Each of us breathes about eight to ten times per minute. When we exercise, the rate increases to 15 to 20 times per minute. Surprisingly, our lungs have no muscles of their own. How, then, is the work of breathing done?
The diaphragm and rib muscles of the chest wall work for the lungs. By changing the size of the chest cavity, these muscles control whether air enters or exits the lungs.
The diaphragm, a broad, thin muscle that stretches across the body between the chest and the abdomen, is responsible for about 75% of the air flow in breathing. At rest, the diaphragm actually bulges upward. When we are about to take a breath of air or inhale, the diaphragm moves downward, thereby increasing the space available (and decreasing total pressure) within the chest. The rib muscles move upward and outward at the same time, increasing the space available for air flow by another 25%. Outside air rushes in to fill this space.
Breathing out, or exhaling, is normally a passive process. As the muscles of the chest and diaphragm relax, the space inside the chest becomes smaller and air moves out of the lungs. When we exhale forcibly, some of these muscles actively help push the air out.
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My Health My World: National Dissemination
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The Environment as a Context for Opportunities in Schools
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Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education