Gas Exchange

Question 1

Why is oxygen always the limiting step?

Answer 1

Because carbon dioxide is much more soluble so it diffuses twenty times faster than oxygen even though it is a larger molecule.

Question 2

How is the slower diffusion rate of oxygen compensated for?

Answer 2

The partial pressure gradient for oxygen is steeper.

Question 3

Explain Graham’s law and what factor means it does not apply to carbon dioxide and oxygen.

Answer 3

Rate of diffusion of a gas is inversely proportional to its molar mass.
Theoretically oxygen should then diffuse more quickly than carbon dioxide as it is a smaller molecule however carbon dioxide is much more soluble so diffuses faster.

Question 4

How is diffusion mechanically facilitated during inhalation?

Answer 4

Distance between blood air interface decreases as lung distends

Question 5

Why is partial pressure of oxygen in alveoli lower compared to external environment?

Answer 5

Because it is being continuously diffused across alveolar wall.

Question 6

What is the partial pressure of carbon dioxide in alveolar gas?

Answer 6

5.3 kPa

Question 7

What is the partial pressure of oxygen in alveolar gas?

Answer 7

13.3 kPa

Question 8

What barriers do the gases have to traverse during exchange?

Answer 8

Epithelial cell of alveoli, tissue fluid, endothelial cell of capillary, plasma, red cell membrane

Question 9

Why is it beneficial that multiple breaths are required to completely exchange alveolar air?

Answer 9

If respiration is temporarily interrupted, blood gas and ph levels are not affected so respiratory control is more stable

Question 10

What is the importance of a ventilation perfusion ratio of one?

Answer 10

Allows for maximal gas exchange

Question 11

What is the anatomical or serial dead space?

Answer 11

The last air that enters the lungs is the first to leave and so never takes place in gas exchange as it is only in the conducting portion of the airways

Question 12

What is alveolar or distributive dead space?

Answer 12

Parts of the alveoli that do not engage in gas exchange due to poor perfusion or dead or damaged alveoli

Question 13

What two components make up physiological dead space?

Answer 13

Anatomical and alveolar dead space

Question 14

What is pulmonary ventilation rate?

Answer 14

Respiration rate x tidal volume

Question 15

What is alveolar ventilation rate?

Answer 15

Pulmonary ventilation rate - dead space ventilation rate

Question 16

What is dead space ventilation rate?

Answer 16

Dead space volume x respiration rate

Question 17

What are the six factors affecting diffusion rate of a gas into a fluid?

Answer 17

1. Molecular weight of the gas
2. Solubility of gas in solution
3. Distance the molecules must diffuse
4. Pressure difference
5. Cross sectional area of the fluid
6. Temperature of the fluid