Gas Exchange and Transport

Gas Exchange and Transport
Partial Pressures
In a gas mixture, each gas species exerts a pressure, the partial pressure of that gas. The sum of the partial pressures of the gases in a mixture equals the total gas pressure.
Partial pressure for an individual gas = the fraction of that gas in the gas mixture × total gas pressure Calculation of Partial Pressure of Oxygen (Po2) in Dry Inspired Air O2 comprises 21% of air; total gas pressure = 760 mm Hg (at sea level)
At high altitude, the Po2 is reduced because barometric pressure is lower.
Correction of Po2 for the Presence of Water Vapor
Dry air entering the lungs becomes completely saturated with water as air passes through moist airways. This displaces some of the other gases and slightly reduces their partial pressures.
Partial pressure of water vapor (PH2o) is 47 mm Hg at body temperature.
Total pressure of gases other than water = 760 mm Hg ? 47 mm Hg
= 713 mm Hg
Therefore, the Po2 in warm, humidified inspired air is
Gas Exchange
Diffusion of Gases
O2 and carbon dioxide (CO2 ) diffuse between alveolar gas and pulmonary capillary blood according to standard physical principles
– The total amount moved per unit of time is proportional to the area available for diffusion and to the difference in partial pressure between alveolar gas and pulmonary capillary blood, and inversely proportional to the thickness of the diffusion barrier.
– Gas will diffuse from the alveoli (higher partial pressures) to the pulmonary capillaries (lower partial pressures) until they equilibrate and no partial pressure gradient exists. As a result, blood entering the pulmonary veins from the pulmonary capillaries has virtually the same partial pressures as gases in the alveoli.
– The diffusion barrier, composed of alveolar epithelial cells (type I pneumocytes) and capillary endothelial cells, is very thin, which ensures that the diffusion distance between alveolar gas and pulmonary capillary blood is very short. This allows blood in the pulmonary capillaries to equilibrate with alveolar gas during the short time (< 1 sec) that the blood is in the capillaries.