3.2 Gas Exchange

Question 1

What are some factors affecting diffusion rate of a gas in a fluid?

Answer 1

Pressure difference
Solubility of the gas in solution 
Cross-sectional area of the fluid 
Distance that the molecules need to diffuse
Molecular weight of the gas 
Temperature of fluid (body assume 37)

Question 2

What can the diffusion coefficient be used to determine?

Answer 2

The relative rates at which different gases will diffuse across the same membrane at the same pressures

Eg CO2 diffuses x20 more quickly than O2

Question 3

How can lower diffusion coefficients be compensated for?

Answer 3

Larger partial pressures

Question 4

Which gas exchange is more impaired in a diseased lung and why?

Answer 4

Oxygen exchange more impaired

Because it has a slower diffusion rate

Question 5

Describe the diffusion barrier for diffusion through gas to alveolar wall

Answer 5

5 cell membranes, 3 layers of cytoplasm, 2 layers of tissue fluid

Epithelial cells of alveolus
Tissue fluid
Endothelial cell of capillary
Plasma
Red cell membrane

Question 6

What is the distance for diffusion between the blood and air in the alveoli and why is this relevant?

Answer 6

0.6 micrometers
Distance decreases during inhalation as lung distends

Allows fast and efficient diffusion

Question 7

What does diffusion resistance depend on?

Answer 7

Nature of the barrier

Nature of the gas

Question 8

How do you work out permeability using diffusion coefficient?

Answer 8

Permeability = (Diffusion coefficient of molecules in the membrane X Solubility in the lipid bilayer) / Thickness of membrane

Question 9

Describe the composition of dry air

Answer 9

Mixture of O2 (21%) and N2 (79%)
Variable amount of H20 depending on relative humidity and temperature
101kPa

Question 10

Describe the composition of tracheal/inhaled air

Answer 10

Inspired air always saturated with water
Water content depends on temperature
- At body temp, water is 6.28kPa
Means that oxygen and nitrogen are slightly diluted

Question 11

Describe the movement of oxygen from the air to the blood in terms of partial pressures

Answer 11

PO2 lower in alveoli than external environment
- Continuous diffusion of O2 across alveolar wall
- Due to diluting effect of CO2 in alveoli
PO2 is higher in alveoli than capillaries so O2 diffuses into blood
O2 combines with Hb and travels in bloodstream and oxy-haemoglobin

Question 12

Describe the movement of carbon dioxide from blood to air

Answer 12

Enters RBC as a waste product and reacts with water -> Form carbonic acid
CA dissociates to bicarbonate IONS and hydrogen ions
Bicarbonate ions diffuse to plasma
H+ buffered by Hb in RBC

PCO2 in capillaries higher thats alveoli -> Exhaled

Question 13

What are the normal partial pressures of oxygen and carbon dioxide in the lung?

Answer 13

PO2: 13.3kPa
PCO2: 5.3kPa

Question 14

What are the partial pressures of oxygen and carbon dioxide in the alveolar capillaries in comparison to alveolar air?

Answer 14

The same

Equilibrium

Question 15

Why does oxygen diffuse into the blood and carbon dioxide diffuse out?

Answer 15

PO2 in alveolar gas > PO2 in returning blood

PCO2 in alveolar gas < PCO2 in returning blood

Question 16

What are the factors affecting rate of gas diffusion through the respiratory membrane?

Answer 16

Thickness of the membrane
Surface area
Diffusion coefficient of the gas in the substance of the membrane
Pressure difference of the gas over the two sides of the membrane

Question 17

What is the diffusion impairment in fibrotic lung disease?

Answer 17

Thickened alveolar membrane
Slows gas exchange

PO2 low
PCO2 normal - crosses alveolar membrane more easily

Question 18

What is the diffusion impairment in pulmonary oedema?

Answer 18

Fluid in the interstitial space increases diffusion distance

PO2 low -> increased diffusion distance
PCO2 normal due to higher CO2 solubility in water

Question 19

Why is some air wasted in every breath?

Answer 19

Because air enters and leaves by the same airways

Last air in is first air out so it doesn’t reach the alveoli

Question 20

What is the serial/anatomical dead space?

Answer 20

Due to the portion of the airways where there is no gaseous exchange -> Conducting portion

150ml

Question 21

What is the distributive dead space?

Answer 21

170ml

Some parts of the lung are not airways or do not support gas exchange (dead/damaged alveoli or alveoli with poor perfusion)
Add this to the serial dead space

Question 22

How do you calculate alveolar ventilation rate?

Answer 22

(Tidal volume - Anatomical dead space) X Respiration rate

Question 23

What is the ideal ventilation-perfusion ratio?

Answer 23

1

V = 350 X 14 = 4.9L/min 
Q = RV (CO) output = 5L 

5/5=1

Question 24

What is the V/Q ratio if V is 0 but there is still perfusion?

Answer 24

0

Question 25

What is the V/Q ratio if V is normal but perfusion is 0?

Answer 25

Infinity