The American journal of physiology
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The circulatory systems of vertebrate animals are closed, and blood leaves and returns to the heart at the same level. It is often concluded, therefore, that the heart works only against the viscous resistance of the system, not against gravity, even in vascular loops above the heart in which the siphon principle operates. However, we argue that the siphon principle does not assist blood flow in superior vascular loops if any of the descending vasculature is collapsible. ⋯ Thus the pressure developed by the heart to establish a given flow rate is independent of the resistance occurring in the partially collapsed vessels. The pressure depends only on the height of the blood column and the resistance in the noncollapsed parts of the system. Simple laboratory models, involving water flow in collapsible tubing, dispel the idea that the siphon principle facilitates blood flow and suggest that previously published results may have been affected by experimental artifact.
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The authors investigated the role of alpha 1- and beta-adrenoceptors on induction of ventricular arrhythmias during thiopental anesthesia in dogs and compared with that during halothane anesthesia. Throughout this study, arrhythmogenic threshold of epinephrine during thiopental anesthesia was designed to be comparable with that during halothane anesthesia. ⋯ During thiopental anesthesia, incidence of arrhythmias with blood pressure elevation by epinephrine, phenylephrine, or angiotensin II was not different, and increasing heart rate by electrical pacing did not replace isoproterenol in the arrhythmogenic interaction between isoproterenol and phenylephrine. The results indicate that blood pressure elevation due to the combined inotropic action of alpha 1- and beta-adrenoceptor agonists is a critical factor in the genesis of thiopental-epinephrine arrhythmias.