thermoregulatory centre, coordinating skin arterioles, sweat glands, hair erector muscles and shivering (7.17).
Limits of the model
- One core compartment for a 70 kg adult: heat capacity 245 kJ K⁻¹, heat from metabolism 100 W at rest, 600 W jogging and 1000 W on a hard run. Skin temperature, clothing, wind and the skin thermoreceptors are folded into one heat-loss term; “radiation” in the heat panel includes convection. Effector gains are illustrative.
- Real set points and limits differ between people. Water loss, ADH and the heart and breathing responses are other loops in 7B and 7C 3. This is not a way to judge anyone’s health.
Guided tour · step 1 of 7
- In Body and air, just below these steps, set Activity to Jog (options: Rest, Jog, Hard run).
- Let the clock run until sweating starts in the simulator and Core temperature levels off.
Working muscles release about six times as much heat. Core temperature rises a little, the hypothalamus detects the warmer blood, and the skin arterioles vasodilate and the sweat glands secrete. Temperature settles about 0.6 °C above the set point and is held there: dynamic equilibrium, not a failure.
Body and air
Questions
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Define homeostasis.
Show suggested answer
Maintenance of a stable internal environment.
Practice question
Retrieval and explanation · suggested answer
What does dynamic equilibrium mean during exercise?
Show suggested answer
Conditions fluctuate around a set point and are continually corrected, rather than staying fixed, while exercise is disturbing them.
Practice question
Retrieval and explanation · suggested answer
Describe two responses coordinated by the hypothalamus when core temperature rises.
Show suggested answer
Any two of: sweating and evaporative cooling, vasodilation of skin arterioles, hairs lying flat, reduced metabolism.
Practice question
Retrieval and explanation · suggested answer
Why is shivering negative feedback?
Show suggested answer
It is a response to a fall in temperature, and the muscle contractions release heat, which opposes the fall.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A runner’s core temperature is slightly above the set point, sweat rate is high, and breathing rate is high. Explain this as dynamic equilibrium rather than as “homeostasis has failed”. Then state one observation that would instead suggest the temperature control was being overwhelmed. Use the lesson mechanisms, not a diagnosis.
Show a hint
Small departure plus active correction is the definition.
Explore the reasoning
Heat from muscle has pushed temperature up, and sweating plus ventilation are corrections in progress, so the system is in dynamic equilibrium. It would look overwhelmed if temperature kept climbing despite sweating, or if sweating had stopped while temperature was still very high. That second pattern is the heat-illness warning on the NHS page, not a specification effector. No personal diagnosis is being made.
Recall this lesson
Lesson recall questions
Question 1 of 5: Why is homeostasis described as dynamic equilibrium during exercise rather than an unchanging state?
Homeostasis in exercise
Practice question
Retrieval and explanation · suggested answer
Why is homeostasis described as dynamic equilibrium during exercise rather than an unchanging state?
Lesson: 7C 4: Thermoregulation and Exercise
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Heat exhaustion is not an extra effector
7C · Thermoregulation and ExerciseThe NHS distinguishes heat exhaustion, with heavy sweating, from heatstroke, where the person is dangerously hot, the skin may be hot and dry, and emergency help is needed.…Specification 7.17Read & use the scienceClose article
Why this matters
The NHS distinguishes heat exhaustion, with heavy sweating, from heatstroke, where the person is dangerously hot, the skin may be hot and dry, and emergency help is needed. Use this only as the boundary of 7.17: negative feedback has a limited capacity. Do not turn the symptom list into required recall, and do not use this page as medical advice for a real person who is unwell. Seek urgent help for real heat illness. 7.17: the hypothalamus coordinates thermoregulation so exercise can continue in dynamic equilibrium.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.17 Homeostasis and thermoregulation during exercise.
Use the science
Why is “hot dry skin means vasodilation has succeeded” the wrong reading?
Explore a possible response
Successful heat loss includes sweating. Hot dry skin, in the NHS contrast, fits heatstroke, where sweating has failed and the situation is an emergency. Vasodilation alone is not success if evaporative cooling has stopped and temperature is still rising.
Explore the source
Heat stress is a failure of balance, not a thermometer fact
7C · Thermoregulation and ExerciseCDC guidance describes heat-related illness when the body cannot keep temperature controlled under a high heat load. Exercise adds metabolic heat while hot or humid air can limit…Specification 7.17Read & use the scienceClose article
Why this matters
CDC guidance describes heat-related illness when the body cannot keep temperature controlled under a high heat load. Exercise adds metabolic heat while hot or humid air can limit heat loss. The 7.17 idea of dynamic equilibrium explains why the hypothalamus must coordinate responses such as sweating and changes in skin blood flow. It does not imply that a body holds one exact number at every moment. A rise in core temperature is an observation; interpreting the cause requires information about workload, environment, hydration and the effectiveness of heat-loss responses. This is especially relevant for outdoor work and sport.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.17 Homeostasis and thermoregulation during exercise.
Use the science
Why can very humid air make sweating less effective even when the hypothalamus increases sweat production?
Explore a possible response
Evaporation carries heat away. Humid air reduces the rate at which sweat evaporates, so producing more sweat may not remove enough heat to maintain the usual temperature range.
Explore the source
- Heat-related illnessesUS Centers for Disease Control and Prevention