About one-fifth (20.9%) of air contains oxygen. In the inspired air, oxygen has partial pressure at 1/5 of the atmospheric pressure, i.e., 159 mm Hg.
Oxygen partial pressure decreases to 100 mm Hg up to alveoli. Oxygen tension in venous blood is 40 mm Hg.
The larger the O2 or CO2 tension difference between the gas phase in the alveolus and plasma in the capillary, the greater the diffusion.
The mixed venous blood entering the pulmonary capillary has a Oxygen Partial Pressure of 40 mm Hg (5.3 kPa), and alveolar Oxygen Partial Pressure is approximately 100 mm Hg (13.3 kPa), thus creating a driving pressure of 60 mm Hg (8 kPa).
When blood flows through the capillary, it takes up oxygen (and delivers CO2), but because oxygen pressure builds up in capillary blood, the diffusion rate slows down and becomes zero when pressure is equilibrated over the alveolar-capillary wall.
In a normal lung at low cardiac output, equilibrium has been reached within 25% to 30% of the capillary distance, and no or little gas transfer takes place in the remaining capillary.
When cardiac output is increased, as during exercise, blood passes through the capillary faster, and a longer distance of the capillary is therefore required before equilibrium is reached, equilibration time being much the same as during resting conditions (lower venous Oxygen Partial Pressure speeds diffusion up in the beginning). Thickened alveolar-capillary membranes will prolong the equilibration process with the possible effect of causing hypoxemia.