Size of Gas Exchange Vessels in the Lung *

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Abstract
The size of vessels that permit the bulk of gas exchange in the lung was determined. After injection of ether dissolved in alcohol into the pulmonary artery of an apneic dog enclosed within a body plethysmograph, pressure in the plethysmograph rises as ether arrives at the site of gas exchange. The time for ether evolution from ether dissolved in alcohol was compared to the time from ether dissolved in oil vehicles. The size of the oil droplet was measured both from in vitro experiments and from microscopic sections of the lung. The droplets are arrested in vessels whose diameter is equal to the droplet diameter. The time for ether evolution from droplets occluding vessels in the vicinity of the anatomic precapillaries is not measurably longer than the time required from injections of ether dissolved in alcohol. The time for ether evolution is delayed in larger vessels because the rate of passage through the vessel wall is diffusion limited. Calculations from a mathematical model of the pulmonary arterial system of a 7 Kg dog while the blood is flowing indicate that less than 0.2% of injected ether equilibrates across the arterial system before reaching from 8 to 20[mu] in diameter (vicinity of anatomic precapillaries), 9% across the precapillaries, and the remainder across the first hundredth length of the anatomic capillaries.