Respiratory Gas Exchange

Question 1

What is diffusion of a gas?

Answer 1

Net movement of a molecule from an area of higher concentration to area of lower concentration

Question 2

How is diffusion calculated?

Answer 2

Fick’s law of diffusion, (surface area X diffusion coefficient X partial pressure of gradient) / thickness of the barrier

Question 3

What is perfusion-limited diffusion?

What are some examples?

Answer 3

The diffusion of the gas is limited by the diffusion in the alveoli, the gases on each side equilibrate before the RBC leaves the capillary and the only way to increase diffusion is to increase CO

Examples: N2O, O2

Question 4

What is diffusion-limited diffusion?

Answer 4

The diffusion of the gas is limited to its diffusibility across the barrier and the surface and thickness area of the barrier. By the time it passes the alveoli, it is not in equilibrium with the alveoli

To increase diffusion you must recruit more pulmonary capillaries

CO has a high affinity for Hgb and will bind to it, there will be a high concentration on the Hgb but almost none in the plasma

Question 5

When might you see diffusion-limited O2 transfer?

Answer 5

During strenuous exercise - get the RBCs flowing through the capillaries at 1/4 of a second

Poor diffusion characteristics at the alveolar capillary barrier, may show some diffusion-limited characteristics
Person with 1/4 normal diffusion barely has perfusion limited transfer
person with 1/8 normal diffusion has diffusion-limited transfer

Question 6

What is the perfusion-diffusion like for CO2?

Answer 6

CO2 is normally perfusion-limited
CO2 diffuses 20x faster than O2

CO2 is diffusion-limiter at 1/4 and 1/8 normal diffusion capacity

Question 7

What are some conditions that decrease diffusion capacity?

Answer 7

1.Thickening of the barrier
Interstitial or alveolar edema
Interstitial or alveolar fibrosis
---Sarcoidosis
---Scleroderma

2.Decreased surface area
Emphysema
Tumors
Low cardiac output
Low pulmonary capillary blood volume

3.Decreased uptake by erythrocytes
Anemia
Low pulmonary capillary blood volume

4.Ventilation-perfusion mismatch

Question 8

How is diffusion capacity measure clinically?

Answer 8

Use CO because the mean pulmonary capillary partial pressure of CO is almost zero, all that needs to be measured are the partial pressures of CO and you calculate diffusion capacity

By rearranging Fick’s equation, diffusion capacity can be calculated.

Since CO is only limited by surface area, diffusibility, and thickness of barrier, by measuring the diffusion capacity we can determine if there is a perfusion or diffusion limited problem

Question 9

How does gas move through the system in terms of bulk flow and diffusion?

Answer 9

1. External air environment - alveoli= by bulk flow
2. Alveoli - pulmonary capillaries= by diffusion
3. Gas in blood through arterioles = bulk flow
4. Through systemic capillaries into tissues = diffusion
5. Venous circulation- Bulk flow
6. Pulmonary capillaries to alveoli = diffusion
7. Alveoli to external environment = bulk

Question 10

What would low diffusion capacity indicate?

Answer 10

This would indicate a diffusion problem

• Diseases associated with interstitial or alveolar fibrosis (sarcoidosis, scleroderma, asbestosis)
• conditions causing interstitial or pulmonary edema