Thermoregulation

What you'll learn

Thermoregulation is the process by which the human body maintains a constant core temperature of approximately 37°C, regardless of external conditions. This topic is a key component of homeostasis in the AQA GCSE Biology specification. You'll explore the mechanisms the body uses to detect temperature changes and the responses triggered to maintain optimal conditions for enzyme activity.

Key terms and definitions

Thermoregulation — the maintenance of a constant internal body temperature (approximately 37°C in humans) through negative feedback mechanisms.

Homeostasis — the regulation of internal conditions within narrow limits, including temperature, blood glucose levels and water balance.

Vasodilation — the widening of blood vessels (particularly arterioles) near the skin surface to increase heat loss through radiation.

Vasoconstriction — the narrowing of blood vessels near the skin surface to reduce heat loss and conserve body heat.

Thermoregulatory centre — the region in the hypothalamus of the brain that monitors and controls body temperature.

Negative feedback — a control mechanism where a change in a condition triggers responses that counteract the initial change, returning the system to its set point.

Shivering — rapid, involuntary muscle contractions that generate heat through increased respiration in muscle cells.

Sweating — the secretion of water (and dissolved salts) onto the skin surface from sweat glands; evaporation of this water removes heat energy from the body.

Core concepts

Why thermoregulation is essential

The human body must maintain a core temperature of approximately 37°C because:

• Enzyme activity — enzymes controlling metabolic reactions have an optimum temperature of around 37°C. If body temperature rises too high, enzymes denature (their active sites change shape permanently), preventing them from catalysing essential reactions. If temperature drops too low, enzyme activity decreases significantly, slowing metabolic processes.

• Cell function — extreme temperatures damage cell membranes and other cellular structures, compromising cell survival.

• Homeostasis — maintaining stable internal conditions allows cells to function efficiently despite changing external environments.

Body temperature that deviates significantly from 37°C can be life-threatening. Hypothermia (core temperature below 35°C) and hyperthermia (core temperature above 40°C) both require urgent medical intervention.

The thermoregulatory centre and temperature detection

The thermoregulatory centre is located in the hypothalamus of the brain. This region acts as the body's thermostat by:

Monitoring blood temperature directly:

• Receptors in the thermoregulatory centre detect the temperature of blood flowing through the brain
• This provides continuous information about core body temperature

Receiving information from temperature receptors:

• Thermoreceptors in the skin detect changes in external temperature
• These receptors send electrical impulses along sensory neurones to the thermoregulatory centre
• This provides early warning of environmental temperature changes before core temperature is affected

Coordinating responses:

• When temperature deviates from the set point (37°C), the thermoregulatory centre triggers appropriate responses
• These responses involve both nervous system signals and hormonal changes
• Multiple mechanisms work together to restore normal temperature

Responses to increased body temperature

When core body temperature rises above 37°C (for example, during exercise, hot weather, or fever), the thermoregulatory centre coordinates several cooling mechanisms:

Vasodilation:

• Arterioles near the skin surface dilate (widen)
• More blood flows through capillaries in the surface layers of the skin
• The skin appears flushed or red
• Heat is transferred from blood to the environment by radiation
• This is the most significant method of heat loss when the body is too warm

Increased sweating:

• Sweat glands secrete more sweat onto the skin surface
• Sweat is mainly water with dissolved salts
• Water evaporates from the skin surface, transferring heat energy from the body to the environment
• Evaporation requires energy (latent heat of vaporisation), which is taken from the body, cooling it down
• In humid conditions, evaporation is less efficient because the air already contains substantial water vapour

Reduced muscle activity:

• Behavioural changes such as reducing physical activity lower metabolic heat production
• This is often a conscious response to feeling too hot

Hair lies flat:

• The erector muscles in the skin relax
• In humans, this has minimal effect due to limited body hair
• In other mammals with thick fur, flattened hair reduces the insulating air layer

Responses to decreased body temperature

When core body temperature falls below 37°C (for example, in cold environments), the thermoregulatory centre triggers warming mechanisms:

Vasoconstriction:

• Arterioles near the skin surface constrict (narrow)
• Less blood flows through capillaries in the surface layers of the skin
• The skin may appear pale
• Less heat is lost by radiation from the skin surface
• Blood is diverted to deeper vessels, keeping vital organs warm

Decreased sweating:

• Sweat gland activity reduces or stops completely
• This prevents cooling through evaporation

Shivering:

• Skeletal muscles contract and relax rapidly and involuntarily
• These muscle contractions require increased cellular respiration
• Respiration releases energy, some of which is transferred as heat
• This heat warms the body

Increased metabolic rate:

• The hormone adrenaline is released
• In prolonged cold exposure, thyroid hormones increase metabolic rate
• Increased respiration in cells releases more heat energy
• This mechanism is more important for long-term cold adaptation

Behavioural responses:

• Putting on warmer clothing
• Moving to a warmer location
• Adopting postures that reduce surface area (curling up)
• Increased physical activity

Hair stands upright (piloerection):

• Erector muscles in the skin contract
• Hairs stand on end, creating "goosebumps"
• In furry mammals, this traps an insulating layer of air
• In humans, this response is largely vestigial (an evolutionary leftover with little current function)

Negative feedback in thermoregulation

Thermoregulation operates through negative feedback, a key homeostatic mechanism:

1. Receptor — thermoreceptors in the skin and thermoregulatory centre detect a change in temperature (either increase or decrease from 37°C)

2. Coordination centre — the thermoregulatory centre in the hypothalamus processes this information

3. Effector — appropriate effectors respond:

   • Blood vessels (vasodilation or vasoconstriction)
   • Sweat glands (increased or decreased secretion)
   • Skeletal muscles (shivering or relaxed)

4. Correction — these responses counteract the initial change, returning body temperature toward 37°C

5. Feedback — as temperature returns to normal, the corrective responses are reduced or stopped

This creates a self-regulating cycle that maintains temperature within narrow limits around the set point.

