Temperature Regulation Response

Why body temperature is controlled

Cells and enzymes work best within a fairly narrow range of conditions. A substantial fall in body temperature slows enzyme-controlled reactions, while a substantial rise can disrupt enzymes and other cell processes. The body therefore regulates its internal temperature, rather than simply matching the temperature of its surroundings.

Temperature can be challenged from outside the body, for example by cold weather or warm clothing, and from inside it, for example when active muscles release more energy as heat during exercise. Thermoregulation responds automatically to either kind of challenge.

Detecting and processing a temperature change

Temperature control follows a coordinated pathway:

1. Thermoreceptors in the skin detect changes in external temperature. They can therefore provide early information about the surroundings.
2. Thermoreceptors in the hypothalamus monitor the temperature of the blood flowing through the brain, providing information about internal temperature.
3. The hypothalamus processes this information. It acts as the coordination centre and compares the condition with the body's normal range.
4. If temperature has moved away from that range, the hypothalamus sends nerve impulses to suitable effectors, including sweat glands and muscles.

These responses are autonomic: they do not have to be started consciously.

Response when the body is too hot

The hypothalamus coordinates responses that increase heat transfer from the body:

• Sweat glands secrete more sweat. As water in sweat evaporates from the skin, it transfers thermal energy from the body to the surroundings, cooling the skin.
• Vasodilation occurs. Muscles in the walls of arterioles supplying the skin relax, widening the vessels. More blood flows through capillaries near the surface, so more thermal energy can be transferred to the surroundings.
• Hair erector muscles relax, so hairs lie flatter. In humans, this response has much less effect than sweating and altered skin blood flow.

Together, these changes reduce body temperature towards its normal range.

Response when the body is too cold

The hypothalamus instead coordinates responses that conserve or generate heat:

• Skeletal muscles shiver. Their rapid contractions increase respiration and release more energy as heat.
• Vasoconstriction occurs. Muscles in the walls of arterioles supplying the skin contract, narrowing the vessels. Less blood flows near the surface, so less thermal energy is transferred to the surroundings.
• Hair erector muscles contract, making hairs stand upright. The trapped layer of air can reduce heat loss, although this has only a small insulating effect in humans.
• Sweating decreases, so less cooling occurs by evaporation.

These changes raise body temperature towards its normal range.

How negative feedback restores the normal range

This control is an example of negative feedback because each set of responses opposes the original change. A rise in temperature triggers cooling responses; a fall triggers warming and heat-conserving responses.

As the temperature returns towards its normal range, thermoreceptors detect the reduced deviation. The hypothalamus then reduces its signals to the effectors, so responses such as shivering or increased sweating diminish. This prevents the corrective response from continuing at full strength after it is no longer needed.

The hot and cold responses are antagonistic because they have opposing effects. For example, vasodilation increases heat transfer at the skin, whereas vasoconstriction reduces it. Having opposing effectors allows control in both directions and keeps internal temperature within a narrow range rather than perfectly fixed at one value.

Control pathway

Temperature change → thermoreceptors in the skin and hypothalamus → processing by the hypothalamus → nerve impulses → effectors → corrective response.

• Skin receptors detect external temperature changes.
• Hypothalamic receptors monitor blood temperature.
• The hypothalamus coordinates autonomic responses.

Too hot

• More sweating: evaporation increases cooling.
• Vasodilation: more blood flows near the skin surface, increasing heat transfer.
• Hairs lie flatter.

Too cold

• Shivering: muscle contractions release more heat.
• Vasoconstriction: less blood flows near the skin surface, reducing heat transfer.
• Hairs stand upright and sweating decreases.

Negative feedback

The response opposes the temperature change. As temperature returns towards its normal range, the corrective response is reduced. Hot and cold responses are antagonistic because they act in opposite directions.