The main cells present within the alveoli are:
The main cells present within the alveoli are:
The diagram above shows the organization of these cells within the alveoli. For clarity in the diagram, the thicknesses of the type I cells and the capillary endothelial cells are exaggerated by about 3X. The actual distance gas must diffuse between the alveolar airspace and blood for the most part is less than about 0.5 µm.
In the figure above, the narrow, yellow layer shown covering the type I cells is a thin layer of fluid. This layer of fluid is absolutely necessary, because the type I cells are living cells. But the surface tension due to the fluid by itself would be a problem, since it is a force tending to collapse the alveoli. Indeed, if the fluid were ordinary interstitial fluid, the surface tension would be far too strong for breathing.
To reduce this surface tension, surfactant is secreted into the fluid by the type II cells, as noted above. The surfactant is 80-90% phospholipid, plus four special proteins. As might be expected, these are amphipathic, just like phospholipid.
The figure to the right shows an experiment in which surface tension was measured on water surfaces of various areas. Notice that surface tension in a surface of pure water is the same at all areas. Next observe that ordinary detergent reduces surface tension (since it is an amphipathic molecule) and that again the surface tension is the same at any area.
However, look at the results when surfactant extracted from lungs is added to water. As expected, it greatly reduces the surface tension. But now there is a strong effect of area. Small areas have the surface tension reduced much more than large areas.
In other words, small alveoli have less surface tension than large alveoli.
Think for a moment and then answer why having less surface tension in small alveoli would be helpful and would reduce the work of breathing.
Less surface tension in small alveoli is also helpful for a second important reason.
Think back to the Law of Laplace (discussed under "Heart Failure"). If alveoli did not have surfactant and thus alveoli of two different sizes had the same surface tension, how would you expect the pressure in the smaller alveolus to compare to the larger?
Thus if alveoli did not have surfactant and surface tension thus were the same in alveoli of all sizes, you would expect air to flow from smaller alveoli into larger alveoli. In other words, the smaller alveoli would tend to collapse. This happens to a small extent in normal people. But since surfactant reduces surface tension much more in small alveoli, this effect is minimized in normal people. But it is a much bigger issue in infant respiratory distress syndrome.