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Airways and alveoli 

Airways and alveoli

Airways and alveoli

Peter D. Wagner


May 30, 2013: This chapter has been re-evaluated and remains up-to-date. No changes have been necessary.

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date: 28 April 2017

The lung is the organ of gas exchange, providing the means of transferring oxygen (O2) from the air to the blood by passive diffusion for subsequent distribution to the tissues, and of similarly removing metabolically produced carbon dioxide (CO2) from the blood, which is then exhaled to the atmosphere.

A large area of contact between alveolar gas and capillary blood is required to ensure sufficient gas flux across the blood–gas barrier to meet metabolic demand: the lungs contain about 300 million very small (radius c.150 µm) alveoli.

After the mainstem bronchi have arisen from the trachea, the airways continue an essentially dichotomous branching pattern until the alveoli are reached. Successive branching of connected conducting pipes to the 16th generation yields in the order of 50 000 to 100 000 airways (called terminal bronchioles), each of which supplies a functional lung unit comprising a further 7 generations of divisions (3 divisions of respiratory bronchioles, then alveolar ducts, finally alveoli).

The lungs are enclosed within the thoracic cavity. Inspiration is driven by contraction of the intercostal muscles and the diaphragm, which expands the ribcage in both anteroposterior and lateral dimensions, such that the pressure inside the thoracic cavity but external to the lungs is reduced to below that of the air, which is thereby drawn in. Expiration to return lung volume to functional residual capacity after inspiration can occur by elastic recoil.

Lung diseases of many types commonly affect each of the steps involved in gas exchange, and the clinical consequences can usually be readily understood if the structure–function relationships are known.

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