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Show: December 5, 2004:

Produced by WFYI Public Media in Association with Indiana University School of Medicine

IU School Medicine
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Medical Mystery: Why Two Nostrils?

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Air date: December 5, 2004

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Facial features tend to come in pairs. We have two eyes, two ears, and two lips. True, we have only one nose, but look closely, and there they are -- two nostrils side by side. Is there any purpose for having two nostrils, or is this just evolution's way of keeping things symmetrical?

Is there a purpose to having two nostrils? Besides the fact that one gaping nostril would look pretty strange, the answer is yes. Without two separate airways through the nose, our sense of smell wouldn’t be nearly as sharp as it is.

Let's say you're out for a walk and you stop to sniff a flower. You inhale, and air containing tiny flower particles rushes up your nose. Sensors in the nose absorb the particles and send smell signals to the brain. Now, although they may look identical, your nostrils go through regular cycles of swelling and shrinking. When the right is swollen, the left is normal, and vice versa. The result is that the swollen nostril inhales a bit more slowly than the non-swollen nostril, which affects how quickly the flower particles reach the inside of your nose.

This is important because there are two basic kinds of smell-producing particles that enter your nose. Some dissolve slowly and others more quickly. When you sniff a flower, the slow-dissolving flower particles need time to settle down inside your nose in order to really register. Breathe them in too fast and many just pass right through without stopping to contribute to that great flowery smell. Meanwhile, the fast dissolving flower bits like to charge in and take over. Caught on a too slow air stream, these particles will dissolve before they really have a chance to make their mark.

And that's why having two nostrils comes in handy. Slowly-dissolving particles do better when inhaled through the swollen, slow breathing nostril. Fast-dissolving particles thrive when taken in through the non-swollen, fast breathing nostril. Working together, your nostrils allow you to smell both kinds of particles.

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