General

The windpipe and the alveoli

Inside the lungs, the windpipe (or trachea) branches out into clusters of small air sacs called alveoli. Their total surface equals a floor space of 90 square metres.

The alveoli are surrounded by tiny blood vessels called capillaries. The blood that enters the lungs via the pulmonary artery is almost depleted of oxygen. The oxygen has been used by the organs and cells of the body.