Chapter 4: Diffusion Of Pulmonary Gases

Question 1

Describe the layers gas passes through in the A/C membrane

Answer 1

Moving from the air in the alveolus (Thick side of ACM)
-Surfactant/fluid layer
-Alveolar epithelium
-Alveolar epithelium basement membrane
-Interstitial space/fluid
-Endothelial basement membrane
-Plasma
-Plasma membrane of RBCs

Question 2

Describe the layers gas passes through in the A/C membrane

Answer 2

Moving from the air in the alveolus (Thin side of ACM)
-surfactant/fluid layer
-alveolar epithelium
-fused alveolar capillary basement membrane
-Endothelium
-plasma
-plasma membrane of RBCs

Question 3

Describe Dalton’s law of partial pressures

Answer 3

The total pressure of a gas is the sum of the partial pressures of the individual gases

So an E cylinder of air has a total pressure of 2200psi
2200psi= 1717.98 psi N2 + 460.9 O2 + 20.46 psi Argon + .66 psi trace gases

So if we know the total pressure and the fraction of the total that is composed by a specific gas, we can then compute the partial pressure of that gas.

(Barometric pressure - PH2O)Fgasx
(760-47)FIO2 = partial pressure of oxygen.

In room air this will be
(760-47)x0.2095 = 149 mm Hg (PO2 is 149 mm Hg)

Question 4

Compute the alveolar gas equation

Answer 4

Alveolar air equation uses DLPP and knowledge of gas exchange across ACM to determine PAO2

On room air again (using .2095 rounded to 0.21):
(760-47)x0.21 = 149.7 mm Hg - PACO2/0.8

If PaCO2 is normal (40mm Hg ) then
(760-47)x0.21 = 149.7 mm Hg - (40/0.8) = 99.7 mm Hg or 100 mm Hg

Question 5

Discuss Henry’s law

Answer 5

Henry’s Law: amount of gas dissolved in a liquid is proportional to partial pressure of the gas and its solubility coefficient
-Solubility coefficient for oxygen in blood is 0.003 mL O2/mm Hg/ dL blood
- CO2 is ~ 24 times more soluble than O2

Question 6

Discuss Graham’s Law

Answer 6

Graham’s law: the larger the molecule the slower it moves across a membrane

Combined Henry’s and Graham’s law mean the diffusion coefficients which determine that CO2 moves across the ACM and into blood about 20 times more easily than oxygen

Question 7

Discuss Fick’s Law

Answer 7

The greater the SA for gas exchange, the greater the diffusion
The thinner the membrane for diffusion, the greater the diffusion
The higher the partial pressure, the higher the diffusion
The higher the diffusion coefficient, the higher the diffusion

Question 8

Discuss when and why Hyperbaric chamber is the treatment of choice

Answer 8

Wound care, bends “decompression sickness”, Carbon Monoxide poisoning

Increases the O2 tension gradient on the venous side of the circulation and throughout the tissues

Question 9

Compare and contrast diffusion limitation vs perfusion limitation

Answer 9

-perfusion limited is where we normally live. If we increase the amount of blood flow past the ACM we increase the amount of diffusion across the ACM

• diffusion limitation occurs when the ACM thickens due to restrictive diseases. The result is it takes longer for equilibration of gases to occur across the ACM. At rest in mild to moderate disease the patient may still have time for equilibration to occur. However, when the patient exercises, resulting in increased cardiac output and reduced time for equilibration, equilibration no longer occurs due to the reduced transit time. Now the PAO2 falls and the patient may experience exertional dyspnea

Question 10

Discuss the most common presentation of chronic restrictive diseases

Answer 10

The lower PAO2 results in exertional dyspnea. This is most commonly the first sign although a dry cough may also appear about the same time