pk4 Alveolar Gases and Diffusion

Question 1

How much air mixes into the FRC per inspiration?

Answer 1

~350ml.

Question 2

What is the average FRC?

Answer 2

2500ml.

Question 3

What is the average total lung capacity?

Answer 3

6L.

Question 4

What are the partial pressures of oxygen and carbon dioxide in the alveoli and arteries?

Answer 4

• PAO2 = 13 kPa.

- PACO2 = 5 kPa.

Question 5

What are the average mixed venous gas tensions of oxygen and carbon dioxide?

Answer 5

• PVO2 = 5kPa.

PVCO2 = 6kPa.

Question 6

What sets alveolar PCO2?

What is the equation for alveolar CO2 using these variables?

Answer 6

1 - Arterial flow of CO2.

2 - Alveolar flow.

• PACO2 = 115kPa * (Arterial flow CO2 / Total gas flow in the alveoli).

Question 7

Why is PaCO2 an effective clinical measure of adequacy of alveolar flow?

Answer 7

• Because if total alveolar flow increases in proportion with arterial flow of CO2, then PACO2 (and hence PaCO2) can be maintained constant at 5kPa.
• If PaCO2 falls below 5kPa then alveolar flow has not increased in proportion with arterial flow.

Question 8

Why are the determinants of alveolar PO2 not the same as the determinants of alveolar PCO2?

Answer 8

• Because inspired PO2 is not zero so must also be taken into account.
• Otherwise the same as for PACO2.

Question 9

What is the equation for alveolar O2?

Answer 9

PAO2 = PIO2 - ( 115kPa x (Arterial flow O2 / Total gas flow in the alveoli).

Question 10

How thick is the alveolar-capillary membrane?

Answer 10

0.2 - 0.6 micrometers.

Question 11

List the layers that comprise the alveolar-capillary membrane.

Answer 11

1 - Capillary epithelium.

2 - Basal lamina.

3 - Alveolar epithelium.

Question 12

Concerning movement of gases across alveoli, through which phases must gases diffuse?

Answer 12

Gaseous and liquid phases.

Question 13

What is Graham’s law?

What are its implications for the diffusion of oxygen and carbon dioxide through gas?

Answer 13

• In a gaseous phase, the rate of diffusion of a gas is inversely proportional to the square root of the mass of its particles (and is also dependent upon the partial pressure difference).
• Since the relative mass of CO2 is 44 and the relative mass of O2 is 32, CO2 will diffuse 0.85x slower than O2 through gas.

Question 14

What is Henry’s law?

What are its implications for the diffusion of gases through liquid?

Answer 14

• Conc. of Gas = Partial Pressure x Solubility Coefficient (α).
• Since the rate of diffusion through a liquid phase depends on the concentration gradient and, according to Henry’s law, a more soluble gas maintains a higher concentration difference, a more soluble gas will also diffuse quicker.

Question 15

List the solubility values for oxygen and carbon dioxide.

How much faster will carbon dioxide diffuse through a liquid than oxygen?

Answer 15

• αCO2 = 5.30 ml/LkPa
• αO2 = 0.23 ml/LkPa
• Carbon dioxide will diffuse 23x faster than oxygen in a liquid.

Question 16

Why does carbon dioxide diffuse faster through the alveolar-capillary membrane than oxygen?

Answer 16

Despite carbon dioxide having a lower difference across the membrane than oxygen (1kPa vs 8kPa), carbon dioxide is more soluble and so diffuses through the liquid phase much quicker.

Question 17

Why is movement of CO across the alveolar-capillary membrane diffusion limited?

Answer 17

• Because of the high CO-binding capacity of Hb.
• Any CO diffusion across the membrane will be bound by Hb and so will not contribute to the partial pressure of CO in the blood.
• Diffusion is therefore not fast enough to saturate the blood.
• Therefore equilibration is not met before the transit time is over.

Question 18

Why is movement of N2O and O2 across the alveolar-capillary membrane perfusion limited?

Answer 18

• Because of the high diffusion rate of N2O across the membrane.
• N2O therefore quickly saturates the blood supply.
• Equilibration is therefore met before the transit time is over.

Question 19

Define transit time.

Answer 19

The time that a red blood cell takes to flow through the pulmonary exchange interface.

Question 20

List 2 reasons for oxygen taking a longer time to equilibrate than normal.

Answer 20

1 - Exercise.

2 - Altitude.

Question 21

What is the equation for the flow of gas across a diffusion barrier?

Answer 21

• Flow = d x (A/T) x (P1-P2).

- Where d = diffusability, A = area, T = thickness and P = partial pressure.

Question 22

What is DL?

How was this value measured?

Answer 22

• The diffusion constant.
• A combination of d, A and T.
• Measured using CO since O2 has a variable P2 therefore is hard to measure (the P2 for CO is almost always 0 because of its high affinity for Hb).

Question 23

List 4 disturbances that reduce DL by reducing area.

Answer 23

1 - Loss of lung tissue.

2 - Airway obstructions.

3 - Capillary obstructions.

4 - Ventilation / perfusion mismatches.

Question 24

List 3 disturbances that reduce DL by increasing thickness / diffusion path length.

Answer 24

1 - Thickened alveolar-capillary membrane (e.g. fibrosis).

2 - Accumulation of lung fluid.

3 - Increased intracapillary distance (e.g. low haemoglobin).

Question 25

Define cyanosis.

Answer 25

Blue discolouration of the skin due to low oxygen saturation.

Question 26

How would a patient with reduced DL present?

Answer 26

• Hypoxic with cyanosis that is aggravated by exercise.
• Increased alveolar ventilation and decreased PaCO2.
• Normal ventilatory capacity.