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M04.07.010 The Normal Lung: Gas Exchange and Partial Pressures
The Normal Lung: Gas Exchange and Partial Pressures
Partial Pressure of a Gas in Ambient Air
The partial pressure of a gas in ambient air can be calculated using the formula:
Formula:
Pgas = Fgas × Patm- Pgas: Partial pressure of the gas
- Fgas: Fraction of the gas in the mixture
- Patm: Atmospheric pressure
For example, the partial pressure of oxygen (PO₂) in ambient air is calculated as:
PO2 = 0.21 × 760 = 160 mm HgPartial Pressure of a Gas in Inspired Air
Inspired air refers to air that has been inhaled, warmed to 37°C, and completely humidified but has not yet engaged in gas exchange. This fresh air is in the anatomical dead space (anatVD) and is about to enter the respiratory zone.
The partial pressure of water vapor (PH₂O) depends only on temperature and is 47 mm Hg at 37°C. Humidification of the air reduces the partial pressures of other gases present.
Formula:
PIgas = Fgas × (Patm – PH2O)For example, the partial pressure of oxygen in inspired air (PIO₂) is:
PIO2 = 0.21 × (760 – 47) = 150 mm HgAlveolar–Blood Gas Exchange
Alveolar and blood gas exchange involves the transfer of gases between the alveoli and pulmonary capillary blood. The pressures of oxygen and carbon dioxide in different compartments can be illustrated in a table:
Table: Pulmonary Capillary Gases
| Compartment | Partial Pressure of O₂ (PO₂) | Partial Pressure of CO₂ (PCO₂) |
|---|---|---|
| Ambient Air | 160 mm Hg | – |
| Inspired Air | 150 mm Hg | – |
| Alveolar Air (PA) | 100 mm Hg | 40 mm Hg |
| End Capillary (Pulmonary) | 100 mm Hg | 40 mm Hg |
| Systemic Arterial Blood | 95 mm Hg | 40 mm Hg |
| Venous Blood (Pv) | 40 mm Hg | 47 mm Hg |
Key Concepts
- Dalton’s Law of Partial Pressures: The total pressure exerted by a mixture of gases is the sum of the pressures exerted independently by each gas in the mixture.
- A-a Gradient: The difference between the alveolar and systemic arterial PO₂ (5-10 mm Hg) provides information about the cause of hypoxemia.
Points to Remember
- The partial pressure of gases in inspired air decreases due to humidification.
- Under normal conditions, the PO₂ and PCO₂ in the alveolar compartment and pulmonary end capillary blood are the same (perfusion-limited).
- A small A-a gradient is normal and helps in diagnosing hypoxemia.
Bibliography
- West, J.B. (2012). Respiratory Physiology: The Essentials. 9th Edition. Lippincott Williams & Wilkins.
- Guyton, A.C., & Hall, J.E. (2016). Textbook of Medical Physiology. 13th Edition. Elsevier.
- Levitzky, M.G. (2018). Pulmonary Physiology. 9th Edition. McGraw-Hill Education.