Thermoregulation (SACE Stage 2 Biology)

What this dot point is asking

You need to explain why temperature must be controlled, identify the control centre and receptors, and describe the specific cooling and warming responses as a negative feedback loop.

Why temperature must be regulated

Body temperature must stay close to the set point of about 37 degrees Celsius because the body's reactions are controlled by enzymes, which have an optimum temperature. Too hot and enzymes denature; too cold and reactions become too slow. Keeping temperature stable keeps metabolism working efficiently.

Humans are endotherms: we generate heat internally and regulate temperature physiologically, unlike ectotherms which rely mainly on behaviour and the environment. Maintaining a stable core temperature lets endotherms stay active across a wide range of external conditions, but it carries a high energy cost because heat must be continually generated by metabolism.

Nervous and hormonal responses working together

Thermoregulation is a good example of the two communication systems cooperating. The fast nervous responses - sweating, shivering and changes in skin blood flow - begin within seconds of the hypothalamus detecting a change, providing immediate correction. The slower hormonal response, in which the hypothalamus stimulates the pituitary to release TSH and the thyroid to release thyroxine, raises the metabolic rate over a longer period to sustain heat production in prolonged cold. Adrenaline from the adrenal glands also raises metabolic rate quickly. Combining a rapid nervous response with a sustained hormonal one gives both speed and endurance, which is why strong answers mention both pathways.

Heat exchange with the environment

Understanding how the body gains and loses heat sharpens these answers. Heat is lost by radiation, conduction, convection and the evaporation of sweat, and is gained from the environment when it is hotter than the body or generated internally by respiration and muscle activity. Vasodilation and vasoconstriction work by changing how much warm blood reaches the skin, altering the rate of radiation and convection from the surface. Sweating relies specifically on evaporation, which is why it becomes ineffective in very humid conditions where sweat cannot evaporate - a frequent exam application.

The control centre

The hypothalamus in the brain is the control centre for temperature. It contains receptors that monitor blood temperature, and it receives information from temperature receptors in the skin. When core temperature moves from the set point, the hypothalamus coordinates a corrective response, communicating through nerves and hormones.

Responses to overheating

When body temperature rises above the set point, the responses increase heat loss:

• Sweating - sweat on the skin evaporates, and evaporation removes heat from the body.
• Vasodilation - blood vessels near the skin surface widen, so more warm blood flows close to the surface and loses heat to the surroundings.
• Reduced metabolic activity and behaviour such as removing clothing or seeking shade.

These responses cool the body back toward the set point, then switch off.

Responses to cooling

When body temperature falls below the set point, the responses conserve and generate heat:

• Vasoconstriction - surface blood vessels narrow, reducing blood flow to the skin and limiting heat loss.
• Shivering - rapid muscle contractions generate heat through respiration.
• Raised metabolic rate - thyroxine and adrenaline can increase heat production.
• Erect hair (piloerection) and behavioural responses such as adding clothing.