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### Course: MCAT>Unit 7

Lesson 11: Respiratory system

# Fick's law of diffusion

Fick's law describes the movement of particles over time. There are a few strategies for maximizing particle movement, such as minimizing the distance the particles have to travel, using smaller molecules, increasing pressure, and increasing surface area. Fick's law can be expressed as an equation, but it is sometimes written in different forms. Created by Rishi Desai.

## Want to join the conversation?

• First things first. This video is just awesome, it truly gave me a very intuitive understanding of how Fick's Law Of Diffusion works! Thank you very much! I'm now a very happy engineering student :-)
Now my question is....
`What are the usual applications of Fick's Law of Diffusion in science and engineering?`
• I'm a respiratory therapist (RRT) in Halifax, Canada, so can give you a medical perspective. Fick's Law is the foundation of a lot of what we do in respiratory therapy and medicine. If you consider the Alveolar Capillary membrane (the junction between alveolar gas, and pulmonary capillary blood), you see Fick's Law at work. The surface area of the lungs is enormous, around the size of a tennis court, this allows a huge amount of diffusion to take place. When we give oxygen to people who need it, we are increasing the partial pressure difference across the membrane (we are increasing the pressure gradient.... meaning more diffusion), that's how breathing higher concentrations of oxygen actually helps people. The diffusion coefficient of gases is important when comparing different gases in the lung. Let's take CO2 and O2. They are similar in molecular weight, but CO2 is about 20x more soluble than O2 in plasma, and therefore diffuses faster! This shows how the diffusion coefficient is important. Lastly, the thickness of the AC membrane is incredibly thin. This allows for very fast diffusion, because as Sal mentioned: Diffusion is inversely proportional to the thickness of the membrane. When this membrane becomes thickened (an example would be pulmonary fibrosis), diffusion is impaired, and the patients experience symptoms of poor diffusion (hypoxemia, hypoxia). Hope that helps!
• Is the dot over the letter (here, V) unique to Fick's law or will it be seen in other applications/formulas?
• The dot over the volume (V) denotes a rate. " A change in quantity over some time." It quite prevailant and ubiquitous throughout science, Various exmples are velocity, acceleration, and so forth. You'll see it thoughout your higher level scince classes in college, when variable beging to run-out.
• Can you explain how to derive diffusion constant (D) = "solubility"/sqrt(Molecular weight), and why?

Also please define "solubility". Are you referring to "Kh" (henry's law constant)?
• Couldn't an increase in temperature also help diffuse the molecules faster?
• I believe that temperature affects D, the diffusion constant.

D=D_0e^(-Q_d/(RT))

where
D_0 is the temperature-independent pre-exponential
Q_d is the activation energy
R is the gas constant
T is the temperature

From Materials Science and Engineering : An Introduction
(1 vote)
• At around he said that "V is kind of the rate of particles moving and when I say rate you know that means that there's some time component" He then said that V may stand for volume here as well.
So my question is, does the variable V have units of time in it and what are the exact units of V?
• In this case, for the example he used to derive Fick's law, it would be molecules/second (or mols/second, if you'd like it to be).

More generally the units could be described as amount/second and so the amount in some applications could be a volume. I grant you this is quite confusing, though maybe not because of volume but because of pressure. It often isn't a traditional pressure like air pressure, which can affect volume. It can be a "diffusion pressure" formed from an unequal distribution of molecules that are trying to get from a place of high concentration to a place of low concentration.
• why he use P1-P2 to describe change in pressure.... Change in something (Pressure, volumen etc.) is not P2-P1..? this is an special case for this formula or a small mistake? Thaks
• This is a special case for this formula. P1 is designated as usually the "higher" pressure or else diffusion will not take place.
• In Fick's Law of Diffusion, V= (ΔP⋅A⋅D)/T
What are the conventional units that we usually use for the variables V, P, A, D, T?
• Most general units of...

V: [amount] / [time] - (rate of change)
P: [amount] / [length]^3 - (quantity per volume)
A: [length]^2 - (area)
D: [length]^2 / [time] - (diffusion constant)
T: [length]
• Is diffusion not temperature dependent? Does changing the temperature have any effect on diffusion?