Chapter

Structure and function

Nicholas W. Morrell

in Oxford Textbook of Medicine

Fifth edition

Published on behalf of Oxford University Press

ISBN: 9780199204854
Published online May 2012 | e-ISBN: 9780199570973 | DOI: http://dx.doi.org/10.1093/med/9780199204854.003.161501

Series: Oxford Textbooks

Structure and function

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The normal pulmonary circulation distributes deoxygenated blood at low pressure and high flow to the pulmonary capillaries for the purposes of gas exchange. The structure of pulmonary blood vessels varies with their function—from large elastic conductance arteries, to small muscular arteries, to thin-walled vessels involved in gas exchange.

Pulmonary vascular resistance (PVR) is about one-tenth of systemic vascular resistance, with the small muscular and partially muscular arteries of 50 to 150 µm diameter being the site of the greatest contribution to resistance. The gas-exchanging capillary surface area (c.125 m2) contains a blood volume of about 150 ml at any one time, with the blood–gas barrier being only 0.2 to 0.3 µm thick at its thinnest part. In the normal pulmonary circulation, a large increase in cardiac output causes only a small rise in mean pulmonary arterial pressure because PVR falls on exercise: this is accomplished by a combination of vascular distensibility and recruitment. Pulmonary blood flow is heterogeneous: gravity causes increased blood flow in the more dependent parts of the lung; within a horizontal region—or within an acinus—blood-flow heterogeneity is imposed by the branching pattern of the vessels.

Many neural and humoral mediators can influence pulmonary vascular tone, including nitric oxide and prostacyclin. Alveolar hypoxia causes constriction of the small pulmonary arteries, whereas systemic arteries dilate when hypoxic: this hypoxic pulmonary vascoconstriction can reduce venous admixture and improve arterial oxygenation in the presence of bronchial obstruction. Despite large regional differences in the matching of ventilation and perfusion within the normal lung, the overall lung ventilation–perfusion ratio is maintained remarkably steady at around 0.85.

Acknowledgement: Much of the chapter written for the third edition of this textbook by the late J S Prichard has been retained here.

Chapter.  2843 words.  Illustrated.

Subjects: Cardiovascular Medicine

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