REBUTTAL FROM DR. BRENGELMANN

Abstract

POINT-COUNTERPOINTREBUTTAL FROM DR. BRENGELMANNPublished Online:01 Nov 2006https://doi.org/10.1152/japplphysiol.00698c.2006MoreSectionsPDF (35 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat Point of agreement:the utility of combining open-loop cardiac and vascular subsystem properties in analyses of cardiovascular stability. For the heart, Pra ↑→ F ↑; for the vasculature, F ↑→ Pra ↓. Connected, they form a negative feedback system that stabilizes at the Pra that causes the heart to pump out the F that causes that Pra. Showing this graphically requires plotting one of the relationships with its independent variable on the y-axis.“Waterfall” relevance?Yes, resistance of a vascular segment or Starling resistor increases as it collapses. But, the (Pms−Pra)/Rv concept applies only to the sloped segment of venous return curves, for which intravascular pressures are positive and apparent Rv is constant, i.e., no vessel collapse. About waterfalls: 1) flow depends only on the flow arriving at the precipice edge; 2) transport to the lower level is due to gravity; and 3) they are not enveloped by flexible elastic walls. Why are we talking about them?Bathtub analogous?In Magder's Fig. 1A, (1) we see the right atrium at the level of the water surface and Pra labeled as equal to MSFP (my Pms). But surface level pressure has to be zero, i.e., equal to atmospheric (Patm). Just as the Fig. 1A tub cartoon does not correspond to the pressures marked on the graph below it, the hydrostatic relationships are incorrect in the other panels (e.g., pressure at the atrium level would be greater than any in the tub). Correcting all pressures to the same level would reveal the pressure gradient associated with flow, but why pursue this? The (Pms−Pra)/Rv concept is not about blood flowing downhill, and flow in the defining experiments was certainly not driven by gravity. And that faucet? How does it know the flow needed to keep the tub full?MSFP (Pms) energy source?Quantitatively, the elastic work that moves blood out of a compartment equals the integral of instantaneous pressure times compartment volume decrement dV. Magder's compartment at Pms, kept at constant volume for steady states, has no dV. No dV, no energy release.To Magder's “what makes the blood go around?” (1, first sentence), I reply not elastic energy from a compartment at Pms; but the work manifested in the integral of P times dV for the ventricles (ignoring for the purposes of the present argument the energy input by vessel compression and expansion due to activity of skeletal and respiratory muscles).REFERENCE1 Magder S. Point: The classical Guyton view that mean systemic pressure, right atrial pressure, and venous resistance govern venous return is correct. J Appl Physiol. In press.Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformationCited ByAn analytical perspective on the venous return controversy7 June 2018 | Canadian Journal of Anesthesia/Journal canadien d'anesthésie, Vol. 65, No. 9Understanding Guyton's venous return curvesDaniel A. Beard, and Eric O. Feigl1 September 2011 | American Journal of Physiology-Heart and Circulatory Physiology, Vol. 301, No. 3 More from this issue > Volume 101Issue 5November 2006Pages 1527-1527 Copyright & PermissionsCopyright © 2006 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.00698c.2006History Published online 1 November 2006 Published in print 1 November 2006 Metrics Downloaded 288 times