Core body temperature is controlled by the thermoregulatory centre in the hypothalamus, a small region at the base of the brain.The hypothalamus receives two kinds of information:
• from its own thermoreceptors, which monitor the temperature of the blood passing through it (a direct measure of core temperature)
• from peripheral thermoreceptors in the skin, via sensory neurones, which give early warning of a change in the external temperature.The thermoregulatory centre has two parts:
• a heat loss centre, activated when the blood is warmer than the norm. It sends impulses that cause vasodilation, more sweating and flattening of the hairs, and it inhibits shivering.
• a heat gain centre, activated when the blood is cooler than the norm. It causes vasoconstriction, less sweating, raising of the hairs, shivering and a higher metabolic rate (via adrenaline and, over a longer time, thyroxine).Most of these responses are brought about by the autonomic nervous system, so they are involuntary. The hypothalamus also sends impulses to the cerebral cortex, making us aware that we are hot or cold, so we change our behaviour.Example: fever. During an infection, chemicals released by the immune system raise the norm (set point) in the hypothalamus, for example to 39 °C. Blood at 37 °C is now detected as too cold, so the heat gain centre causes shivering and vasoconstriction. The person feels cold even while their temperature is rising. When the infection clears, the set point returns to normal, the heat loss centre is activated and the person sweats.
• from its own thermoreceptors, which monitor the temperature of the blood passing through it (a direct measure of core temperature)
• from peripheral thermoreceptors in the skin, via sensory neurones, which give early warning of a change in the external temperature.The thermoregulatory centre has two parts:
• a heat loss centre, activated when the blood is warmer than the norm. It sends impulses that cause vasodilation, more sweating and flattening of the hairs, and it inhibits shivering.
• a heat gain centre, activated when the blood is cooler than the norm. It causes vasoconstriction, less sweating, raising of the hairs, shivering and a higher metabolic rate (via adrenaline and, over a longer time, thyroxine).Most of these responses are brought about by the autonomic nervous system, so they are involuntary. The hypothalamus also sends impulses to the cerebral cortex, making us aware that we are hot or cold, so we change our behaviour.Example: fever. During an infection, chemicals released by the immune system raise the norm (set point) in the hypothalamus, for example to 39 °C. Blood at 37 °C is now detected as too cold, so the heat gain centre causes shivering and vasoconstriction. The person feels cold even while their temperature is rising. When the infection clears, the set point returns to normal, the heat loss centre is activated and the person sweats.