Abstract
Air that enters the respiratory system is first conditioned: it is filtered, warmed, and humidified by the tracheobronchial tree before the air arrives at the parts of the tree than gas exchange. Gas flow in the respiratory system is governed by pressure differences, and these are driven by expansion or compression of the volume of the lungs. During restful breathing, the diaphragm provides the force for expansion, while expiration is passive and requires no muscles. The external intercostals aid inspiration. The abdominal muscles and internal intercostals aid forceful expiration. Muscles move the chest wall and not the lungs, but the lungs follow because the thin fluid in the pleura pastes the lungs to the chest wall. The lungs have a compliance that is produced by elastic fibers and by surface tension. The surface tension is reduced by surfactant, whose lack in newborns causes respiratory distress syndrome. The surfactant helps stabilize alveoli size, keeps the alveoli dry, and reduces the work of breathing. The chest wall acts like a spring: at low volumes it recoils to a higher volume; at high volumes it recoils to a smaller volume. The lungs always tend to collapse. Put together, the balance of lung collapse and chest expansion is the FRC—functional residual capacity.
Published Version
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