Your nose has a bit of built-in redundancy. Although you could survive perfectly well with a single nostril, your nose holds two parallel breathing passages that are divided by a thin wall of cartilage called the septum. These passages meet at the back of your throat, where they take a single tunnel down to your lungs.
So why do you have two nostrils? Most people don’t spend much time comparing them, and simply assume that the second one is there to take over when the first one is blocked—say, by a nasty cold or an antisocial finger. But the reality is subtler and stranger.
Your two nostrils shift their workload back and forth in a delicate dance called the nasal cycle. At any moment, most of the air you inhale travels through just one nostril, while a much smaller amount seeps in through the other. At some point, the nasal cycle reverses course and the workload shifts to the other nostril. The length of time between nostril switching varies, depending on the individual and various other factors, but each cycle usually lasts from 40 minutes to several hours. (That’s why you often have intermittent periods of easy breathing even when you suffer from a heavy cold. In this case, one nostril is plugged more than the other.)
For the longest time, scientists had no idea why the nasal cycle took place. After all, your nostrils aren’t doing much work, so there’s no reason they need hours of idle time. But recent research has uncovered the apparent reason: nostril switching improves your sense of smell.
To understand why, you need to realize that the nasal cycle changes the way air passes through your nose. In your dominant nostril, the air moves very quickly. In your other nostril, it seeps through more slowly.
This difference is important because odor-causing chemicals vary in the amount of time they take to dissolve through the mucus that lines your nasal cavity. Chemicals that dissolve quickly have the strongest effect in a fast-moving airstream that spreads them out over as many odor receptors as possible. But chemicals that dissolve slowly are easier to smell in a slow-moving airstream. If the air rushes by too quickly, the chemicals will be whisked away before they’ve reached any odor receptors. That’s why your nose has both a fast road and a slow lane. Quite simply, the combination of two nostrils with different airflows gives you a more detailed“smell picture” of the world around you. Incidentally, your nose performs nostril switching using special erectile tissue that swells to narrow the passageway in the nostril that’s taking a break. Collectors of bizarre body trivia take note: It’s the same sort of erectile tissue that’s at work in male and female reproductive organs.
Source of Information : Oreilly - Your Body Missing Manual