Gas Transport Through The Respiratory System
When oxygen enters the alveoli, it diffuses (moves) into
the capillaries due to a difference in pressure (there is
more pressure in the alveoli than the blood), and
concentration (there is more oxygen in the alveoli than
the blood).
In the red blood cell, oxygen displaces hydrogen in a
hemoglobin molecule to form oxyhemoglobin.
When the blood reaches a body cell low in oxygen, a drop
in pressure and a concentration difference force
hemoglobin to release oxygen, and it diffuses into the
body cell.
At the same time, carbon dioxide diffuses into the blood
(due to concentration difference) and an enzyme in the
red blood cell combines CO2 and water to make carbonic
acid, which helps maintain the blood pH.
Carbonic acid is stored in the plasma of the blood until it
reaches the lungs, when it is turned back into CO2 and it
diffuses into the alveoli to be exhaled.
Note: Gases naturally move from:
 areas of high pressure to low pressure.
 Areas of high concentration (mass of substance per
unit volume) to low concentration – this is called
diffusion.