I’m aware a stuffy nose is caused not by mucus, but by “swollen blood vessels in the nasal passages”. But why does such swelling happen? How exactly does a cold causes this?
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When your body detects that a foreign substance has entered the nose (including things like viruses, pet dander, pollen, and other airborne irritants) your immune system releases histamines. Histamines bind to alpha and beta receptors in the mucosa at a molecular level and tell the body to increase blood flow to the sinuses and turbinates. The blood vessels and capillaries become larger and you get a stuffy nose. Antihistamines block this action by antagonistically binding to these same receptors which prevents histamine from binding and causing a reaction. Antihistamines will reduce the itch you feel, while nasal decongestants will actively shrink the blood vessels and capillaries so you can breathe again.
ELI5 version: Cold air makes your nose try to warm the arm.
ELI20: cold causes mast cells to do a histamine release which causes your blood vessels to become leaky, causing increased swelling which then warms the air due to increased blood flow.
The cold virus attacks the lining of your nose. Your immune system in response sends signals to the blood vessels in the nose to increase blood flow and to get a little more leaky. Leaky blood vessels and increased blood flow overall allow your immune system easier access to the tissues. Blood vessels don’t have convenient little airlocks for the immune cells to exit. Instead white blood cells for example must squeeze between the cells lining the blood vessel. Kinda like cutting open a chain link fence to sneak through.
The end result of all this increase leaking and blood flow is what you call “swelling”. You have probably seen the same thing around a cut on your hand. But there isn’t much extra room inside your nose, so the swelling on one side runs into the swelling on the other side and pretty soon you can’t move any air through. There could be other benefits to this swelling. For example it’s much harder to get more viruses into your nose if the tissue is swollen shut. Or it could just be an unfortunate side effect of the inflammation.
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