This question was asked by Bilal from Glasgow, UK.
The inside of your nose is full of tiny blood vessels and hairs called sillia (pronounced SILL-ee-uh). It also has a lot of mucus (pronounced MYOO-kus), which you might call snot. This keeps the lining of your nose moist and warms up the air you breathe in before it travels to the lungs. It also captures dust, dirts and germs before they can reach your lungs and make it harder for you to breathe. The sillia help to move the snot and all the trapped debris towards the front of the nose, or the back of the throat.
When you have a cold the nose produces even more mucus as it tries to stop any new germs from entering your body. The extra snot runs out of your nose or down your throat; sometimes it gets caught up in cavities inside your skull called sinuses, which is what gives you that stuffy feeling in your head. If you have certain allergies your body reacts to the allergens (the things you’re allergic to, like pollen or animal hair) as though they were germs, and you may find that your nose is suddenly full of snot again.
When you’re breathing in cold air, the blood vessels in your nose become wider so that they can warm the air before it goes into your lungs. Unfortunately increasing the blood flow to your nose like this stimulates the production of mucus, so your nose begins to run. Sometimes you may notice that you have a lot more snot when you’ve been crying; this is because some of the fluid from your tear ducts drains down into your nose and thins out the mucus, making it more runny.
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