When we consider how much air we inhale and exhale over a period of time, the first thing we think of is the minute ventilation, the amount of air we breathe in and out over a period of 60 seconds. This is what we feel—the touch of the breath in the nostrils, in and out, over a period of one minute. All you have to do to calculate your minute ventilation is measure your tidal volume and multiply that value times the number of breaths you take per minute. According to textbooks, this would be 500 ml per breath times u breaths per minute, and this equals 6,000 ml per minute.
The minute ventilation does not tell us everything we need to know however, because what is most important is not the amount of air thai moves in and out of the nose or mouth, but the amount of air that gets past the anatomic dead space into the alveoli. This is also measured over a peri« of one minute and is called, logically enough, the alveolar ventilation. It 1 our primary concern when we want to know how breathing affects the content of oxygen and carbon dioxide in the blood, and that is our main interest it yoga breathing exercises. To calculate the alveolar ventilation, subtract th< size of the anatomic dead space from the tidal volume before multiplying b\ the respiratory frequency. For example, 500 ml of tidal volume minus 150 m of anatomic dead space equals 350 ml per breath, and 350 ml per breath time 12 breaths per minute yields an alveolar ventilation of 4,200 ml per minute
The values given for lung volumes and capacities, as well as for minut and alveolar ventilation, are only textbook examples—it is not uncommor to breathe more rapidly and take in a smaller tidal volume for each breath If you watch a dozen people closely in casual situations, such as when the} are sitting on a bus with their arms folded across their chests, you can easily count the breaths they take per minute, and it is usually faster than tin textbook standard of 12 breaths per minute: 24-30 breaths per minute is ; lot more common. This is of no great consequence because everyone simph adjusts their tidal volume so that their alveolar- ventilation stays within ; normal range (fig. 2.14). In meditation the rate of breathing generalh seems to slow down, but it can still vary widely and may either be faster 01 slower than the standards cited in the medical literature on respiration. Here too, you adjust the rate of breathing and the tidal volume so that the alveolar ventilation comes in line with the metabolic requirements of the practice.
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