During normal spontaneous inspiration pulmonary blood flow increases in spite of an increase in resistance to flow in the pulmonary bed. The enhancement of pulmonary flow is caused by an augmentation of venous return due to thoracic aspiration. The right heart acts as a moderator for the pulmonary flow by temporarily storing part of the large influx of venous blood during inspiration and ejecting the stored part during expiration and the expiratory pause.O N E of the oldest controversies in the field of pulmonary circulation concerns the effect of spontaneous respiration on blood flow in the pulmonary artery. According to pulmonary vascular bed perfusion experiments pulmonary flow diminishes due to an increase in resistance when the lungs are expanded by negative pressure in the closed chest, such as would occur during spontaneous inspiration.1 ' s On the other hand, Baxter and Pearse* found that, in the intact animal, pulmonary flow was augmented during inspiration. These contradictory findings could be explained only if one assumes that during inspiration venous return increases to such an extent that it augments right heart output in spite of the higher pulmonary bed resistance. Recently it has been demonstrated that venous return actually can increase with inspiration. 4 ' 6 The correlation of such flow augmentation with changes in pulmonary flow has not been established, however, and it was the purpose of this investigation to study this problem. METHODIn acute experiments on dogs, blood flow was simultaneously measured in both the main pulmonary artery and the superior vena eava with two 5734 vacuum tube bristle flowmeters. The superior caval flow was taken as representative of venous return since it has been found in former experiments' that superior and inferior caval flows show the same From the Departments of Physiology and Surgery, School of Medicine, Western Reserve University and University Hospitals of Cleveland.Aided by grants from the Life Insurance Medical Research Fund and the Cleveland Area Heart Society.Received for publication December 6, 1954.directional changes during the respiratory and cardiac cycle. Flow in the superior cava was recorded with a bristle flowmeter of a design described previously. 7 ' 8 The cross-sectional area of the vein was fixed for volume flow determination by the ringlike head of the flow cannula inserted into the vessel at the entrance of the right atrium. The internal diameter of the cannula head was 12 mm. Right heart output was recorded by a modified bristle flowmeter inserted into the main pulmonary artery.' The bristle wasintroducedintothetrunk of the pulmonary artery through a buttonhole incision without blood loss, and the cross sectional area of the vessel was kept constant by passing a suitable metal band around it.Nine dogs, ranging from IS to 34 Kg. in weight, were anesthetized with 1.5 mg/kg morphine sulfate and 15 mg/Kg. sodium pentobarbital. The animals were fixed in the right lateral position and the chest was entered on the left side between th...

Intraventricular pressures were measured during brief periods of mitral orifice occlusion to determine whether or not the ventricle can exert a sucking force during diastole. This procedure was based on the premise that with a wide open mitral orifice negative diastolic transmural pressures cannot be detected in the ventricle because the adjoining atrium has collapsible walls. During mitral orifice closure, negative intraventricular pressures were recorded, and Ringer's solution was sucked against the force of gravity into the ventricle. It is concluded that the mammalian ventricle is capable of sucking blood from the atrium into its cavity.

Unequivocal evidence in favor of the concept that a ventricular diastolic vis a fronte contributes to ventricular filling has been established only in the following conditions: a. When the ventricle contains an abnormally small residual volume (rat, dog, turtle) b. When the ventricle contains a more nearly normal, residual volume due to ejection against the resistance of a fluid column (dog, turtle). The quantitative contribution of the ventricular vis a fronte to ventricular filling at various residual volumes and various levels of cardiac activity is still unknown. The physiological significance of diastolic suction for the return flow of blood can therefore not yet he evaluated.

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