Oxygen cascade

Master answer

• Fraction of O₂ in atmosphere = 0.21
• Oxygen cascade:

1. Oxygen in the atmosphere

• Oxygen in the atmosphere at sea-level
  • Atmospheric pressure = 760mmHg
  • pO₂  = 760mmHg x 0.21 = 160mmHg

2. Oxygen in the bronchi

• Inspired air at sea level is humidified and saturated by H₂O vapour at the isothermic saturation boundary 3cm distal to the carina
  • pO₂ = 0.21 (760mmHg – 47mmHg) = 150mmHg

3. Oxygen in the alveoli

• In the alveoli, inspired air is diluted with CO₂ diffusing into the alveoli from the alveolar capillaries.
  • p_AO₂  = 0.21(760mmHg – 47mmHg) – p_aCO₂/R + f
    • where, R = respiratory quotient, approx. 0.8
    • f = correctional factor
    • p_aCO₂ = 40mmHg
    • p_AO₂ = 100mmHg

4. Oxygen in the arterial blood

• Oxygen diffuses from the alveoli to pulmonary capillaries (solubility coefficient 0.024) however p_aO₂ is lower than p_AO₂ due to venous admixture
• Factors contributing to venous admixture
  • V/Q mismatch
  • Shunt
• The degree of venous admixture can be calculated by

• Oxygen in carried in blood as a predominant Hb bound portion and a minor diffused portion, governed by the oxyhaemoglobin dissociation curve
• In healthy adult, A-a gradient is no more than 15mmHg

5. Oxygen in the cytosol and mitochondria

• Oxygen diffuses from capillary blood across the plasma membrane (according to Fick’s law of diffusion) into the cell cytosol and then across the mitochondrial membranes into the mitochondria
• There is paucity of evidence to determine mitochondrial pO₂ but estimated to be between 5mmHg and up to 40 mmHg in recent studies

Sakurai / Gladwin 2016

Answer by Gladwin, Sakurai