The air sacs are covered by an etch of very fine blood vessels, known as the capillaries [capillaries: approx. 1000 blood vessels = 1 capillary network per air sac]. This is where the gas exchange takes place: exchange of oxygen [O₂] from the air sacs and carbon dioxide [CO₂] from the blood vessels.

In this process O₂ streams [diffusion] from the inhaled air, passes through the wall of the air sacs and through those of the finest blood vessels. In medicine, those layers are grouped as the alveolo capillary membrane.

In the blood the O₂ attaches to red blood cells [erythrocytes]. The latter "drives" the O₂ through the blood circulation like a cab to every possible destination. To muscles or other organs (heart, brain etc.) the blood delivers O₂, nutrients and other important substances and takes away the waste products. The most important waste product is CO₂. It is produced in the cells (e.g. muscle cells) whenever the body performs work and also during the organs' usual business. In exchange for oxygen CO₂ is attached to the red blood cells and pumped back the same way it came. Having arrived in the capillaries, at the air sacs, it streams through the walls of the blood vessels (creating room for new O₂) as well as through the walls of the air sacs and can then be breathed off from there.

Supplementary information