Thermoregulation

AQA spec ref: 3.6.4 - Thermoregulation

Body temperature profoundly affects enzyme activity, membrane fluidity, and the rate of virtually all metabolic reactions. Maintaining a stable core temperature is therefore essential to life, particularly in mammals and birds. The mechanisms of thermoregulation are a classic example of negative feedback - the same principle that governs blood glucose control and osmoregulation.

Why Temperature Regulation Matters

Enzymes have an optimum temperature - typically around 37°C in humans. Above this, excess kinetic energy causes the tertiary structure of the enzyme to denature (hydrogen bonds and other interactions break, altering the active site). Below the optimum, reduced kinetic energy slows the rate of enzyme-substrate collisions. Maintaining a narrow core temperature keeps enzyme-catalysed reactions proceeding at their maximum rate.

Membrane fluidity is also temperature-dependent: too cold and phospholipid bilayers become rigid and impermeable; too hot and they become excessively fluid and leaky.

Ectotherms vs Endotherms

Ectotherms (e.g. reptiles, fish, insects) rely primarily on external heat sources to regulate body temperature. Their metabolic rate is lower and more variable. They use behavioural strategies (basking in sun, seeking shade) rather than physiological ones. Their core temperature tracks ambient temperature.

Endotherms (mammals and birds) generate heat internally through metabolism - primarily from respiration in metabolically active tissues (liver, muscle, brain). They can maintain a stable core temperature independent of the environment, allowing activity in cold conditions. The cost is a high metabolic rate and constant need for food.

The Hypothalamus - The Thermostat

In mammals, thermoregulation is controlled by the hypothalamus in the brain. It acts as a thermostat with a set point (~37°C in humans).

The hypothalamus receives input from two sources:

• Peripheral thermoreceptors - in the skin; detect changes in external temperature
• Central thermoreceptors - within the hypothalamus itself; monitor the temperature of the blood flowing through it (core temperature)

Changes from the set point trigger responses via the autonomic nervous system (and some hormonal routes) to restore temperature. This is negative feedback - the response opposes the change.

Responses to Overheating (Too Hot)

When core temperature rises above the set point:

Sweating

Sweat glands secrete sweat (mostly water + dissolved salts) onto the skin surface. As water evaporates, it absorbs latent heat of vaporisation from the skin, cooling it. Even a small volume of evaporated sweat removes a large amount of heat because water has a high latent heat of vaporisation. See Water.

Vasodilation

Arterioles in the skin dilate - their smooth muscle relaxes, increasing the diameter of the vessel and reducing resistance. More blood flows through the superficial capillaries near the skin surface. The skin becomes flushed (red). Heat is lost to the environment by radiation and convection. This is not blood being diverted - the arterioles dilate to increase overall blood flow near the surface.

Reduction in metabolic rate

Less heat is generated by reducing the rate of metabolic reactions.

Flattening of hair/fur

The erector pili muscles relax → hairs lie flat → little insulating air layer is trapped → heat dissipates more easily. Less significant in humans (evolutionary remnant - "goosebumps" when cold is the reverse).

Behavioural responses

Seeking shade, reducing activity, panting (dogs, rodents - evaporation from the mouth/respiratory tract).

Responses to Cooling (Too Cold)

When core temperature falls below the set point:

Vasoconstriction

Arterioles in the skin constrict - smooth muscle contracts, narrowing the vessel. Less blood flows near the skin surface. Heat is retained in the core. The skin becomes pale. This is the counter-current heat exchange effect - blood in deeper vessels retains heat rather than radiating it.

Shivering

Skeletal muscles contract rapidly and involuntarily (antagonistic muscle pairs contract simultaneously). No mechanical work is done, so almost all the energy from ATP hydrolysis is released as heat. This raises core temperature rapidly.

Increased metabolic rate

The liver increases its rate of respiration, generating more heat. Adrenaline and thyroxine (long-term) increase metabolic rate.

Piloerection (raising of hairs)

Erector pili muscles contract → hairs stand on end → a layer of still air is trapped between the hairs → air is a poor conductor → insulation increases → less heat lost. More effective in other mammals with dense fur; less effective in humans.

Behavioural responses

Huddling, wearing clothing, reducing surface area exposed.

Summary of the Negative Feedback Loop

```

Core temperature rises/falls

↓

Detected by thermoreceptors (skin + hypothalamus)

↓

Hypothalamus coordinates response via autonomic NS

↓

Effectors: sweat glands, arterioles, skeletal muscle, liver

↓

Temperature restored to set point

↓

Negative feedback - response switched off

```

Comparing Vasodilation and Vasoconstriction

Summary

• Endotherms maintain core temperature via the hypothalamus detecting changes→negative feedback via autonomic NS
• Too hot: vasodilation, sweating, hair flattening, reduced metabolic rate
• Too cold: vasoconstriction, shivering, piloerection, increased metabolic rate
• Vasodilation=arteriole smooth muscle relaxes→more blood near surface→more heat loss
• Vasoconstriction=arteriole smooth muscle contracts→less blood near surface→less heat loss
• Sweating works via latent heat of vaporisation of water

AQA Exam Tips

• Vasodilation/vasoconstriction: always describe the mechanism in terms of smooth muscle in arteriole walls. "Blood vessels dilate" is insufficient - state: "smooth muscle in arteriole walls relaxes → lumen widens → more blood flows through capillaries near skin surface → more heat lost by radiation."
• Shivering mechanism: "antagonistic muscles contract simultaneously → no useful work done → energy released as heat." Do not just say "muscles contract."
• Sweating: link to water's high latent heat of vaporisation - a key property of water. The mark scheme expects: evaporation of sweat → latent heat absorbed from skin → skin cools.
• Negative feedback: always state the loop explicitly - stimulus → receptor → coordinator → effector → response opposes the change → returns to set point.
• Piloerection: the mechanism must include "traps a layer of still air" → "air is a poor conductor/insulator" → "reduces heat loss." Saying "hairs stand up to keep warm" without the mechanism gets no marks.
• Ectotherm vs endotherm questions: ectotherms are not "cold-blooded" - their temperature matches the environment. They cannot maintain a stable temperature through internal heat generation.