Respiratory: Physiology - Gas diffusion

Question 1

Fick’s law equation

Answer 1

Vgas ∝ (A/T) x D x (P1 - P2)
D ∝ (solubility/√MW)

Question 2

Describe Fick’s law

Answer 2

Rate of diffusion is proportional to surface area available for diffusion and pressure gradient, and inversely proportional to diffusing membrane thickness
Diffusion rate is also proportional to solubility and inversely proportional to the square root of the molecular weight

Question 3

Is CO diffusion- or perfusion-limited? How?

Answer 3

Diffusion-limited
Binds Hb rapidly and avidly such that the partial pressure of CO in the blood is unchanged
Gas transfer is therefore not limited by the amount of blood available

Question 4

Is NO diffusion- or perfusion-limited? How?

Answer 4

Perfusion-limited
Does not bind Hb and so partial pressure in blood rises rapidly, causing back-pressure such that its transfer is dependent on amount of available blood

Question 5

Is O2 diffusion- or perfusion-limited? How?

Answer 5

Perfusion-limited under normal circumstances (partial pressures in alveoli and capillary have equilibrated within 0.25s or ~1/3 of the time of the RBC’s passage through the capillary)
In disease states may be diffusion-limited (e.g. where there is abnormal thickening of the blood-gas barrier)

Question 6

What is the normal PO2 of mixed venous blood entering the capillary?

Answer 6

40mmHg

Question 7

What is the normal alveolar PO2?

Answer 7

100mmHg

Question 8

List three factors which may challenge the diffusion process of O2

Answer 8

1. Exercise (blood spends less time in capillary)
2. Alveolar hypoxia (reduced pressure gradient)
3. Blood-gas barrier thickening (decreased rate of diffusion)

Question 9

What is the normal diffusing capacity for CO at rest? How does this change during exercise?

Answer 9

25ml/min/mmHg
Increases 2-3x during exercise due to increased capillary recruitment and distension

Question 10

What gas is used to measure diffusing capacity of the lung and why?

Answer 10

CO as it is strictly diffusion-limited

Question 11

Outline the formula (based on Fick’s law) used to calculate the diffusing capacity of the lung

Answer 11

DL = VCO/PACO

Question 12

What are the two components that make up diffusing capacity?

Answer 12

1. Diffusion through blood-gas barrier (including plasma and RBC interior)
2. Reaction rate with Hb

Question 13

What is the typical ratio of resistance to O2 diffusion due to reaction rate vs due to diffusion across blood-gas barrier?

Answer 13

Resistance to O2 diffusion due to Hb reaction rate is approximately equal to resistance to O2 diffusion due to diffusion across blood-gas barrier

Question 14

What can be done to change the reaction rate of CO?

Answer 14

Increase PO2 as O2 competes for Hb binding

Question 15

Is CO2 diffusion limited? Why or why not?

Answer 15

No, because it is highly soluble

Question 16

How much faster is the rate of CO2 diffusion vs O2 diffusion?

Answer 16

~20x