If atmospheric pressure is lower, the partial pressure of a gas will be proportionately decreased. In Salt Lake City, the atmospheric pressure is 647 mm Hg. However, in the lung carbon dioxide exits the blood to raise the carbon dioxide content of the air. The partial pressure of carbon dioxide in the lung air sacs is around 40 mm Hg. Because this carbon dioxide gas must displace oxygen and nitrogen, the partial pressure of oxygen in the lung air sac will be lower than in outside air.
Water vapor enters the air when it s exposed to water.
The maximum amount of water vapor in the air varies with temperature. As further water vapor enters the air, other water must condense out the water content of the air is limited to 47 mm Hg. Therefore in the lung, where air is totally water-saturated, the partial pressure of water vapor would be 47 mm Hg. Back to Index. During respiration, air becomes saturated with water vapor by the time it enters the alveolar sac.