Can you measure the partial pressure of oxygen in the alveolus?

The partial pressure gradient of O₂ between the alveolus and the pulmonary capillaries is one of the key factors that determine the rate of O₂ diffusion across the alveolar–capillary barrier (see Chapter 10). Unfortunately, it is not possible to directly measure the P_AO₂. Instead, P_AO₂ can be estimated using the alveolar gas equation (AGE).

What is the AGE?

The AGE¹ allows P_AO₂ to be estimated from variables that are easily measured.

Key equation: simplified AGE

PAO2=FiO2PB−PSVPwater−PaCO2R

where F_iO₂ for dry air is 20.93%; P_B at sea level is 101.325 kPa. Inspired air becomes fully saturated with water vapour by the time it reaches the carina. P_{SVP water} is the saturated vapour pressure of water, which at body temperature is 6.3 kPa. P_aCO₂ is measured in kilopascals. R is the respiratory quotient, usually taken as 0.8.

The AGE tells us that P_AO₂ is essentially dependent on three variables:

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  The inspired fraction of O₂. According to the AGE, increasing F_iO₂ will result in a greater P_AO₂, thus increasing the pressure gradient across the alveolar–capillary barrier; the rate of O₂ diffusion will increase (Fick’s law – see Chapter 10).

  
  
• 
  

  Barometric pressure. P_B decreases exponentially with ascent to altitude (see Chapter 87). According to the AGE, as the elevation above sea level increases, the fall in P_B results in a lower P_AO₂ and thus a reduced rate of O₂ diffusion across the alveolar–capillary barrier.

  
  
• 
  

  Alveolar ventilation. As P_aCO₂ is inversely proportional to V̇_A (see Chapter 11):

  
  
  
  
  1. 
     

     – An increase in V̇_A (hyperventilation) results in a decrease in P_aCO₂. According to the AGE, P_AO₂ will increase, thus increasing the rate of O₂ diffusion across the alveolar–capillary barrier.

     
     
  2. 
     

     – A decrease in V̇_A (hypoventilation) results in an increase in P_aCO₂. According to the AGE, P_AO₂ will decrease, thus reducing the rate of O₂ diffusion across the alveolar–capillary barrier.

  
  

What is the respiratory quotient? Why does it differ for different dietary substrates?

The respiratory quotient R is the ratio of CO₂ production and O₂ consumption:

R=rate ofCO2producedrate ofO2consumed

Related posts:

Chapter 9 – Carbon Dioxide Transport Chapter 16 – Ventilation–Perfusion Zones in the Lung Chapter 26 – Anaesthesia and the Lung Chapter 37 – Venous System Chapter 40 – Cardiovascular Reflexes Chapter 48 – Cerebral Blood Flow