3. Gas exchange

Question 1

describe the diffusion pathway between alveolar gas and alveolar capillary blood

Answer 1

1. alveolar epithelial cell
2. interstitial fluid
3. capillary endothelial cell
4. plasma
5. red cell membrane

overall thickness of barrier = 0.6 micron

Question 2

what is the pO2 and pCO2 of alveolar air in the normal lung

Answer 2

• pO2 = 13.3 kPa

- pCO2 = 5.3 kPa

Question 3

how is pO2 and pCO2 in blood arriving and leaving alveolar capillaries different to pO2 and pCO2 in alveolar air

Answer 3

mixed venous blood gas content:

• pO2 in blood arriving at capillaries (5.3 kPa) < pO2 of alveolar air… allows O2 diffusion into blood
• pCO2 in blood arriving at capillaries (6.1) > pCO2 of alveolar air… allows CO2 diffusion out of blood
• blood leaving alveolar capillaries is in equilibrium with alveolar air so has same pO2 and pCO2

Question 4

why is the pressure difference required to cause CO2 diffusion much lower than that required to cause O2 diffusion - what is the effect of this in lung disease

Answer 4

CO2 can diffuse 20x faster than O2 as is significantly more soluble in water

O2 exchange thus more impaired than CO2 exchange in a diseased lung

Question 5

name 4 factors affecting rate of gas diffusion through the respiratory membrane

Answer 5

1. membrane thickness
2. membrane surface area
3. diffusion coefficient of the gas in the membrane substance (dependent on gas’s solubility and square root of molecular weight)
4. partial pressure difference of gas on either side of membrane (ie in alveoli and in pulmonary capillary bood)

Question 6

explain why gas diffusion is impaired in

1. fibrotic lung disease
2. pulmonary oedema
3. emphysema

how does this affect capillary pO2 and pCO2

Answer 6

1. lung fibrosis: thickened alveolar membrane - slows gas exchange so low capillary pO2 but normal pCO2 as CO2 crosses alveolar membrane more easily
2. pulmonary oedema: fluid in interstitial space increases diffusion distance - slows gas exchange so low capillary pO2 but normal pCO2 as CO2 has higher solubility in water
3. emphysema: alveoli destruction reduces surface area for gas exchange - slows gas exchange so low capillary pO2 but normal pCO2 as CO2 crosses alveolar membrane more easily

Question 7

explain what is meant by the term ‘physiological dead space’

Answer 7

Volume of air that is inhaled but does not take part in gas exchange (approx. 0.17 L) because:

1. remains in conducting airways = ANATOMICAL DEAD SPACE (0.15 L)
2. reaches alveoli that are dead/damaged or not/under-perfused = DISTRIBUTIVE DEAD SPACE

Question 8

what is the difference between alveolar ventilation rate (AVR) and pulmonary ventilation rate

Answer 8

• pulmonary ventilation rate = resp. rate (normally 14-16 breaths/min)
• AVR = PVR - dead space ventilation
• dead space ventilation = dead space volume x resp. rate

Question 9

what is the approximate normal AVR

Answer 9

AVR = 350 ml x 14 (RR) = 4.9 L/min

tidal volume - anatomical dead space = 0.5 L - 0.15 L = 350 ml

Question 10

what is the V/Q ratio - when might this be abnormal?

Answer 10

• is the ventilation/perfusion ratio (ie amount of air reaching alveoli and amount of blood reaching alveoli)
• normally =1 as AVR = 4.9L/min and CO = 5L/min
• can get V/Q mismatch when alveolar subset have:
  1. reduced ventilation, e.g. early stage asthma, early stage COPD, neonatal resp. distress syndrome
  2. reduced perfusion, e.g. PE