AJP - Regulatory, Integrative and Comparative Physiology, Vol 262, Issue 5 725-R732, Copyright © 1992 by American Physiological Society

ARTICLES

Gravity and the circulation: "open" vs. "closed" systems

J. W. Hicks and H. S. Badeer
Department of Biomedical Sciences, School of Medicine, Creighton University, Omaha, Nebraska 68178-0224.

The elementary principles of liquid dynamics are described by the equations of Bernoulli and Poiseuille. Bernoulli's equation deals with nonviscous liquids under steady streamline flow. Pressures in such flows are related to gravity and/or acceleration. Changes in elevation affect the gravitational potential energy of the liquid and the velocity of flow determines the kinetic energy. The sum of these three factors represented in the Bernoulli equation remains constant, but the variables are interconvertible. In contrast, the Poiseuille equation describes the pressures related to viscous resistance only, and the energy of flow is dissipated as heat. A combination of the two equations describes the flow in tubes more realistically than either equation alone. In "open" systems gravity hinders uphill flow and causes downhill flow, in which the liquid acts as a falling body. In contrast, in "closed" systems, like the circulation, gravity does not hinder uphill flow nor does it cause downhill flow, because gravity acts equally on the ascending and descending limbs of the circuit. Furthermore, in closed systems, the liquid cannot "fall" by gravity from higher levels of gravitational potential to lower levels of potential. Flow, up or down, must be induced by some source of energy against the resistance of the circuit. In the case of the circulation, the pumping action of the heart supplies the needed energy gradients. Flow in collapsible tubes, like veins, obeys the same basic laws of liquid dynamics except that transmural pressures near zero or below zero reduce markedly the cross-sectional area of the tube, which increases the viscous resistance to flow.(ABSTRACT TRUNCATED AT 250 WORDS)

This article has been cited by other articles:

				T. Nishiyasu, S. Hayashida, A. Kitano, K. Nagashima, and M. Ichinose Effects of posture on peripheral vascular responses to lower body positive pressure Am J Physiol Heart Circ Physiol, July 1, 2007; 293(1): H670 - H676. [Abstract] [Full Text] [PDF]

				J. Gisolf, A. Gisolf, J. J. van Lieshout, and J. M. Karemaker The siphon controversy: an integration of concepts and the brain as baffle Am J Physiol Regulatory Integrative Comp Physiol, August 1, 2005; 289(2): R627 - R629. [Full Text] [PDF]

				J. W. Hicks and J. R. Munis The siphon controversy counterpoint: the brain need not be "baffling" Am J Physiol Regulatory Integrative Comp Physiol, August 1, 2005; 289(2): R629 - R632. [Full Text] [PDF]

				E. A. Dawson, N. H. Secher, M. K. Dalsgaard, S. Ogoh, C. C. Yoshiga, J. Gonzalez-Alonso, A. Steensberg, and P. B. Raven Standing up to the challenge of standing: a siphon does not support cerebral blood flow in humans Am J Physiol Regulatory Integrative Comp Physiol, October 1, 2004; 287(4): R911 - R914. [Abstract] [Full Text] [PDF]

				H. S. BADEER and D. P. SWAIN Is the Water-Tower Analogy Justified? Advan Physiol Educ, September 1, 2001; 25(3): 187 - 188. [Full Text] [PDF]

				Q. Fu, S. Iwase, Y. Niimi, A. Kamiya, J. Kawanokuchi, J. Cui, T. Mano, and A. Suzumura Effects of lower body positive pressure on muscle sympathetic nerve activity response to head-up tilt Am J Physiol Regulatory Integrative Comp Physiol, September 1, 2001; 281(3): R778 - R785. [Abstract] [Full Text] [PDF]

				H. S. Badeer HEMODYNAMICS FOR MEDICAL STUDENTS Advan Physiol Educ, March 1, 2001; 25(1): 44 - 52. [Abstract] [Full Text] [PDF]

				O. Bailliart, A. Capderou, B. P. Cholley, C. Kays, D. Riviere, P. Techoueyres, J.-L. Lachaud, and P. Vaida Changes in lower limb volume in humans during parabolic flight J Appl Physiol, December 1, 1998; 85(6): 2100 - 2105. [Abstract] [Full Text] [PDF]

				T. Nishiyasu, K. Nagashima, E. R. Nadel, and G. W. Mack Effects of posture on cardiovascular responses to lower body positive pressure at rest and during dynamic exercise J Appl Physiol, July 1, 1998; 85(1): 160 - 167. [Abstract] [Full Text] [PDF]