The author tries to relax while his resting metabolic rate is measured.

Credit: Vasiliki Costarelli.

University of California at Berkeley. (“Imagine a golden light moving up your body, each part of you relaxing as it bathes in the glow.”) It was all very restful—spring sunshine seeped through the blinds, and the only noises were the hum of the building and a whispered conversation between Anderson and Dr. Costarelli. I tried to stay awake. And every 30 seconds, tubes in the hood carried some air away to an analysis chamber, which measured the amount of oxygen I breathed in.

For one of those inhaled oxygen molecules, it was a short, simple trip through my nose and down my windpipe. Then the path forked at my bronchioles, leading down into my lungs. These air passages divide again and again, into a labyrinth of ever-narrowing tubes. Eventually, after crossing more than 20 such junctions, the molecule reached a blind alley—one of the lungs’ air sacs, called an alveolus, where gases move into and out of the blood. The membrane of an alveolus is no barrier to a molecule as small as oxygen, and it slipped easily out of the lungs and into the bloodstream. Instantly, a vastly larger molecule

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