Respiratory: Gas Exchange

Question 1

What are the factors affecting diffusion of a gas?

Answer 1

• the pressure difference across the barrier
• the solubility of the gas in solution
• the surface area of the barrier
• the diffusion distance

Question 2

What is more soluble, CO2 or O2?

Answer 2

CO2 is 20x more soluble so the partial pressure gradient is smaller. This is because oxygen is small and therefore fast so CO2 is more soluble. In a diseased lung oxygen gas exchange is therefore more impaired.

Question 3

What are the layers making up the diffusion barrier?

Answer 3

RBC membrane 
Capillary endothelium 
Capillary basement membrane 
Epithelial cell of alveolus 
Fluid and surfactant layer

Question 4

What factors affect diffusion resistance?

Answer 4

The nature of the barrier - permeability, thickness

The nature of the gas

Question 5

What are the different ways that CO2 is transported in the blood?

Answer 5

5% dissolved
5% as carboxyhaemoglobin on proteins
90% as bicarbonate ions

Question 6

What happens once CO2 enters a RBC?

Answer 6

CO2 enters RBCs as a waste product then reacts with water to form carbonic acid.
Carbonic acid dissociates into bicarbonate ions and hydrogen ions.
Bicarbonate ions enter plasma, h+ buffered by Hb in the RBC