Thermoregulation

Spec Mapping — OCR H420 Module 5.1.1 — Communication and homeostasis, content statements covering temperature regulation in ectotherms and endotherms, including the role of the hypothalamus and the autonomic effectors that mediate heat loss and heat gain in mammals (refer to the official OCR H420 specification document for exact wording).

Temperature influences every biochemical process in a cell — enzyme activity, membrane fluidity, diffusion rates and the rate of respiration all depend on it. Keeping body temperature within a narrow range is therefore critical for any active animal. Different animals solve this problem in different ways: ectotherms rely mainly on behaviour to exchange heat with the environment, while endotherms use metabolic heat and a suite of physiological mechanisms coordinated by the hypothalamus. This lesson covers the mechanisms of thermoregulation in both groups, with particular emphasis on the detailed mammalian responses required for OCR A-Level Biology A specification module 5.1.1(f)–(g).

Claude Bernard's nineteenth-century milieu intérieur and Walter Cannon's twentieth-century homeostasis both took thermoregulation as their flagship case study — the maintenance of core body temperature is in many ways the paradigm of negative feedback in physiology. Cannon's 1929 paper described thermoregulation as "the most general expression" of homeostatic control because the same architecture (thermoreceptor → hypothalamic integrator → autonomic effector → heat loss or gain → restored set point) recurs unchanged across all endothermic vertebrates. Modern undergraduate physiology texts paraphrase Cannon's analysis closely; the verbatim quotations are not reproduced here.

Key Definitions:

• Ectotherm — an animal whose body temperature depends largely on heat from the environment.
• Endotherm — an animal that maintains body temperature primarily using metabolic heat.
• Thermoregulation — the maintenance of body temperature within a range that allows enzymes and other proteins to function.
• Hypothalamus — the region of the brain that acts as the coordinator of mammalian thermoregulation.

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Why Temperature Matters

Enzymes have an optimum temperature (around 37 °C for most human enzymes). Below the optimum, rates are slow because molecules have insufficient kinetic energy. Above the optimum, the shape of the active site becomes distorted; by about 40–45 °C, most human enzymes are denatured beyond repair. Small temperature changes therefore have disproportionate effects on metabolism. A deviation of ~5 °C from normal can be fatal in mammals.

Temperature also affects membrane fluidity: at higher temperatures, phospholipids move more freely and membranes become leakier; at lower temperatures, they become more rigid and less permeable. The Q₁₀ (temperature coefficient) of most metabolic reactions is ~2, meaning that rates double with every 10 °C rise — hence the dramatic effect of even small temperature changes on metabolism.

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Ectotherms

Ectotherms gain most of their body heat from the environment. Reptiles, amphibians, fish and invertebrates are ectothermic. They tend to have lower resting metabolic rates than endotherms of similar size, and they do not need to eat continuously to fuel heat production.

Behavioural Thermoregulation in a Lizard

Consider a lizard on a cool morning. Its body temperature is close to air temperature — perhaps 15 °C — and at this temperature it is sluggish because metabolic reactions proceed slowly. The lizard warms up by:

• Basking on a rock in direct sunlight, orienting its body to maximise the surface area exposed to the sun.
• Darkening its skin (in species that can change colour) to absorb more radiation.
• Flattening its body against the warm rock to increase conductive heat gain.

Once warmed to its preferred operating temperature (~35 °C in many lizards), it becomes active. If it starts to overheat during the day, it:

• Retreats into the shade or into a burrow.
• Orients so that only a small part of its body faces the sun.
• Pants to lose heat by evaporation.
• Climbs off hot rocks onto cooler surfaces.

flowchart LR
    A[Cold lizard] --> B[Basks in sun]
    B --> C[Body temperature rises]
    C --> D[Active lizard]
    D --> E[Overheating]
    E --> F[Retreats to shade]
    F --> G[Body temperature falls]
    G --> D

Advantages and Disadvantages of Ectothermy

Advantages:

• Less energy is spent on generating heat, so ectotherms require less food.
• A larger proportion of energy intake can be devoted to growth and reproduction.
• They can survive long periods without food (e.g., pythons).

Disadvantages:

• Activity depends on ambient conditions — early mornings and winters limit movement.
• Escape from predators may be impossible when cold.
• Ectotherms are generally confined to warmer climates.

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Endotherms

Endotherms (mammals and birds) generate most of their body heat through metabolism and maintain it using physiological mechanisms. They can remain active regardless of the ambient temperature, but pay a much higher energy cost.

The Hypothalamus as the Thermostat

The hypothalamus contains two sets of thermoreceptors:

• Central thermoreceptors — detect the temperature of the blood flowing through the hypothalamus itself. This gives a measure of core body temperature.
• Peripheral thermoreceptors — in the skin, detect external temperature changes and send signals via sensory neurones to the hypothalamus.

The hypothalamus has two functional centres:

• The heat loss centre is activated when core temperature rises. It sends impulses via the autonomic nervous system to effectors that promote heat loss.
• The heat gain centre is activated when core temperature falls, triggering effectors that conserve or generate heat.

