system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can one of your nostrils be completely blocked up then when you turn onto your side, opposite to that of the blocked nostril, it almost instantly clears up?
gpt
Congestion in your nose is primarily caused by the filling up of tissues (“turbinates”) in your nostrils with blood. They are similar to erectile tissue.

The nostril that is on the bottom has higher blood pressure and therefore is more likely to fill up.

You will notice that if you stand up they will both clear up more because the blood drains out of your head more toward your feet.