PULMONARY GAS EXCHANGE

SUMMARY

1. Atmospheric pressure (P_b) at sea level, at 59° F, is 760 mmHg.

2. The component gases of the atmosphere each exert a consistent partial pressure and contribute to the atmospheric pressure.

3. Partial pressure O₂ = F_iO₂ * P_b = 0.209 x 760 mmHg, where F_iO₂ = fraction of inspired oxygen & P_b = atmospheric pressure. This pressure is called the P_iO₂ (inspired).

4. The F_iO₂ remains constant regardless of the altitude.

5. The respiratory quotient (0.8) is the minute production of CO₂/minute consumption of O₂. This quotient allows us to use the measurable P_aCO₂ (arterial) in the alveolar gas equation instead of the P_ACO₂ (alveolar), which we can't readily measure.

6. The alveolar gas equation calculates the partial pressure of O₂ in the alveoli: P_AO₂ = [(P_b- P_H2O) * F_iO₂] - [P_aCO₂/0.8]

Nb: we see that the F_iO₂ is still multiplied by the P_b but only after its value is decreased to account for the water vapor. The second term decreases this product by an amount that takes into account the O₂:CO₂ exchange in the alveoli.

Reference(s)

Wilkinson, I., Furmedge, D. and Sinharay, R. (2017). Oxford handbook of clinical medicine. Oxford: Oxford University Press. Get it on Amazon.
Feather, A., Randall, D. and Waterhouse, M. (2020). Kumar And Clark’s Clinical Medicine. 10th ed. S.L.: Elsevier Health Sciences. Get it on Amazon.
Hannaman, R. A., Bullock, L., Hatchell, C. A., & Yoffe, M. (2016). Internal medicine review core curriculum, 2017-2018. CO Springs, CO: MedStudy.
Therapeutic Guidelines. Melbourne: Therapeutic Guidelines Limited. https://www.tg.org.au [Accessed 2021].