Effect of Respiration on Pulmonary Capillary Blood Flow in Man

Pulmonary capillary blood flow was measured in man during slow breathing by a modification of the body plethysmograph technique for measuring N 2 O uptake. In seated subjects breathing slowly, flow was significantly higher during inhalation. In supine subjects whose legs were raised, the difference between inhalation and exhalation was not significant. Flow was usually greater during tidal inhalation than exhalation, but there was considerable variation. The changes in flow were not directly related to intrathoracic pressures or lung volumes. The results suggest that it is the amount and pressure of the venous blood available for aspiration into the thorax that influences pulmonary capillary blood flow during the course of a respiratory cycle.