Alveolar Gas Exchange
Alveolar gas exchange is the transfer of oxygen and carbon dioxide between the gas in the alveoli and the blood in the surrounding pulmonary capillaries. The alveolus is the functional unit where inspired air meets venous blood, and the partial pressures of gases within it set the gradients that drive the exchange.
Definition
Alveolar gas exchange is the diffusive transfer of oxygen from alveolar gas into pulmonary capillary blood and of carbon dioxide from blood into alveolar gas, governed by the alveolar partial pressures of those gases.
Scope
This topic covers how alveolar gas composition is established, the alveolar gas equation as the link between inspired oxygen, carbon dioxide elimination, and alveolar oxygen tension, and the concepts of alveolar ventilation, dead space, and shunt that determine how completely capillary blood equilibrates with alveolar gas. It is physiological reference material, not clinical guidance.
Core questions
- What determines the partial pressure of oxygen in alveolar gas?
- How does alveolar ventilation set arterial carbon dioxide tension?
- Why is some inspired air 'wasted' as dead space, and how does that affect exchange?
- How do shunt and venous admixture lower the oxygen content of blood leaving the lung?
Key concepts
- Alveolar partial pressures (PaO2, PaCO2 of alveolar gas)
- Alveolar gas equation
- Alveolar ventilation
- Anatomical and physiological dead space
- Shunt and venous admixture
- Respiratory exchange ratio
Key theories
- Alveolar gas equation
- The alveolar partial pressure of oxygen is derived from inspired oxygen tension minus carbon dioxide elimination scaled by the respiratory exchange ratio; this relationship, formalized in the ideal alveolar air analysis, lets alveolar oxygen be estimated from measurable quantities.
Mechanisms
With each breath, inspired air is warmed, humidified, and diluted by resident alveolar gas, so alveolar oxygen tension is lower than inspired and alveolar carbon dioxide tension reflects the balance of metabolic carbon dioxide production and alveolar ventilation. Oxygen and carbon dioxide then diffuse across the thin alveolar-capillary barrier down their partial-pressure gradients until capillary blood approaches equilibrium with alveolar gas. Regions that are ventilated but not perfused contribute dead space; regions perfused but not ventilated contribute shunt. The alveolar gas equation expresses the dependence of alveolar oxygen tension on inspired oxygen, carbon dioxide elimination, and the respiratory exchange ratio.
Clinical relevance
The alveolar gas framework underlies the interpretation of arterial blood gases and the alveolar-arterial oxygen difference used to characterize gas-exchange impairment. This entry describes that physiology for reference and does not provide diagnostic thresholds or treatment recommendations.
Evidence & guidelines
The concepts here are standard respiratory physiology, supported by the foundational ideal alveolar air analysis and by integrative reviews of pulmonary gas exchange. The material is descriptive physiology rather than guideline-governed practice.
History
The quantitative treatment of alveolar gas dates to Riley and Cournand's mid-century work, which defined an 'ideal' alveolar compartment against which real lungs could be compared, allowing dead space and shunt to be estimated from blood and gas measurements. Subsequent reviews integrated these ideas with continuous V/Q distributions and regional imaging.
Key figures
- Richard Riley
- André Cournand
- John B. West
Related topics
Seminal works
- riley-cournand-1949
- petersson-glenny-2014
Frequently asked questions
- Why is alveolar oxygen lower than the oxygen in the air we breathe?
- Inspired air is humidified, adding water vapour that dilutes it, and oxygen is continuously taken up by the blood while carbon dioxide is added, so alveolar oxygen tension settles below the inspired value.
- What is dead space?
- Dead space is the part of each breath that does not take part in gas exchange, either because it stays in the conducting airways (anatomical) or because it reaches alveoli that are ventilated but poorly perfused (physiological).