Respiratory regulation of splanchnic and systemic venous return

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Abstract
This report presents measurements of respiratory changes in instantaneous blood flow and pressure in closed-chest dogs under anesthesia. Sites of measurements included thoracic and abdominal venae cavae, portal vein, and ascending aorta with "on-line"'' computation of hepatic outflow (true splanchinic outflow) by a continuous subtraction of supra-and infrahepatic inferior caval flow. Measurements showed respiratory alternation in systemic and splanchnic contributions to venous return in the common thoracic segment of inferior vena cava. Increased venous return during inspiration represented primarily systemic flow whereas splanchnic flow predominated during expiratory pauses. Alternation resulted from arrest of splanchnic outflow coinciding with downstream inspiratory fall in intraluminal caval pressure. There was an increase in liver volume during each inspiration. Isolating the liver from diaphragmatic impact of paralyzing the diaphragm prevented mechanical compression of the organ and resulted in concomitant inspiratory acceleration of splanchnic and systemic venous returns with wider oscillations in volume of thoracic vena cava. The model proposed suggests the normal respiratory alternation as an effective mechanism to prevent abrupt respiratory changes in the level of the central venous reservoir and thus provide a more steady supply to the right heart.

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