Pressure and Flow—Chickens and Eggs

James R. Munis

in Just Enough Physiology

Published on behalf of © Mayo Foundation for Medical Education and Research

Published in print December 2011 | ISBN: 9780199797790
Published online June 2012 | e-ISBN: 9780199929665 | DOI:

Series: Mayo Clinic Scientific Press

Pressure and Flow—Chickens and Eggs

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We tend to assume that when 2 things are associated with each other, one must be causing the other. Nothing could be further from the truth, though. Because we're used to seeing the independent variable (‘cause’) plotted on the x-axis and the dependent variable (‘effect’) on the y-axis, this equation and graph suggest that the pressure gradient causes the paddle wheel flow rate. That, of course, is nonsense. This type of specious thinking is intended to warn you away from assuming that relationships necessarily imply causality. As you've learned already, pressure is not the same thing as energy, and pressure by itself cannot perform work or generate flow. However, flow generated by pressure-volume work (either by the heart or a mechanical pump) certainly can create pressure gradients. In this sort of chicken (flow) or egg (pressure) question, if the only energy-containing term is flow, then I'll say that the chicken came first.

Chapter.  1201 words.  Illustrated.

Subjects: Professional Development in Medicine

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