Why do we need to maintain a constant internal environment?Monitoring the body's temperature – Higher

Detecting changes in external temperature

Temperature receptors in the skin detect changes in the external temperature. Neurons transmit this information as nerve impulses to the brain. The area in the brain that receives this information is the thermoregulatory centre. It is found in the hypothalamus.

The hypothalamus also has temperature receptor cells which detect changes in the temperature of the blood flowing through the brain.

If the temperature is above or below 37°C, the hypothalamus sends electrical nerve impulses to effectors, which are mainly found in the skin. This causes a response that brings body temperature back to 37°C.

Temperature control and negative feedback

The hypothalamus constantly monitors the temperature of the blood. The constant monitoring of the temperature of the blood means that negative feedback can happen.

Too cold

If the temperature falls below 37°C then it sends nerve impulses to effectors, the skin and muscles, to cause:

• shivering
• vasoconstriction in blood vessels that supply skin capillaries with blood - this reduces blood flow near the skin's surface and reduces heat loss
• erector muscles in hair follicles to contract causing hairs to stand on end and trap warm air next to the skin
• sweat glands to stop secreting sweat

The blood temperature returns back to the set level and the mechanisms are switched off.

Too hot

If the temperature goes above 37°C then the muscles and skin receive signals to cause:

• shivering to stop
• vasodilation to happen to increase blood flow near the skin's surface and increase heat loss
• erector muscles in hair follicles to relax, releasing any warm air trapped next to the skin
• sweat gland to start secreting sweat, which will evaporate from the surface of the skin, removing heat energy

The blood temperature returns back to the set level and the mechanisms are switched off.