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The respiratory system 

The respiratory system
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date: 21 January 2022

The lungs contain 200–300 million alveoli that are reached via 23 generations of airways. The volume in the lungs after an ordinary expiration is called functional residual capacity (FRC) and is approximately 3–4 L. The lung is elastic and force (pressure) is needed to expand it and to overcome the resistance to gas flow in the airways. This pressure can be measured as pleural minus alveolar pressure. The inspired volume goes mainly to dependent, lower lung regions, but with increasing age and in obstructive lung disease airways may close in dependent lung regions during expiration, impeding oxygenation of the blood. With lowered functional residual capacity,airways may be continuously closed with subsequent gas adsorbtion from the closed off alveoli. Perfusion of the lung goes also mainly to dependent regions, but there is in addition, possibly more important, a non-gravitational inhomogeneity. A ventilation-perfusion mismatch may ensue that impedes oxygenation and CO2 removal, but can to some extent be corrected for by hypoxic pulmonary vasoconstriction.

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