A thermal regulator in the brain, after receiving messages from temperature sensors in the skin, automatically alerts blood vessels there to constrict. “You can see this when someone suddenly goes into a very cold building, they go pale or their skin mottles,” says Prof. Eccles. The next stage is shivering, which will raise body temperature by generating heat.
At the same time, blood vessels constrict in the nose and throat, where bacteria and viruses often lurk. “If you were to look into a throat, you could see it go from a nice pink-red to a very pale color,” says Prof. Eccles. “This happens within a few seconds to conserve the heat that we lose to the air we breathe out.”
When blood flow diminishes, the white blood cells that typically fight bacteria and viruses do too, allowing these latent risk factors to easily bloom into a full-blown cold. “If there isn’t as much blood flow to the throat, there aren’t enough white blood cells to ward off infection,” Prof. Eccles says.