You always breathe with only one nostril

Your nostrils operate alternately, it is called "nasal cycle". Right now one of them is working in full, while the other "moonlight". Every few hours, the nervous system that monitors the heart rate, digestion, and a host of other processes, restarts the nasal cycle and starts to work the other nostril. Opening and closing the nostrils partly is due to the swelling and deflation of the corpus cavernosum in the nose. By the way, from the cavernous bodies are sexual organs, so it can be compared to the swelling of erection.

When you have a cold, "active" nostril provides you with oxygen and "passive" is filled with mucus. This explains the fact that one nostril is always laid more than the other, unless, of course, a runny nose is not so intense, from both nostrils, flowing like tap.

via factroom.ru