If the partial pressure of oxygen in the atmosphere is 159 mmHg and that inside human alveoli is 104 mmHg, and if these values are fixed or don't change very much, then why does exhaled air have oxygen in it? Or when we compress our lungs this value changes?
Oxygen should not move from a higher partial pressure to a lower partial pressure no matter what other gases are exhaled because when we inhale, the pressure inside the lungs becomes 1-3mm Hg lower than the atmosphere; then air starts to enter and at a point when it stops entering this is the moment when the total pressure outside and inside becomes equal, and oxygen and other gases diffuse in the blood and carbon dioxide in the alveoli; which is why the partial pressure in the alveoli is 104 mmHg. But when we exhale the lungs start to compress and thus pressure inside the lungs increases and air moves outwards. But how is it possible that only the partial pressure of oxygen doesn't change and the pressure of all other gases inside the lungs increases?
It should also increase because if our expired air has oxygen in it then it should also increase?