The red blood cells inside the capillaries release their oxygen to pass through the capillary walls, which goes to the alveoli.

What happens in the lungs/Gas exchange

#5

We inhale oxygen, which goes into the alveoli.

Then the oxygen passes through the surrounding capillaries into the blood.

While that is happening, the carbon dioxide passes from the blood, through the capillaries, into the alveoli.

Finally we exhale the carbon dioxide into the air.