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Every Olympic endurance coach in the world now tapes their athletes' mouths shut at night because a Swedish lab proved in 1995 that the nose produces a gas the mouth cannot, and that single gas determines whether your blood absorbs 100% of the oxygen you inhale or only 82%.

The gas is nitric oxide.

The lab was the Karolinska Institute in Stockholm. The discovery was published in Nature Medicine that same year, and it quietly rewrote everything respiratory physiology thought it knew about why humans have a nose in the first place.

Here is what they actually found.

The empty air-filled cavities inside your skull, the ones anatomy textbooks called evolutionary leftovers for a hundred years, are not empty and not useless.

The lining of those sinuses contains an enzyme called inducible nitric oxide synthase. It runs continuously. It produces large amounts of nitric oxide gas. That gas sits in your nasal cavity at concentrations hundreds of times higher than anywhere else in your body.