From the Research
The oxygen concentration in expired air is higher than in alveoli because expired air is a mixture of alveolar air and dead space air. When we breathe out, the first portion of air that exits comes from the anatomical dead space (airways like the trachea, bronchi, and bronchioles) which did not participate in gas exchange. This dead space air has essentially the same composition as inspired air, with an oxygen concentration of about 21% 1. As exhalation continues, alveolar air (which has a lower oxygen concentration of about 14-15% due to oxygen being absorbed into the bloodstream) follows. The resulting expired air is a blend of these two components, giving it an intermediate oxygen concentration of approximately 16-17%. This mixing effect explains why expired air contains more oxygen than alveolar air, despite oxygen being removed during respiration. Understanding this concept is important for interpreting pulmonary function tests and understanding respiratory physiology in both normal and disease states. The most recent and highest quality study on gas exchange and ventilation-perfusion relationships in the lung supports this concept, highlighting the importance of considering dead space air in the analysis of expired air composition 1. Other studies on pulmonary rehabilitation and diffusing capacity of the lung for carbon monoxide also provide relevant information on respiratory physiology, but do not directly address the question of why the oxygen concentration in expired air is higher than in alveoli 2, 3, 4, 5. However, these studies do emphasize the importance of considering multiple factors, including ventilation-perfusion mismatch and diffusion limitation, in the assessment of respiratory function. In clinical practice, understanding the factors that influence expired air composition is crucial for interpreting pulmonary function tests and making informed decisions about patient care. For example, a patient with chronic obstructive pulmonary disease (COPD) may have a higher oxygen concentration in expired air due to increased dead space ventilation, which can impact the interpretation of pulmonary function tests and the management of the disease. Therefore, considering the mixing of dead space air and alveolar air is essential for understanding the oxygen concentration in expired air and for making accurate diagnoses and treatment decisions in patients with respiratory diseases.