Gas exchange

How does oxygen get from the air into the blood and around the body?

There are about 350 millions of tiny air sacs in the lungs. 

This is where gas exchange happens.

These air sacs are called alveoli (pl) alveolus (sg)

The alveoli are surrounded by a network of tiny blood vessels called capillaries.

red blood cell plasma blood diffusion air

Oxygen moves

from the air in the alveolus to the blood in the capillary.

Oxygen passes across the wall of the alveolus by diffusion. 

Oxygen is carried in the blood mainly in red blood cells. 

red blood cell plasma blood diffusion air

Carbon dioxide moves

from the blood in the capillary to the  air in the alveolus.

Carbon dioxide is carried in the blood mainly in plasma.

Carbon dioxide passes across the wall of the capillary by diffusion.