A stuffy nose feels as if something has physically plugged the airway.
Sometimes mucus is part of the problem. But a major reason the nose feels blocked is that the tissue lining the nasal passages becomes swollen.
The hallway itself gets narrower.
Your nose contains tissue that can swell
The inside of the nose contains a rich supply of blood vessels.
When you have a cold, allergies, or another irritation, chemical signals can cause those blood vessels to widen. More blood moves into the tissue, and the lining swells.
Even a modest amount of swelling matters because the nasal passages are already fairly narrow.
You do not need a giant wad of mucus to make breathing feel difficult.
Why does your body make more mucus too?
Mucus helps trap particles and keep the nasal lining moist.
During an infection or allergic reaction, the nose may increase mucus production as part of its effort to trap and remove irritants.
That gives you two things at once: swollen tissue and extra fluid.
No wonder breathing through your nose can suddenly feel like trying to use a straw somebody stepped on.
Why is one nostril sometimes more blocked than the other?
Even healthy noses alternate somewhat between sides.
This is called the nasal cycle. Blood flow naturally changes from one side of the nose to the other over the course of the day.
Most of the time you barely notice.
When you already have congestion, the normal cycle can make one side feel dramatically more blocked for a while.
Lie on your side and gravity can make the lower nostril feel worse too.
Why do allergies cause congestion?
Allergies happen when the immune system reacts to substances such as pollen, dust mites, or animal dander.
The resulting chemical signals can cause swelling, itching, sneezing, and watery mucus.
The allergen is not physically filling the nose. Your own immune response is changing the tissues inside it.
Why does a cold do the same thing?
Viruses can infect cells lining the respiratory tract.
The immune response to that infection causes inflammation. Blood vessels widen, tissues swell, and mucus production can increase.
That is also why congestion can last even when you have already blown your nose repeatedly. You can remove mucus, but blowing your nose does not instantly shrink swollen tissue.
Is every stuffy nose caused by a cold or allergy?
No.
Dry air, smoke, strong odors, medications, structural problems, hormonal changes, and other conditions can affect nasal airflow too.
Congestion that is severe, persistent, one-sided for a long time, or accompanied by significant pain or other concerning symptoms can deserve medical evaluation.
For the ordinary stuffy nose, though, the explanation is simple.
The nose is not always clogged with material.
Sometimes the walls moved inward.
The same body systems connect with Why do we sneeze?, another everyday effect with its own underlying mechanism.
And that’s the short explanation.
Related explanations: Why do we sneeze?, Why does snot turn green?, and Why does your nose run when you cry?