Alveoli of the Lungs

The oxygen exchange in the lungs takes place across the membranes of small balloon-like structures called alveoli attached to the branches of the bronchial passages. These alveoli inflate and deflate with inhalation and exhalation. The behavior of the alveoli is largely dictated by LaPlace's law and surface tension. It takes some effort to breathe in because these tiny balloons must be inflated, but the elastic recoil of the tiny balloons assists us in the process of exhalation. If the elastic recoil of the alveoli is compromised, as in the case of emphysema, then it is difficult to exhale forcibly.

Inflating the Alveoli

The amount of net pressure required for inflation is dictated by the surface tension and radii of the tiny balloon-like alveoli. During inhalation the radii of the alveoli increase from about 0.05 mm to 0.1 mm . The normal mucous tissue fluid surrounding the alveoli has a nominal surface tension of about 50 dynes/cm so the required net outward pressure is:

The remarkable property of the surfactant which coats the alveoli is that it reduces the surface tension by a factor of about 15 so that the 1 mmHg pressure differential is sufficient to inflate the alveoli. Other factors affecting the remarkable efficiency of oxygen transport across the lung membranes is characterized in Fick's Law.

Surfactant Role in Respiration

One of the remarkable phenomena in the process of respiration is the role of the fluid coating the walls of the alveoli of the lungs. This fluid, called a surfactant, lowers the surface tension of the balloon-like alveoli by about a factor of 15 compared to the normal mucous tissue fluid in which they are immersed. There appears to be a nearly constant amount of this surfactant per alveolus, so that when the alveoli are deflated it is more concentrated on the surface. Since the surface-tension-lowering effect of the surfactant depends on this concentration, it diminishes the required pressure for inflation of the alveoli at their most critical phase. For a given surface tension, the pressure to inflate a smaller bubble is greater. It is the surfactant which makes possible the inflation of the alveoli with only about 1 mmHg of pressure excess over their surroundings. The baby's first breath depends upon this surfactant and is made more difficult in premature infants by the incomplete formation of the surfactant.

Alveoli and Exhalation

The alveoli of the lungs act much like balloons in that there is some effort involved to inflate them, but when the inflating pressure is released, the recoil of the elastic walls provides the pressure necessary to deflate them. The lungs are suspended in the thoracic cavity which is normally at a slight negative pressure. When the diaphragm is lowered, that pressure becomes more negative and the lungs expand into the cavity. Air from the atmosphere moves into the resulting partial vacuum and inflates the alveoli. One is aware of the effort, but it is not extreme as in the case of the baby's first breath . Once the alveoli are fully inflated, exhalation can be accomplished by merely relaxing the diaphragm, since the wall tension in all the tiny alveoli will act to force the air out of them. By forcing the diaphragm upward, we can exhale forcefully by adding the diaphragm effort to the recoil of the elastic alveoli. In diseases like emphysema, the elasticity of the alveoli is lost and exhalation becomes a laborious process.