Cooling Mechanisms (Response to Heat)

When core body temperature rises above ~37 °C:

• Vasodilation — arterioles supplying the skin capillaries dilate. More warm blood flows near the surface, so heat is lost by radiation.
• Sweating — sweat glands secrete sweat onto the skin surface. As the water evaporates, it removes latent heat of vaporisation (about 2.4 kJ g⁻¹), cooling the body.
• Flattening of hair (hair erector muscles relax) — reduces the insulating layer of trapped air, allowing heat to escape.
• Reduced metabolic activity — thyroxine secretion is reduced (long-term response), lowering basal metabolic rate.
• Behavioural responses — moving into shade, removing clothing, reducing activity.

Warming Mechanisms (Response to Cold)

When core body temperature falls below ~37 °C:

• Vasoconstriction — skin arterioles constrict, reducing blood flow to the skin. Less heat is lost by radiation. Blood is shunted to deeper tissues via arteriovenous shunt vessels.
• Piloerection — hair erector muscles contract, raising hairs to trap a thicker insulating layer of air. (In humans, this creates "goose bumps" but has little functional effect because we are not hairy enough.)
• Shivering — involuntary rapid contraction of skeletal muscles. Muscle contraction is inefficient, so most of the energy released is heat. Shivering can increase heat production fivefold.
• Increased metabolic rate — adrenaline and thyroxine secretion increase, raising the rate of respiration in tissues such as the liver and muscles.
• Non-shivering thermogenesis — in infants and hibernating mammals, brown adipose tissue contains mitochondria with a protein called thermogenin (UCP1), which uncouples oxidative phosphorylation, releasing the energy of respiration as heat rather than ATP.
• Behavioural responses — curling up, seeking shelter, adding clothing, increasing activity.

Summary Table

Mechanism	Response to heat	Response to cold
Skin arterioles	Dilate (vasodilation)	Constrict (vasoconstriction)
Sweat glands	Active	Inactive
Hair erector muscles	Relaxed	Contracted (piloerection)
Skeletal muscle	Relaxed	Shivering
Metabolic rate	Reduced	Increased
Behavioural	Move to shade, remove clothing	Huddle, shelter, increase activity

flowchart TB
    A[Core temperature rises] --> B[Hypothalamus heat loss centre]
    B --> C[Vasodilation]
    B --> D[Sweating]
    B --> E[Reduced metabolic rate]
    C --> F[Heat lost to environment]
    D --> F
    E --> F
    F --> G[Temperature returns to set point]
    H[Core temperature falls] --> I[Hypothalamus heat gain centre]
    I --> J[Vasoconstriction]
    I --> K[Shivering]
    I --> L[Piloerection]
    I --> M[Increased metabolic rate]
    J --> N[Heat retained / generated]
    K --> N
    L --> N
    M --> N
    N --> G

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Comparing Ectotherms and Endotherms

Feature	Ectotherm	Endotherm
Source of body heat	Mainly environmental	Mainly metabolic
Metabolic rate	Low	High
Activity at low temperature	Limited	Maintained
Food requirements	Low	High
Body temperature constancy	Variable	Constant
Climatic range	Often restricted	Wide
Example	Lizard, snake, fish	Mouse, human, sparrow

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Exam Tip

Be precise about vasodilation and vasoconstriction. Blood vessels in the skin dilate or constrict — the capillaries themselves do not change. OCR mark schemes frequently penalise "capillaries dilate" because capillaries do not have smooth muscle. The correct effector is the arteriole.

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Common Exam Mistakes

• Saying "blood is warmer at the skin surface". Blood flow is redirected closer to the surface during vasodilation, but the blood itself does not change temperature instantly.
• Describing ectotherms as "cold-blooded". Their blood is often warm — they simply rely on environmental heat to warm it.
• Confusing hair erector muscles with hairs themselves. The muscles contract; the hairs are lifted passively.
• Claiming humans rely heavily on piloerection. Goose bumps are evolutionary relics; we are too hairless for them to insulate effectively.
• Saying sweating cools through "cold sweat". Sweating cools by the evaporation of water; without evaporation, sweat does not cool the body.
• Forgetting behavioural responses in endotherms. Mammals and birds also use behaviour — huddling, basking, seeking shade — in addition to physiological mechanisms.

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Quick Recap

• Ectotherms regulate body temperature primarily by behaviour, exchanging heat with the environment.
• Endotherms generate heat metabolically and use physiological mechanisms — vasodilation, sweating, piloerection, shivering — coordinated by the hypothalamus.
• The hypothalamus contains thermoreceptors and acts as the coordinator, with separate heat loss and heat gain centres.
• Sweating, vasodilation and reduced metabolism cool the body; vasoconstriction, shivering, piloerection and increased metabolism warm it.
• Endothermy is energetically expensive but allows activity across a wide range of climates.

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Visualising Thermoregulatory Effectors in a Mammal