Alveolar Oxygen

  • Qualitatively, the partial pressure of oxygen within the alveoli is determined by two opposing processes. The first involves entry of oxygen into the alveolus from the external environment and is determined by the rate of alveolar ventilation as well as the starting partial pressure of oxygen in the external environment. The second involves the rate at which oxygen diffuses into the pulmonary capillaries and is carried away, a process discussed in Oxygen Pulmonary Gas Exchange. The balance between these two processes, which bring oxygen into the alveolus and then carry it away, ultimately determines the steady-state value of alveolar oxygen partial pressure.
  • Quantitatively, the partial pressure of alveolar oxygen can be calculated using the "Alveolar Gas Equation" given below. The alveolar oxygen tension is of significant physiological importance as it largely determines the partial pressure of arterial oxygen. This is because the partial pressure of oxygen in the alveoli largely equilibrates with that in capillary blood by the end of the pulmonary capillaries, as discussed in Oxygen Pulmonary Gas Exchange. Consequently, derangements of the alveolar oxygen tension can and will lead to significant derangements in the arterial oxygen tension.
  • Overview
    • Although the derivation for the Alveolar Gas Equation is complex and not discussed here, below we provide a largely informal qualitative explanation for the structure of the equation. Given our discussion above it may be initially unclear why terms for alveolar ventilation and the rate of oxygen diffusion into the pulmonary capillaries are not included in the equation. The student can think of these variables as subsumed under the term for alveolar carbon dioxide as discussed further below.
  • Alveolar Gas Equation
    • PAO2 = PIO2 - PACO2/R
    • Where:
    • PAO2 = Partial pressure of alveolar oxygen
    • PIO2 = Partial pressure of inspired air
    • PACO2 = Partial pressure of Alveolar Carbon Dioxide
    • R = Respiratory Exchange Ratio
  • Influence of Inspired Air Oxygen Tension
    • As can be seen from the alveolar gas equation the partial pressure of inspired oxygen (PIO2) is an important variable that determines the partial pressure of alveolar oxygen. This is intuitively intelligible as higher inspired oxygen partial pressure will allow for higher alveolar partial pressures of oxygen; conversely, when the PIO2 is low, this yields a lower starting point for what the PAO2 can ultimately be, regardless of other factors. PIO2 is normally 150 mm Hg at sea level and is slightly lower than the actual partial pressure of oxygen in the external atmosphere due to dilution of external air with water vapor as it is humidified during passage through the upper respiratory tract.
  • Influence Alveolar Ventilation
    • The alveolar ventilation rate powerfully influences the partial pressure of alveolar oxygen. Higher values of alveolar ventilation result in more rapid refreshing of the alveolar air with oxygen-rich inspired air and thus increase the alveolar oxygen tension. In contrast, at lower levels of alveolar ventilation, the alveolar air is refreshed less often and thus is rendered oxygen-poor. However, as mentioned above, alveolar ventilation is not a direct variable in the alveolar gas equation and is subsumed under the alveolar carbon dioxide variable (PACO2).
    • As described in alveolar carbon dioxide, PACO2 is inversely proportional to the alveolar ventilation rate. Thus higher alveolar ventilation rates reduce the value for PACO2 and thus result in increased values for PAO2. An interesting thought experiment is to ask the consequence of a lung with infinite alveolar ventilation. In this scenario, the value for PACO2 will approach zero and thus the PAO2 will approach that of inspired air.
  • Influence of Oxygen Diffusion
    • As discussed in Oxygen Pulmonary Gas Exchange the rate of oxygen diffusion into the pulmonary capillaries from the alveoli is dependent on the oxygen tension gradient between the blood that initially enters the pulmonary capillaries and that in the alveoli. When the oxygen tension in the initial pulmonary capillary blood is lower, the alveolar-capillary oxygen tension gradient is widened which in turn enhances the rate of oxygen diffusion into the capillaries. The principal physiological variable which influences the oxygen tension within the incoming pulmonary capillary blood is the metabolic rate of the body. Recall that metabolic activity by the peripheral tissues consumes oxygen and releases carbon dioxide. To do so, peripheral cells extract oxygen from the circulation and thus reduce the partial pressure of oxygen in the venous blood which ultimately perfuses the pulmonary capillaries. Consequently, higher rates of metabolism result in lower incoming pulmonary capillary oxygen tensions and thus enhanced diffusion of oxygen from alveoli into the pulmonary capillaries.
    • As seen above, the metabolic rate is not a variable in the alveolar gas equation; however, it still exerts its influence by modulating the partial pressure alveolar carbon dioxide. As described in the alveolar carbon dioxide page, the partial pressure of alveolar carbon dioxide is proportional to the metabolic rate of the body as measured by the quantitative metabolic production of CO2. Consequently, enhanced metabolic activity increases the value of alveolar carbon dioxide (PACO2) and thus reduces the value for PAO2 in the equation above.
  • Respiratory Exchange Ratio
    • One issue with the discussion of metabolism above is that the quantitative metabolic production of CO2 by the body is often not numerically equivalent to the metabolic production of O2. Because the value of alveolar carbon dioxide is calculated using the the rate of CO2 production, this may lead to errors when the PACO2 is used to a proxy variable for the metabolic consumption of O2 in calculating the alveolar oxygen tension. The Respiratory Exchange Ratio (R) included in the alveolar gas equation is a variable which corrects for this issue. The respiratory exchange ratio can vary and is dependent on the primary nutrients that the body is metabolizing.
    • For example, when the body primarily metabolizes carbohydrates, the respiratory exchange ratio is approximately 1.0; however, when fats are primarily utilized, the value drops to 0.7. The dependence of the ratio on the nutrients metabolized has to do with the number of oxygen molecules required to fully metabolize the nutrient to carbon dioxide. Because carbohydrates inherently possess a large number of oxygen molecules, roughly one molecule of oxygen is required to produce one molecule of carbon dioxide; in contrast, because fats possess nearly no oxygen molecules, each molecule of oxygen produces only 0.7 molecules of carbon dioxide. In a healthy individual on an average diet, the respiratory exchage ratio is roughly 0.8.