Paul McCulloch

A difficult concept for students to understand is how the recoil forces of the lung and recoil forces of the chest wall balance each other to determine the relaxation pressure-volume relationship of the lung and chest wall system. At most lung volumes, the recoil of the lung is inwardly directed, whereas the recoil of the chest wall is outwardly directed. When the two recoil forces are of equal magnitude, but in opposite directions, the lung and chest wall system is in dynamic equilibrium. The lung volume where this occurs is functional residual capacity (FRC). What follows is a description of a simple model that can demonstrate this phenomenon, as well as demonstrate active muscular inspiration and passive expiration. By changing the model slightly, the effect of a change in lung recoil, due to either pulmonary fibrosis or emphysema, can also be demonstrated. This model is based on the pulmonary ventilation teaching aids presented by Stockert (1, 2).

The model is constructed of four aluminum lab-frame rods, three clamp holders, a heavy base, a free-swinging thermometer clamp, and four rubber bands (Fig. 1A). When describing the assembled model (Fig. 1B), indicate that the left vertical rod represents 0% vital capacity (VC), the free-swinging rod in the middle represents the chest wall, and the right vertical rod represents 70% VC. Then, hold up two rubber bands that respectively represent the elastic recoil of the lung tissue and elastic recoil of the chest wall. Attach the rubber bands to the moveable “chest wall” such that the two rubber bands are pulling in opposite directions (Fig. 2A). The place where the chest wall is now positioned is FRC, with the elastic recoils of the lung and chest wall balancing each other.

To demonstrate an inspiration, physically move the bottom of the chest wall rod toward a larger lung volume (i.e., to the right, toward the 70% VC rod; Fig. 2B). The energy that is required to do this is analogous to the energy provided by the muscles of inspiration (i.e., the diaphragm and external intercostal muscles) during an inspiration. This will also stretch the rubber band representing the lung elastic recoil, which will pull back on the chest wall rod. To demonstrate a passive expiration, simply release the chest wall rod, and the chest wall returns to FRC (Fig. 2C). The energy that produces this movement is the stored energy in the stretched rubber band representing lung elastic recoil, and thus no muscular energy is needed during a passive expiration. More advanced demonstrations could indicate the recoil forces during an inspiration to a large lung volume (i.e., >70% VC), where lung and chest wall recoils would now both be directed inward, or during an active expiration to a lung volume less than FRC (i.e., to RV), where the chest wall recoil would increasingly oppose the decreasing lung volume.

By adding or changing rubber bands, pulmonary disease states can also be demonstrated with this model. Adding a second rubber band to increase elastic recoil of the lung would represent pulmonary fibrosis (Fig. 2D). In this case, FRC is reduced, and it is much more difficult to produce an inspiratory movement of the chest wall because of the increased lung elastic recoil. Removing the two lung recoil rubber bands and replacing them with a single, larger rubber band with less elastic recoil would represent emphysema (Fig. 2E). In this case, both FRC and the compliance of the lung are increased, making it easier to produce an inspiratory movement of the chest wall. A pneumothorax can be demonstrated by unhooking the rubber band representing the lung elastic recoil from the chest wall rod and holding it in position on the 0% VC aluminum rod (Fig. 2F). The rubber band will recoil inward, demonstrating a collapsed lung, while the chest wall rod will be moved outward by the chest wall recoil, demonstrating unopposed chest inflation. Note that the rubber band representing the collapsed lung does not completely collapse to a zero volume; the maintained “inflation” of the “lung” during a pneumothorax represents the lung’s minimal volume.

This presentation is highly visual and easily demonstrates a difficult topic for students to understand. It has been used during lectures to students in medical, pharmacy, physician’s assistant, and physical therapy programs. Most students seem to like the demonstration, and I have received positive feedback from students in all programs. Once having viewed the demonstration, students seem to easily grasp this difficult concept.

Fig. 1

Lung and chest wall recoil forces in unassembled (A) and assembled (B) form.

Fig. 2

Schema of the model, illustrating its use. See text for details.