Partial pressure gradients affect oxygenation because gases diffuse from an area of high partial pressure to one of lower partial pressure.
Oxygen moves into the tissues by diffusion across the alveoli and the pulmonary capillaries.
So, the #P_"O₂"# in the alveoli must be kept at a higher level than in the blood. We do this by breathing — continuously bringing fresh air (21 % #"O"_2#) into the lungs and alveoli.
At sea level, atmospheric pressure is 760 mmHg.
So #P_"O₂"# = 0.21 × 760 mmHg = 160 mmHg.
By the time the inspired air reaches the alveoli, it contains water vapour and #"CO"_2#.
The #P_"O₂"# drops to about 104 mmHg.
#"O"_2# easily diffuses across the thin alveolar membrane into the capillary blood.
By the time it reaches the tissues, #P_"O₂"# has dropped to about 95 mmHg.
#P_"O₂"# in cells about 40 mmHg, so oxygen diffuses from the blood to the cells when it reaches the capillaries.
#P_"O₂"# in the blood drops to 40 mmHg, and the deoxygenated blood returns to the heart, where it is pumped to the lungs for re-oxygenation.
