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We have made simultaneous measurements of the instantaneous flow in the main pulmonary artery using an electromagnetic flowmeter and the pulmonary capillaries using the nitrous oxide body plethysmograph method in six lightly anesthetized and spontaneously breathing dogs. We measured the transmission of the flow wave in the pulmonary arterial tree at a variety of heart rates and during alveolar hypoxia. Fourier analysis enabled us to study transmission in the intact animal in a way similar to that of imposed oscillations of flow in the isolated organ. The amplitude of the pulmonary capillary flow pulse was approximately haJf that in the main pulmonary artery. There was progressive decrease in amplitude from the first to the fourth harmonic in both the pulmonary arterial and pulmonary capillary flow pulses. Harmonics above the fourth were small and variable in amplitude. Transmission between pulmonary valve and capillaries was independent of heart rate. Hypoxia did not affect transmission. Assuming that the increase in resistance in this instance was due to precapillary vasoconstriction, this finding suggests that other factors may offset the effect of increase in resistance upon transmission.ADDITIONAL KEY WORDS flow wave transmission pulmonary arterial flow pulse pulmonary blood flow Fourier analysis pulmonary capillary flow pulse pulmonary vascular resistance• We are reporting in this paper for the first time simultaneous measurements of the pulmonary artery and pulmonary capillary blood flow in spontaneously breathing, rightly anesthetized dogs. Our aim was to study the

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