The role of the skin in thermoregulation

The skin is the primary effector organ for thermoregulation, containing several structures involved in temperature control:

Blood vessels:

• Extensive network of arterioles and capillaries
• Can vasodilate or vasoconstrict to control heat loss
• Located in the dermis layer of skin

Sweat glands:

• Coiled tubes extending from the dermis to the skin surface
• Secrete sweat through pores
• Under control of the nervous system

Hair and erector muscles:

• Each hair has an attached erector muscle (arrector pili)
• Muscle contraction raises the hair

Subcutaneous fat layer:

• Layer of adipose tissue beneath the dermis
• Acts as insulation, reducing heat loss
• Thickness varies between individuals

Large surface area:

• The skin's extensive surface area allows significant heat exchange with the environment
• Heat transfer occurs through radiation, conduction, convection and evaporation

Worked examples

Example 1: Explaining vasodilation (4 marks)

Question: Explain how vasodilation helps reduce body temperature when a person exercises in warm weather.

Mark scheme answer:

• Arterioles (near the skin surface) dilate/widen ✓
• More blood flows through capillaries in the skin ✓
• More heat is transferred from blood to the environment / heat is lost by radiation ✓
• (Body/core) temperature decreases ✓

Examiner note: Notice that each mark point makes a distinct, specific statement. Vague answers like "blood vessels get bigger" without mentioning arterioles or the location may not gain full credit. The question asks you to "explain how" so you must describe the mechanism and the outcome.

Example 2: Coordinated response (6 marks)

Question: A person moves from a warm room into cold weather outside. Describe how their body responds to maintain a constant body temperature.

Mark scheme answer:

• Thermoreceptors in skin detect temperature decrease ✓
• (Impulses sent to) thermoregulatory centre in hypothalamus/brain ✓
• Vasoconstriction (of arterioles near skin surface) ✓
• Less blood flows through skin capillaries / less heat lost by radiation ✓
• Shivering / muscles contract (rapidly) ✓
• (Muscle contraction requires) respiration which releases heat/energy (as heat) ✓

Examiner note: For a 6-mark question, aim for at least 6 distinct points. Start with detection (receptors), then coordination (thermoregulatory centre), then multiple effector responses. Don't repeat the same point in different words.

Example 3: Evaluating effectiveness (4 marks)

Question: Sweating is less effective at cooling the body in humid conditions than in dry conditions. Explain why.

Mark scheme answer:

• Sweating cools the body through evaporation (of water from skin) ✓
• Evaporation requires energy / heat energy is transferred from the body ✓
• In humid conditions, the air already contains lots of water vapour ✓
• This reduces the rate of evaporation / water cannot evaporate as easily (into saturated air) ✓

Examiner note: The question requires you to first explain the normal mechanism, then explain why humidity interferes with it. This tests understanding, not just recall.

Common mistakes and how to avoid them

• Confusing vasodilation and vasoconstriction — Remember: vasoDILATION = DIlate (widen) for cooling; vasoconSTRICTION = constrict (narrow) for warming. Don't say "capillaries dilate" — it's the arterioles that change diameter, affecting blood flow through capillaries.

• Stating that temperature receptors are only in the skin — Receptors exist both in the skin (detecting external changes) AND in the thermoregulatory centre (detecting blood temperature). Both sources of information are important.

• Writing that shivering "creates energy" — Energy cannot be created. Shivering increases the rate of respiration in muscle cells, which releases energy that was stored in glucose. Some of this energy is transferred as heat.

• Forgetting the role of the thermoregulatory centre — Always include the coordination centre when describing the full response. Detection → Coordination → Response is the standard homeostasis pathway.

• Vague descriptions of sweating — Don't just say "sweating cools you down." Explain that water evaporates from the skin surface and that evaporation requires energy, which is taken from the body as heat.

• Overestimating the importance of hairs in humans — While you should know that hairs stand up when cold (piloerection/goosebumps), recognise this is largely ineffective in humans due to our sparse body hair. It's much more significant in furry mammals.

Exam technique for "Thermoregulation"

• Command words matter — "Describe" requires you to state what happens; "Explain" requires you to say how or why it happens, often linking cause and effect. For thermoregulation, explanations should connect the mechanism to the temperature change achieved.

• Use precise terminology — Always use the correct scientific terms: vasodilation, vasoconstriction, thermoregulatory centre, hypothalamus, arterioles. Avoid casual language like "blood vessels open up."

• Structure answers logically — For homeostasis questions, follow the receptor → coordination centre → effector → response sequence. This ensures you cover all aspects and helps you identify where marks are allocated.

• Link mechanisms to outcomes — Don't just describe what happens (e.g., "arterioles dilate"). Explain the consequence (e.g., "more blood flows near the skin surface, so more heat is lost by radiation, reducing body temperature").

Quick revision summary

Thermoregulation maintains body temperature at approximately 37°C through negative feedback. Thermoreceptors in the skin and thermoregulatory centre (hypothalamus) detect temperature changes. When too hot, the body responds through vasodilation and increased sweating to increase heat loss. When too cold, vasoconstriction reduces heat loss, while shivering generates heat through increased respiration in muscles. Multiple mechanisms work together, coordinated by the thermoregulatory centre, to maintain the optimal temperature for enzyme activity and cell function.