Principles of gas exchange

Question 1

What’s the difference in gas partial pressure in atmospheric vs alveolar gas?

Answer 1

• O2 and N2 are lower in alveolar air due to water vapour diluting the gas (humidified air)
• there is almost no CO2 in atmospheric air, but there is 27mmHg in expelled air

Question 2

How is the O2 concentration controlled in alveolar air?

Answer 2

1. rate of O2 entry into the lungs
2. rate of O2 absorption into the capillaries

Question 3

What are the components of expelled air?

Answer 3

dead space air (humidified) and alveolar air

Question 4

What’s the advantage of having slow replacement of alveolar air by atmospheric air?

Answer 4

prevent sudden change in gas concentrations in the blood

Question 5

Name 4 reasons that can explain the differences between atmospheric and alveolar air composition.

Answer 5

1. alveolar air is only partially replaced by atmospheric air by each breath
2. O2 is constantly being absorbed from the alveolar air
3. CO2 is constantly diffused from the pulmonary blood into the alveoli
4. dry atmospheric air is humidified before reaching the alveoli

Question 6

What are the layers that O2 must go through to into the capillary?

Answer 6

1. fluid layer/ surfactant
2. alveolar epithelium
3. epithelial basement membrane
4. interstitial space
5. capillary basement membrane
6. capillary endothelium

Question 7

What are some factors that can effect the rate of gas diffusion?

Answer 7

1. membrane thickness - ex. extra fluid? fibrosis?
2. total surface area - ex. emphysema coalescing the alveoli
3. diffusion coefficient
4. pressure difference across the respiratory membrane

Question 8

Which gas has a greater diffusion capacity at rest, CO2 or O2?

Answer 8

CO2

Question 9

How does the diffusion capacity increase for O2 during strenuous exercise?

Answer 9

1. increasing the surface area: opening up closed pulmonary capillaries and dilation of open capillaries
2. improved ventilation/perfusion ratio

Question 10

What’s the concept of V/Q mismatch?

Answer 10

V/Q > 1: more ventilation than perfusion. Ex. physiological dead space
V/Q <1: more perfusion than ventilation. Ex. physiological shunt
V/Q = normal: optimal gas exchange