The Two Doctors Fick

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

The Two Doctors Fick

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We often confuse the ‘Fick principle’ with ‘Fick's law of diffusion.’ They are not the same. Ironically, Fick borrowed heavily from already known physical laws when he first described both his law of diffusion and his principle. Borrowing from Ohm's law of electricity, Fick applied concepts of diffusion and transfer across a resistance to formulate a law of diffusion that could be applied to gas or solute transfer across a membrane. Whether we are talking about transfer across the alveolar-capillary membrane or across a dialysis membrane, the concept is the same. The concept is similar to electricity—you have a transfer rate, resistance, and a gradient. Now let's consider the Fick principle. On the basis of another physical law he understood that, in the steady state, the difference between the amount of oxygen going into a tissue bed minus that leaving the tissue bed must be equal to the oxygen consumed. With a little reworking, this became the Fick principle: Cardiac output = O2 consumption / (arterial O2 - venous O2).

Chapter.  1807 words.  Illustrated.

Subjects: Professional Development in Medicine

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