7-33. "DEAD AIR"
None of the air found in the upper and lower passageways plays a part in actual
respiration. Thus, this air is often referred to as "dead air." During quiet breathing, it
amounts to about two-fifths of the total air volume exchanged.
7-34. PULMONARY ALVEOLI
External respiration is the exchange of gases between the air and the blood.
External respiration takes place in the alveoli (alveolus, singular). 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.
a. 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.
b. In each lung, there are billions of alveoli.
c. Numerous blood capillaries are adjacent to the walls of the alveoli.
d. 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
e. 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.
Section IX. LUNGS AND PLEURAL CAVITIES
In the thoracic cavity is a pair of lungs. Each lung is an individual organ
containing the branching elements of one side of the respiratory tree, the connected
alveoli, and the corresponding pulmonary NAVL. As with the other organs, the tissues
are held together with fibrous connective tissue (FCT).
a. The lungs are located within individual serous cavities, called the pleural
cavities. The lungs with their pleural cavities constitute the major contents of the
thoracic cavity. The pleural cavities help to provide lubrication.