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Why is it more difficult to breathe at high altitudes?
Blood and Circulation
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A pair of lungs filled with air contains the same volume of air as in eight large soda bottles—more than 1.5 gallons (6 liters). In order to accommodate this volume, muscles raise and expand the rib cage while the diaphragm, a domed muscular sheet under the lungs, flattens with each breath inhaled.
It is difficult to breathe at high altitudes because there is less oxygen available in the atmosphere. If the concentration of oxygen in the alveoli drops, the amount of oxygen in the blood drops. At altitudes of 9,843 feet (3,000 meters) or more, people often feel lightheaded, especially if they are exercising and placing extra demands on the cardiovascular and respiratory systems.