4. Gas exchange

Question 1

Total pressure

Answer 1

The sum of all Partial Pressures

Question 2

What does partial pressure of a gas depend on?

Answer 2

– Fractional concentration of the gas
– Total pressure of gas mixture
Pgas = % gas x Ptotal

Question 3

Composition of Air

Answer 3

– 79% Nitrogen
– 21% Oxygen
– Trace amounts carbon dioxide, helium, argon, etc.
– Water can be a factor depending on humidity (i.e. at 100% humidity: 6.2%)

Question 4

Go through gas comp pg 6

Answer 4

6/13

Question 5

Diagram air flow

Answer 5

Question 6

Fick’s Law - rate of diffusion

Answer 6

• Vgas: rate of diffusion
• A: surface area
(50-100m2 in the lung)
• T: thickness
(0.2-0.5µm)
• DP: pressure difference
• D: diffusion constant

Question 7

Fick’s Law Diffusion

Answer 7

• D: diffusion constant
• S: gas solubility
• CO2:O2 solubility
24:1
• MW: melecular weight
• CO2:O2 molecular weight 44:32

Question 8

Solubility of oxygen in water diagram

Answer 8

Question 9

Solubility of CO2 in water diagram

Answer 9

Question 10

Compare the diffusion constants and total diffusion of CO2 and O2 in the lung?

Answer 10

– D(CO2) = 24/Ö44 = 3.62
– D(O2) = 1/Ö32 = 0.18
• CO2 diffusion constant is 20.5 times that of O2
– V(CO2) = 3.62 x 6 = 21.7
– V(O2) = 0.18 x 60 = 10.6
• CO2 diffusion rate is 2 times that of O2

Question 11

Equilibrium rates

Answer 11

• O2 & CO2 equilibrate at similar rates
• At rest: blood spends 0.75 sec in the capillary
• Normal equilibration within 1/3 of capillary transit (0.25 sec)
• During intense exercise: time reduced to 0.25 sec
• Diffusion process affected by:
• Exercise
– Highly trained athletes: risk of Exercise Induced Arterial Hypoxemia (EAIH)
• Thickening of blood-gas barrier

Question 12

What is pulmonary oedema

Answer 12

Fluid accumulation in alveoli and/or interstitial space

Question 13

What does pulmonary oedema cause?

Answer 13

• Impairs diffusion (higher distance from alveoli to blood)
• Leakage in unprotected capillaries
• Increases work of breathing (decreased lung compliance)
• Arterial blood: lower PO2 and higher PCO2

Question 14

Causes of pulmonary oedema x 2 and why

Answer 14

• Increased capillary pressure – Via left heart failure (inability to supply sufficient BF)
• Reduced atmospheric pressure – At altitude

Question 15

Treatment for pulmonary oedema

Answer 15

Administering oxygen and diuretics