1) alveoli and the blood,
Atmospheric air, the air we inhale, consists of 79% nitrogen, 21% oxygen, and 0.03% carbon dioxide.
The air we exhale is made up of the same proportion of nitrous oxide, 16% oxygen and 5% carbon dioxide.
It is noted that the concentration of O2 in the air (and alveoli) is higher than that which is in the capillary.
Note that the concentration of CO2 in the air is lower than that in the capillary.
So by the principle of diffusion, the most concentrated goes to the least concentrated. Oxygen is taken from the lungs since its proportion in the air after passing through the lungs has decreased.
The blood in the lungs has been enriched with oxygen and depleted of carbon dioxide.
2) blood and the body tissues
In the tissues, it is always the same principle of difference of pressure, except that here, the concentration gradient of the gases are reversed relative to the blood and the lungs (outside air).
As it goes through our body, the blood releases O2 molecules.
These molecules bind to the cells, which in return, release CO2.
Again it is thanks to the difference in partial pressures between the blood and tissues that can be done this O2 removal and the release of CO2.
To live the cell consumes O2.
Once the O2 is delivered to the cell, hemoglobin continues its carrier role by bringing the combined form of CO2 back to the lungs
