As the blue blood flows through the main pulmonary artery to the lungs the pulmonary artery branches to smaller and smaller pulmonary arteries and then those branch into very tiny blood vessels called capillaries. The capillaries come around and encircle the walls of the air sacs (aveoli).

The aveoli contain high oxygen content and low carbon dioxide content. The blue blood contains low oxygen content and high carbon dioxide content. At the surface of contact with the aveoli, the blue blood releases its carbon dioxide to the air sac and gets oxygen from it and becomes red.

The process of air oxygenation starts with inspiring (inhaling) fresh air which has a high concentration of oxygen and a low concentration of carbon dioxide. The blood coming back from the body (blue blood) comes in contact with the wall of the air sac where it gives its carbon dioxide and gets oxygen becoming red. This fresh blood (red blood) returns back to the Left Atrium to be circulated to the body.