On an average day, a typical person’s nose will produce about one quart of mucus/fluid (just under one liter). Most all of this snot generally gets passed back into your throat and swallowed, often without you even really being too conscious of it. When you’re breathing cold air though, the rate of mucus production goes up significantly, causing some of that snot to come out the front of your nose, rather than back in your throat.
What’s going on here is the blood supply to your nose actually increases as a response to the cold air, via tiny blood vessels in your nose dilating to increase the blood flow. This helps keep your nose warm as you breathe, as well as begins to warm the cold air you’re breathing before it enters your lungs.
This increased blood flow doesn’t just help warm the air though, it also has a side effect of providing a lot more blood than normal to the glands which produce the mucus in your nose. This, in turn, causes them to start producing snot at a much higher rate than normal, which causes your nose to run when you’re breathing the cold air.
Once you’re back in a warm air environment, the blood vessels in your nose will constrict and the glands that produce the mucus/liquid mix will go back to their normal rate of around four cups of snot per day.
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- Your nose runs when you’re crying because the tears from the tear glands under your eyelids drain into your nose, where they mix with mucus to form very liquidy snot.
- Allergies can cause your nose to run because the body reacts to them the same way it reacts to viruses and the like. Namely, by kicking the mucus glands into overdrive to try to stop as much of the allergen as possible from entering your body.
- As alluded to, the same type of thing is happening when you get a cold or similar sickness. The body increases mucus production rate to try to keep as many of the germs as possible out of your body.
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