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