Plasma-the longest portion of the diffusion path. This design permits gas equilibration during the less than 1 sec a red cell spends in a pulmonary capillary. Barometric pressure is the pressure exerted by the gas in the atmosphere above the point of measurement. The atmospheric pressure always exists in any open system. During respiration air movement is accomplished by creating transient pressure gradients between the air in the lungs and that in the atmosphere.
P B can vary with atmospheric conditions and most notably with altitude.
An additional component of a gas mixture in the body is water vapor pressure. Vapor pressure is defined as the pressure exerted by the vapor phase of a liquid or solid in equilibrium with its own gas phase at a given temperature. It is a function of temperature and is independent of P B . Although room air may contain only small amounts of water vapor, it becomes saturated at 37 o C when drawn through the nose, mouth, and pharynx. 149.2 torr Partial Pressures of Gases in Liquid.