Macklin1 calls attention to the fact that the blood vessels in the normal lung are uniquely situated in that they are surrounded by expanding and contracting air spaces and that this environment influences their behavior, causing with each respiratory cycle a rhythmic increase and decrease. In inspiration the arteries and veins become elongated and widened by the traction of the stroma connecting their walls with the enveloping air spaces, while in expiration these vessels undergo a shortening and narrowing by reason of the recoil of their specialized elastic fibers. Thus in respiration their content of blood is increased and in expiration it is decreased. This action is regarded as a sort of auxiliary hemic pump or pulmonary "accessory heart" whose role is most striking in the vigorously exercised body of the healthy youthful person or animal. To support these views Macklin offers x-ray evidence consisting of the arterial and
CAPACITY OF THE PULMONARY ARTERIES AND VEINS. JAMA. 1946;132(5):285–286. doi:10.1001/jama.1946.02870400033011
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