The result is that the atelectatic lobe is displaced as the other lobes expand. Thus the lower lobes are displaced posteriorly and inferiorly by the upper and middle lobes. Similar arguments may be made for the other lobes. Since all lobes are attached to the remainder of the body, at the hilum, all lobes will collapse toward the hilum. Diagrams of the characteristic patterns of collapse are illustrated on the next two pages.
Passive atelectasis is the result of a different process.
The natural state of the isolated lung is collapse, i.e., atelectasis. This is because of elastic fibers in the pulmonary parenchyma which tend to collapse the lung. A particular type of inspiration, the sigh, is most important in preventing the lung from becoming atelectatic. A sigh transiently inflates the lung to total lung capacity, preventing passive atelectasis. Gravity adds an additional force to the normal elastic forces of the lung which tends to collapse the lung.