in the lungs, oxygen passes through the walls of the and into the blood.

I would go with Alveoli

Alveoli- any of the many tiny air sacs of the lungs which allow rapid gaseous effects.

hope i helped :)

We don’t know what you want to ask us

The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)