The Science of Yoga

misapprehension neglected what turns out to be one of yoga’s main sources of leverage over the unruly currents of human emotion—the manipulation of the body’s relationship with carbon dioxide, which both Paul and Gune had begun to glimpse.
    Today, after many decades of research, it is fairly easy to distinguish between fact and fiction, despite the complexities of respiratory physiology. The big picture is the best place to start.
    The atmosphere of our planet is 21 percent oxygen. That’s a lot. In comparison, the levels of carbon dioxide are five hundred times smaller. The human body exploits this ocean by means of hemoglobin—the remarkable protein inside our red blood cells that soaks up oxygen like a sponge and carries it from the lungs to the tissues. Typically, the refreshed hemoglobin of a resting person is nearly saturated with oxygen, holding virtually as much as it possibly can. The usual figure for the level of absorption is 97 percent.
    For yoga, the glut of oxygen in the air and the saturation of hemoglobin in the lungs mean that fast or slow breathing does little to change the levels that enter the bloodstream—as Gune found at his ashram and as I found at the University of Wisconsin. The vital gas is available in large quantities no matter what.
    The body’s consumption of oxygen does go up and down. But science demonstrates that it does so in response to changes of muscle activity, metabolism, and heart rate—not breathing styles. As we saw in the last chapter, cardiovascular fitness can raise peak oxygen consumption.
    The story with carbon dioxide in the bloodstream is dramatically different and variable. Consider a person breathing in a relaxed way. Fresh air mixes in the lungs with stale air, creating an inner environment where carbon dioxide levels remain fairly high. This person, in typical fashion, ventilates the lungs so inefficiently that each relaxed inhalation replaces less than 10 percent of the gas.
    Now consider what happensif that individual starts to breath fast. Blasts of fresh air with extraordinarily low concentrations of carbon dioxide (three-hundredths of 1 percent of the atmosphere) rush into the lungs, lowering the inner levels. Nature seeks to equalize the concentrations. So diffusion quickly draws more carbon dioxide out of the bloodstream and into the lungs. The result is that the body’s levels plunge.
    This view is anything but contentious. It is the standard account found in hundreds of medical textbooks as well as official pronouncements of the U.S. Navy. Its responsibilities for thousands of professional divers make it a global authority on human respiration. Fast breathing “lowers body stores of carbon dioxide,” the Navy’s diving manual states, “without significantly increasing oxygen stores.”
    The common name for fast breathing is hyperventilation, and the common danger is passing out. It can also result in dizziness, headaches, light-headedness, slurred speech, and numbness or tingling in the lips, hands, and feet. The drop in carbon dioxide influences mood in many ways. One is through respiratory alkalosis. It heightens the excitability of nerves and muscles—so much so that many circuits short out, producing tingling in the hands and spasms in the muscles.
    Yogis often feel such sensations after doing many rounds of Bhastrika, a fast-breathing exercise known as Breath of Fire or Bellows Breath, after its Sanskrit definition. Bhastrika puts the emphasis on exhalation rather than inhalation, like the bellows of a blacksmith. In Light on Pranayama , Iyengar says the repeated blasts of the exercise create “a feeling of exhilaration.”
    While exhilaration and nerve excitement go up, fast breathing does something else that has critical repercussions for mood, mental outlook, and potentially health—it robs the brain of oxygen.
    The reason is that the surge in carbon dioxide causes blood vessels in the brain to contract, reducing the flow of oxygen and producing lightheadedness and perhaps blurred vision. Other symptoms include dizziness and giddiness. In extreme cases, a person can hallucinate or pass out.
    What this means in plain English—as crazy as it sounds, as counterintuitive as it seems, as contrary to the teachings of popular yoga as it appears—is that fast breathing lowers the flow of oxygen to the brain, and does so dramatically. Scientists have found that it cuts levels roughly in half. That plunge is why people faint.
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