A qualitative analogy for respiratory mechanics.

The geometric configuration and mechanical properties of the integral elements of the respiratory system, as well as the modus operandi of the interacting parts in the ventilation process, comprise a hard-to-visualize system, making the mechanics of pulmonary ventilation a really confusing topic for students and a difficult task for the teacher (1, 9, 10, 13). To clarify, consolidate, or even gain a broader view of this issue, didactic models using balloons (3), a bottle of water (11), a glass cylinder (9), and computer software (1) have been described in the literature, providing excellent pedagogical tools. However, these tools are designed to be applied after a theoretical approach to the issue has already been given in the classroom. Here, to improve and facilitate the learning-teaching process during theoretical classes, an analogy for respiratory mechanics is presented. The features of two very familiar elements, syringes and springs, and their predictable behaviors are systematically mapped onto the interacting parts of the respiratory system and the ventilation process. To introduce this analogy, it is important to consider the following points. First, expansion of the lungs is carried out by skeletal muscle contraction, but there is no direct connection between the respiratory muscles and the lungs. Second, the lungs are very elastic structures. Third, even at minimum pulmonary volume the elastic components of the lungs are stretched and therefore tend to recoil. Fourth, the ligaments at costovertebral joints, such as the radiate and costotransverse, are elastic and tend to recoil during respiratory excursions of the chest wall, pulling the ribs to a given position of equilibrium. Fifth, between these two elastic structures (the lungs and the chest wall) there is a hermetic space: the pleural cavity. The mechanical behavior of this airtight space follows Boyle’s law, meaning that its pressure is inversely proportional to its volume. Sixth, the visceral and parietal layers of the pleura, the sealing structures of the pleural cavity, are tightly attached to the lung surface and to the internal surface of the chest wall, respectively. In this way, the movements of either the lungs or chest wall change the volume of the pleural cavity and, hence, intrapleural pressure. Finally, during embryogenesis, the pulmonary epithelium secretes liquid within the lungs, so they are fluid-filled structures (7). This liquid exerts an internal distending pressure on the lungs, which overcomes the elastic lung recoil, maintaining them in a distended state throughout fetal life. At birth, the replacement of this liquid with air by aeration removes the distending pressure and generates surface tension at the air-liquid interface. These events greatly increase lung recoil, causing the lungs to collapse away from the thoracic cage. As a result, intrapleural pressure is 5 cmH2O below atmospheric pressure. Here, we take these seven elements together and gradually organize them into a simple analogy to explain to undergraduate students the movements of the chest wall and lungs that underlie alveolar ventilation. Figure 1 shows the framework of the model, which consists of a syringe and a pair of springs. To set the basic configuration, it is necessary (Fig. 1A) to 1) cut the needle coupler (dotted line) of the syringe, 2) cut the remaining tip from the barrel (dashed line), 3) make a hole in the center of the barrel (arrow) with a diameter smaller than the inner diameter of the needle coupler, and 4) solder the needle coupler over the hole (Fig. 1B). The next step is to introduce another plunger through the opened end of the barrel and then close the needle coupler to make a sealed space between the rubber plunger heads (Fig. 1C). This setup allows students to visualize, in a very intuitive manner, how the movements of two physical structures can be coupled by pressure differences. In a simple example, when the right plunger (RP) is pulled, the left plunger (LP) slides passively inside the barrel, following the movement (Fig. 1D). This can be explained by Boyle’s law: as the RP was pulled, the volume of the airtight space between the rubber heads increases, and, consequently, pressure decreases proportionally, becoming less than atmospheric pressure. The force generated by the pressure difference pushes the LP, so that it follows the RP’s movement. This same principle governs biomechanical interactions between the chest wall and lungs, i.e., even though there is no solid ligament between these two structures, their movements are coupled together by the pleural cavity. To start building an analogy between the two-plunger syringe system and the respiratory system, let us consider the LP and RP as the lungs and chest wall, respectively. Specifically, we can correlate the movements of the plungers with breathing movements of the lungs and chest wall. In this sense, we refer to the LP and RP as the LPL and RPCW, where the subscripts L and CW denote the lungs and chest wall, respectively. Also, the rubber heads represent the pleural layers. Note that the rubber heads are firmly attached to the plungers, just as the pleural layers are attached to the chest wall and lungs. To improve the model, we can introduce an elastic component. As we have pointed out, both the lungs and chest wall present elastic properties, which are modeled by a pair of springs coupled to the thumb rests of the plungers (Fig. 1E). By stretching the springs and fixing their ends (Fig. 1F), we can obtain a configuration from which we make the following considerations. As the plungers are pulled by stretching the springs, the volume of the sealed space between the rubber heads increases, creating a compartment of subatmospheric pressure (just as intrapleural pressure at birth). This negative pressure exerts vacuum traction over both rubber heads (dashed arrows in Fig. 1F) with a tendency to hold them Address for reprint requests and other correspondence: V. Baptista, Dept. de Ciências Fisiológicas, Centro de Ciências Biológicas, Universidade Federal de Santa Catarina, Campus Trindade, Florianópolis, Santa Catarina 88040-970, Brazil (e-mail: [email protected]). Adv Physiol Educ 34: 239–243, 2010; doi:10.1152/advan.00014.2010.