The Respiratory System and the Lungs - Oxygen requirements

How much air do we breathe, and how much oxygen do we absorb into our body from the air? A normal, moderately active person breathes in and out (a complete respiration or breath cycle) about 18 times a minute; that is, the diaphragm contracts and relaxes 18 times a minute, or something over 25,000 times every day. At about four-fifths of a pint of air per breath cycle, this means that we inhale and exhale about 20,000 pints, or 10,000 quarts, or 2,500 gallons of air every day.

Only a very small proportion of this volume is oxygen that finds its way into our bloodstream: about a pint every minute in normal, quiet breathing, a little over 1,400 pints, or 700 quarts, or 175 gallons of oxygen every day. The amount of oxygen our lungs are capable of delivering to our body, however, varies tremendously: during sleep a person may need only a half-pint of oxygen per minute, half the average, while the lungs of a hard-driving athlete striving to break the mile record can deliver up to five quarts to the bloodstream—ten times the average.

At any given time, there are about two quarts of oxygen circulating in our blood. This is why a stoppage of breathing has an upper time limit of about four minutes before it causes irreversible damage or death. With our body needing about a pint of oxygen every minute for normal functioning, we have about four minutes before we use up the oxygen dissolved in our blood and other tissues.

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