Show Summary Details
Page of

Physiology 

Physiology
Page of

PRINTED FROM OXFORD MEDICINE ONLINE (www.oxfordmedicine.com). © Oxford University Press, 2020. All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Medicine Online for personal use (for details see Privacy Policy and Legal Notice).

date: 03 December 2020

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.

Access to the complete content on Oxford Medicine Online requires a subscription or purchase. Public users are able to search the site and view the abstracts for each book and chapter without a subscription.

Please subscribe or login to access full text content.

If you have purchased a print title that contains an access token, please see the token for information about how to register your code.

For questions on access or troubleshooting, please check our FAQs, and if you can't find the answer there, please contact us.