We've learned that air comes down your
trachea into your
bronchi, then to
bronchioles and finally to your
alveoli where the oxygen you need goes into your blood.
The place in your alveoli where the
oxygen in the air goes into your blood is called the
blood-air barrier.
The air you breathe in has oxygen (sometimes called O2) that your body takes into your blood and sends all over your body.
Your body uses up the oxygen and sends back carbon dioxide (sometimes called CO2) in your blood to get rid of when you breathe out.
The blood-air barrier is where the oxygen goes in the blood, and the carbon dioxide comes out of the blood.
You can think of it like your body dumping out the old garbage CO2, and pickup up the new fresh O2 you need.

(from: wikipedia -
blood-air barrier)
Kid Facts - Blast from the past: Parietal Bones