26 - 30 Physics SBAs for the Primary FRCA
Question 26
You are performing a gas induction on an anxious three year old child, when the student ODP asks why you've put the sevoflurane all the way up to 8%.
Which of the following is true regarding Fick's law of diffusion?
- The rate of diffusion across a membrane is exponentially proportional to its cross-sectional area
- The rate of diffusion across a membrane is directly proportional to the concentration gradient across it
- The rate of diffusion across a membrane is directly proportional to the thickness
- Fick's first law describes how molecular diffusion causes the concentration to change over time
- Fick's second law describes the movement of molecules down a concentration gradient
Answer
- The rate of diffusion across a membrane is directly proportional to the concentration gradient across it
Adolf Fick proposed two laws in 1855, both related to the diffusion of molecules down a concentration gradient.
In short:
- Rate of diffusion is directly proportional to the area of the membrane
- It is also directly proportional to the concentration gradient
- It is inversely proportional to the membrane thickness
His first law talks about the movement of molecules down a concentration gradient, while the second law describes how this movement then causes the concentration gradient to change over time.
An example:
- The lung has an enormous surface area, which speeds up gas exchange
- Anything that reduces the surface area (lobectomy, collapse, emphysema) will thus reduce efficiency of gas exchange
- Infection and oedema will thicken the effective alveolar-capillary membrane, therefore reducing the rate of gas exchange further
- This can be countered clinically by simply increasing the concentration gradient (FiO2)
- It's also the reason you whack the sevo up to 8% for a faster gas induction