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Why does your nose run when it's cold?

The human nose works a bit like thermostatic heater: when it gets cold, warm blood rushes through the nose. 

Cold triggers a stimulus similar to that of dust. The nose therefore also reacts with stuffiness. Cold reactions also fall under the responsibility of the nose, which has to fulfill a total of three tasks: filtering air, humidifying it and also warming it when it is cold.

This system is similar to thermostatic heating. When the air is cold, temperature sensors in the nose report an under-temperature to the brain. The control center then sends a heating command, and warm blood flows into the so-called turbinates that run through the inside of the nose. They swell and warm the air so that it doesn't reach the lower airways as icy. 

At the same time as the turbinates swell, the secretion production in the nose increases. The fluid, which is formed even under normal conditions to keep the mucous membranes moist, usually flow backwards - the nose "runs backwards" without you noticing it. In the case of high production and a swollen nose, this no longer works sufficiently, the nasal secretion flows forward - the nose runs.

The increased mucus production has an important function: It protects and cleans the nose. The nasal secretions prevent foreign particles from getting stuck in the nose. 

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