Med. Phys. Discussion (new posts)

Fick: Re: Fick

Wed, 17 Jan 2007 23:09:32 +0000

Email to Dr. Benos:
I…meant to ask about Fick's law. There has been a lot of confusion about it, and I spent about an hour looking in different textbooks and different internet sites and different lecture scripts, noticing that there were two incompatible equations. I finally realized that people define J differently, some define it as flux and some as flow (i.e. without area in the denominator). Should we assume 'J' to be flux?

Email from Dr. Benos
You hit the nail on the head. There is rampant confusion out there. I wouldn’t assume anything; you need to ask for clarification. Flux = Flow/area. The most basic form of Fick’s law is: J (flux) = P (permeability coefficient) times delta C (concentration). If P is in cm/s, and delta C is moles/cm3, then J is flux in units of moles/cm2 x s. Another, more explicit form is:

Flow (in moles/s) = [D A(partition coefficient) (delta C)]/delta x, where D is the diffusion coefficient (in cm2/s), A is the area (in cm2), partition coefficient (unitless, and is defined as C in oil or lipid/C in water), and delta x is the thickness. Remember :

P = [D(partition coefficient)]/delta x You’ll see that the units all work out. Hope this helps!

Dale Benos

Fun with K+: Fun with K+

Wed, 17 Jan 2007 22:13:46 +0000

Here is an email from Dr. Benos is response to a hypothetical test question:

EMAIL TO DR. BENOS
Dr. Benos,
A number of us in the physiology class are divided into two groups over what we think the right answer is for the following question. If this question comes up on the board exam, which of the following answers would you suggest we put?

In a typical cell with normal, physiological concentrations of solutes and a membrane potential of -70 mV, potassium moves out of the cell down its electrochemical gradient. Which of the following is true about this movement?
A) Potassium is moving up its chemical gradient and down its electrical gradient.
B) Potassium is moving up its chemical gradient and up its electrical gradient.
C) Potassium is moving down its chemical gradient and up its electrical gradient.
D) Potassium is moving down its chemical gradient and down its electrical gradient.

RESPONSE FROM DR. BENOS:
A and B are clearly incorrect because the chemical gradient for K is directed outward. The electrical gradient is negative inside the cell, which would tend to retard the outward movement of K. However, the outward chemical gradient exceeds the inward electrical gradient such that the electrochemical gradient is directed outward, the direction of K movement. So the correct answer is C (although I don’t think this question is framed particularly well).

DJB

Fick: Fick

Wed, 17 Jan 2007 07:45:36 +0000

Be careful with Fick's equation as listed in transcripts—sometimes the professor uses J to mean flow, and sometimes the professor uses J to mean flux. Flux is flow/area.

PHYS01-12-07a: PHYS01-12-07a

Wed, 17 Jan 2007 03:51:38 +0000

I think it should say phase 2 (page 487 in our text)
"phase 2 is dependent on the continued entry of Ca and Na ions through their major channels…"
"phase 3 depends on Ik"
-caroline ragheb