From Inviting Guests into the World (#295) by Rabbi Avigdor Miller
It pays from time to time to gain some understanding, even superficially, of what takes place in your body. We begin with the air. Even the purest air in the world is not fit to enter the interior of your body. It has to be prepared for entrance into your lung. First of all, it must be cleansed. Secondly, it must be moistened. And thirdly, it must be warm.
When you breathe in the air, first it goes into your nasal passages. You have hairs in your nose, and the hairs filter out coarser pieces of dust. They keep out the flies and mosquitoes too. The nasal passages are wet all the time (that's why your nose runs: to keep it wet all the time!). The fly paper action of these surfaces catches dust.
It pays to gain some understanding, even superficially, of what takes place
As you draw the air in further, it reaches the sinuses. Why do you have sinuses? They only seem to cause us trouble. But if we wouldn't have sinuses, there wouldn't be places where the germs could lodge. The sinuses twist, and as the air goes in and out of the twisting passages, it sticks against the wall (it has a sticky mucus on it).
As the air proceeds in, it's being warmed, because you have a lot of blood vessels in the nose passages. That's why your nose bleeds frequently; there are more blood vessels in the surface of the nasal passages than most other places. These blood vessels warm the air as it comes in. And because the naval passages are well watered, the air becomes moist and clean and it's warm.
And in case anything does come past all these guards, all these barriers, it irritates the mucus lining of the nose passages and this irritation causes you to expel it. But you don't expel things so simply. In order to expel something that's lodged in the air passages, you sneeze. But sneezing is a miraculous and complicated system. First you breathe deeply, and then you give a violent sneeze and it dislodges. A baby just born does it perfectly, so you see he's born with that wisdom in him.