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
