West's 9th ed - Chapter 3 - Diffusion (1)

Question 1

Fick’s Law of Diffusion states that the rate of diffusion of gas through a tissue is proportional to each of these three: 1) ? 2) ? 3) ?

Answer 1

1) The surface area of the tissue 2) The difference in partial pressure of the gas between the two sides of the tissue 3) The solubility of the gas in the tissue

Question 2

Fick’s Law of Diffusion states that the rate of diffusion of gas through a tissue is INVERSELY proportional to each of these two: 1) ? 2) ?

Answer 2

1) The thickness of the tissue 2) The square root of the molecular weight of the gas

Question 3

Which gas is said to be diffusion limited?

Answer 3

Carbon monoxide. This is because CO is taken up so quickly by Hb that its partial pressure in plasma is virtually eliminated as soon as it gets into the blood. So the major factor affecting its transfer across the blood-gas barrier is the diffusion properties of the blood-gas barrier (even if perfusion is poor).

Question 4

Which gas is said to be perfusion limited?

Answer 4

Nitrous oxide. This is because just about all of the nitrous oxide entering the blood remains in the plasma (not bound to Hb), causing a rise in partial pressure of NO2. Once the blood’s partial pressure of NO2 is equal to that of the alveolus, the only way to transfer more NO2 across the blood-gas barrier is to increase perfusion (bringing more blood that hasn’t yet seen the NO2).

Question 5

Is oxygen diffusion limited or perfusion limited? (assuming that you’re breathing normal air)

Answer 5

Both. In healthy people, it is perfusion limited (so when you’re exercising you increase HR and thereby increase perfusion to the lungs). In people with pulmonary disease that thickens the blood-gas barrier, oxygen diffusion becomes both diffusion limited and perfusion limited.

Question 6

In healthy people, how long does it take for the RBC to attain a partial pressure of O2 almost equal to that of the alveolus?

Answer 6

About 1/3 of the 0.75 seconds it spends in the capillary (at rest).

Question 7

What is the equation for measuring diffusing capacity?

Answer 7

D_L = diffusing capacity for a given gas. Note that for CO, the partial pressure in the plasma is virtually zero (because it is all taken up by Hb), and therefore it can be ignored.

Question 8

Which gas diffuses through tissue faster: CO₂ or O₂?

Answer 8

CO₂ diffuses through tissue about 20 times faster than O₂ because it is much more soluble. Remember that diffusion capacity is proportional to the solubility of the gas.

Question 9

Name three situations in which the diffusion process becomes challenged, and therefore oxygen becomes more diffusion-limited.

Answer 9

1) Exercise
2) Alveolar hypoxia (either by reduced FiO2 or by high altitude)
3) Thickening of the blood-gas barrier