The partial pressure of oxygen (O2) in ambient air changes with altitude. Mount Whitney in Sequoia National Park/Inyo National Forest (14,505 feet; barometric pressure, 437 mm Hg) represents the highest point, while Badwater Basin in Death Valley National Park (282 feet; barometric pressure, 768 mm Hg) represents the lowest point in the contiguous United States. At Mount Whitney, the partial pressure of O2 in ambient air is calculated to be 92 mm Hg, whereas at Death Valley's Badwater Basin, it is calculated to be 161 mm Hg.

It is important to note that the fraction of inspired oxygen (FiO2) remains constant at different altitudes; only the barometric pressure changes. However, these variations in oxygen tension significantly affect arterial blood gas values.

During inspiration, ambient air enters the nasopharynx and laryngopharynx, where it becomes warmed to body temperature and humidified. By the time inspired air reaches the glottis, it becomes saturated with water vapor. This water vapor exerts a partial pressure and dilutes the total pressure in which other gases are distributed. At body temperature, the water vapor pressure is 47 mm Hg.

To calculate the partial pressures of O2 and N2 in a humidified mixture, the water vapor partial pressure needs to be subtracted from the total barometric pressure. In the conducting airways, starting from the trachea, the partial pressure of O2 is calculated to be 150 mm Hg, while the partial pressure of N2 is calculated similarly to be 563 mm Hg. It is important to note that the total pressure remains constant at 760 mm Hg (150 + 563 + 47 mm Hg), and the fractions of O2 and N2 remain unchanged. However, the water vapor pressure reduces the partial pressures of O2 and N2. It is also worth mentioning that when calculating the partial pressure of ambient air, water vapor is ignored, and the air is considered "dry." The conducting airways do not participate in gas exchange, so the partial pressures of O2, N2, and water vapor remain unchanged until the air reaches the alveolus.

Once the inspired air reaches the alveolus, O2 is transported across the alveolar membrane into the capillary bed, and CO2 moves from the capillary bed into the alveolus. The process by which this occurs is described elsewhere. At the end of inspiration with the glottis open, the total pressure in the alveolus is atmospheric. Therefore, the partial pressures of the gases in the alveolus must equal the atmospheric pressure. However, the composition of the gas mixture changes. The fraction of O2 in the alveolus decreases, and the fraction of CO2 increases. The fractions of N2, argon, and water vapor do not change since N2 and argon are inert gases, and the inspired gas is already fully saturated with water vapor at body temperature.

As a consequence of gas exchange, the partial pressure of O2 in the alveolus (PAO2) is determined by the alveolar gas equation, also known as the ideal alveolar oxygen equation:

Equation 23.13
CHAPTER 23 Ventilation, Perfusion, and Ventilation/Perfusion Relationships
PO2 159 150 102 90 40 PCO2 0 040 40 46 PH2O, 37°C 0 47 47 47 47 PN2 601 563 571
The partial pressure of O2 in ambient air varies with altitude. The highest and lowest points in the contiguous United States are Mount Whitney in Sequoia National Park/ Inyo National Forest (14,505 feet; barometric pressure, 437 mm Hg) and Badwater Basin in Death Valley National Park (282 feet; barometric pressure, 768 mm Hg). On Mount Whitney, the partial pressure of O2 in ambient air is calculated as follows: . = 92 mm Hg whereas in Death Valley Badwater Basin, the partial pressure of oxygen is calculated as follows: . = 161mm Hg
Note that the FiO2 does not vary at different altitudes; only the barometric pressure varies. These differences in oxygen tension have profound effects on arterial blood gas values.
As inspiration begins, ambient air is brought into the nasopharynx and laryngopharynx, where it becomes warmed to body temperature and humidified. Inspired air becomes saturated with water vapor by the time it reaches the glottis. Water vapor exerts a partial pressure and dilutes the total pressure in which the other gases are distributed. Water vapor pressure at body temperature is 47 mm Hg. To calculate the partial pressures of O2 and N2 in a humidified mixture, the water vapor partial pressure must be subtracted from the total barometric pressure. Thus in the conducting airways, which begin in the trachea, the partial pressure of O2 is calculated as follows:
Equation 23.10 = . = 150 mm Hg and the partial pressure of N2 is calculated similarly:
Equation 23.11 = 563 mm Hg Note that the total pressure remains constant at 760 mm Hg (150 + 563 + 47 mm Hg) and that the fractions of O2 and N2 are unchanged. Water vapor pressure, however, reduces the partial pressures of O2 and N2. Note also that in the calculation of the partial pressure of ambient air (
Eq. 23.9), water vapor is ignored, and ambient air is considered “dry.” The conducting airways do not participate in gas exchange. Therefore, the partial pressures of O2, N2, and water vapor remain unchanged in the airways until the air reaches the alveolus.
When the inspired air reaches the alveolus, O2 is transported across the alveolar membrane into the capillary bed, and CO2 moves from the capillary bed into the alveolus. The process by which this occurs is described in . At the end of inspiration and with the glottis open, the total pressure in the alveolus is atmospheric; thus, the partial pressures of the gases in the alveolus must equal the total pressure, which in this case is atmospheric. The composition of the gas mixture, however, is changed and can be described as follows:
Equation 23.12 1 0. = FO2 + FN2 + FH2O + FCO2 + Fargon and other gases where N2 and argon are inert gases, and therefore the fraction of these gases in the alveolus does not change from ambient fractions. The fraction of water vapor also does not change because the inspired gas is already fully saturated with water vapor and is at body temperature. As a consequence of gas exchange, however, the fraction of O2 in the alveolus decreases, and the fraction of CO2 in the alveolus increases. Because of changes in the fractions of O2 and CO2, the partial pressures exerted by these gases also change. The partial pressure of O2 in the alveolus (PAO2) is given by the alveolar gas equation, which is also called the ideal alveolar oxygen equation:
Equation 23.13
CHAPTER 23 Ventilation, Perfusion, and Ventilation/Perfusion Relationships
PO2 159 150 102 90 40 PCO2 0 040 40 46 PH2O, 37°C 0 47 47 47 47 PN2 601 563 571