External respiration is the exchange of gases between the air and the blood.
External respiration takes place in the alveoli.
The alveoli are small, spherical sacs that are continuous with the terminal elements of the branches of the respiratory tree. As we indicated earlier, external respiration is a surface phenomenon in which the gases pass through the wall of the alveolus.
Since there is a critical relationship between volume and surface area, the inflated alveolus is spherical. The alveolus is also of a particular size that is ideal for the efficiency of external respiration. In each lung, there are billions of alveoli.
Numerous blood capillaries are adjacent to the walls of the alveoli. To facilitate the exchange of gases between the air in the alveolus and the blood in the capillaries, the wall of the alveolus contains a special chemical known as surfactant.
The inner surfaces of the alveoli must be kept wet to make the transfer of gases possible. Because these surfaces are wet, one of the major fluid losses of the body is with the exhaled air.