Oxygen Cascade

• P_O2 of inspired air is 20.93% x (760 - 47) = 149mmHg

• 760mmHg is barometric pressure, 47 is water vapour of moist inspired gas, 20.93% is the fraction of inspired air which is oxygen

• PO₂ is ~100mmHg by the time it reaches the alveoli

• This is because O₂ is continually being removed by pulmonary capillary blood
• Stays within ~3mmHg of this level as the tidal volume replenishment is small compared to the total volume in the lung
• Rate of removal of O₂ constant at resting conditions
• Level of alveolar ventilation largely determines the alveolar PO₂ and PCO₂

• PO₂ is much lower in tissue capillaries and mitochondria

• Variable due to the proximity of tissues to vessels, but PO₂ can be as low as 1mmHg

• 4 causes of hypoxaemia - hypoventilation, diffusion limitation, shunt, ventilation-perfusion inequality