Fick's Law and What Sets the Rate of Exchange
Rate of diffusion rises with surface area and with the concentration difference, and falls with the distance crossed. The relationship turns comparisons between exchange surfaces into arithmetic.
What a learner can do afterwards
- States the three quantities in the relationship and which way each one pushes the rate.
- Predicts the effect on the rate of doubling the diffusion distance.
- Uses the relationship to explain why an alveolus wall is one cell thick.
1 · Read
The diffusion relationship has three quantities. Surface area pushes the rate up: more room to cross means faster exchange. Concentration difference pushes the rate up too: a bigger gap drives more molecules across. Distance is the odd one out: a longer trip pulls the rate down. Two dials help when turned up, one harms.
Your lungs show the first and third dials at work. Each lung holds around 300 million tiny alveoli, giving about 75 square meters of surface, roughly half a tennis court. Each alveolus wall is only one cell thick, the shortest distance a wall can be. Huge area plus tiny distance is exactly why gas exchange runs fast enough to keep you alive.
Distance works in strict inverse. Double the thickness of a membrane and the rate halves: 12 units per second becomes 6. Triple the distance and the rate divides by 3: 9 units per second becomes 3. Halve the distance and the rate doubles instead. Learn it as dividing by the change.
In exam arithmetic hold two quantities steady and move one. Bigger area or bigger difference with the rest fixed means faster. Longer distance with the rest fixed means slower. If area doubles while distance doubles too, the two changes cancel and the rate stays the same.
Area and difference drive the rate up, distance drags it down, and doubling both area and distance cancels out.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.