Lecture 29: Gas exchange

Question 1

Give the full equation of Fick’s law

Answer 1

V(flow rate) = DL (diffusion constant) * (PA-Pc)(pressure gradient)

V = kAs/(a*sqrt(MW)) * (PA-Pc)

Question 2

What 6 factors determine the diffusion capacity?

Answer 2

Molecular Weight (Graham’s Law): increased molecular weight, decreases diffusion
Henry’s law: SolubilityxPO2: solubility inversely proportional to temperature, diffusion is proportional to solubility, also proportional to pressure of the gas.
Dalton’s law (partial pressure of gas)
Tissue thickness
Diffusion distance
k - how gas interacts with the barrier

Question 3

Give a formula for DL based on Dm and Vc

Answer 3

1/DL=1/Dm + 1/thetaVc where Dm is the diffusion across the membrane through parallel resistors, while Vc is the capillary blood volume and theta is the uptake of the gas by hemoglobin

Question 4

Describe how to measure pulmonary diffusion capacity

Answer 4

Maximal expiration, then breathe in gas of known composition of CO and He. Use He dilution to calculate the PCO breathed in, as well as the percentage of air that is CO. Then hold breath to let air equilibrate, then breathe out, and calculate the percentage of CO, as well as the final PCO. Percentage difference indicates how much CO went across the barrier. Through Fick’s simplified Law, DL= V/(PA-PC), V= total lung volume * (percentage change)

Question 5

Describe perfusion vs. diffusion limits to gas exchange. Why is O2 perfusion limited and CO is diffusion limited (3 reasons)? How does perfusion/diffusion limiting change with exercise and altitude?

Answer 5

use partial pressure in capillary vs length of capillary graph. If line reaches PA (gas equilibrated) then that gas is perfusion limited.
O2 vs CO: greater pressure gradient for O2, greater original saturation for O2, quicker uptake of O2 by Hb.
With exercise, greater CO so would expect to become diffusion limited, but there is distention and recruitment (increase area, decrease DL) so it remains perfusion limited
With altitude, the alveolar pressure lowers, so there is a lower O2 gradient, the lower O2 pressure also means the loading is at a different part of the dissociation curve, so there is more O2 uptake before saturation, so it can become diffusion limited