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ILLUMINATIONS
Program in Physical Therapy, California State Univeristy, Sacramento, Sacramento, CA 95819–6020 E-mail: bstockert{at}csus.edu
[Reprinted with permission from HAPS-EDucator, a publication of the Human Anatomy & Physiology Society (http://www.hapsweb.org/)]
The biomechanical properties of the lungs and chest wall play an important role in ventilation. The biomechanical properties of compliance, elasticity, and recoil force are fundamental to an understanding of ventilation, but they are difficult concepts for many students to understand. Changes in these properties occur with some lung disorders, resulting in a change in the pattern of ventilation. The following information is designed to provide instructors with an inexpensive and effective aid for teaching these biomechanical properties. The materials needed for this demonstration are salad tongs and several rubber bands of various thicknesses (see Fig. 1).
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In contrast, the normal chest wall exhibits outwardly directed elastic recoil, i.e., the chest wall is in a stretched position and trying to increase in size. An outwardly directed recoil force is present in salad tongs when they are used to pick up salad. If the outwardly directed recoil force of the chest wall (or salad tongs) is unopposed, it will increase in size. The unopposed chest wall will generally increase in volume (recoil) to 
70% of the vital capacity. This can occur with a pneumothorax.
Normally, the inwardly directed recoil force of the lungs opposes the outwardly directed recoil force of the chest wall. When the muscles of respiration are relaxed, the inwardly and outwardly directed recoil forces reach a balance. This "balance point" occurs at the end of a quiet exhalation. The volume of air left in the lungs at the balance point is equal to the functional residual capacity, i.e., the residual volume plus the expiratory reserve volume. The balance point concept can be demonstrated by placing a single rubber band around the two arms of the salad tongs. If you have chosen the right-size rubber band and salad tongs, the apparatus will find a balance between inwardly and outwardly directed recoil forces (see Fig. 2). You may need to try several different rubber band sizes to make this work optimally with the salad tong.
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The salad tong and rubber band contraption can be used to explain several changes that occur with obstructive and restrictive pulmonary disorders. Some resistive pulmonary disorders produce fibrosis in the lungs, resulting in an increase in elasticity (and a decrease in compliance). Adding a second rubber band to the salad tong can be used to show the effect of pulmonary fibrosis. The students can visually see that there are more rubber bands present (an increase in elasticity and a decrease in compliance). This situation makes inhalation more difficult by requiring more muscular effort to move the chest wall (spread the arms of the salad tongs) and inflate the lungs (stretch the rubber bands). The increase in inwardly directed recoil force also helps to explain why people with this condition can exhale so quickly.
Obstructive pulmonary disorders, e.g., emphysema, produce anatomical changes in the lungs that result in a decrease in the elasticity (an increase in compliance) of the lungs. This can be demonstrated by using a thinner rubber band than was used for the normal state (see Fig. 3). The students can see that the thinner rubber band has fewer elastic fibers and is more compliant. When a thinner rubber band is used with the salad tongs, several changes in ventilatory mechanics can be demonstrated. First, inhalation is easier because the lungs are more compliant; i.e., less effort is required to spread the arms of the salad tongs. Second, the inwardly directed recoil force of the lungs, used to drive exhalation, is reduced, resulting in a prolonged exhalation. Third, a "new" balance point is found between the rubber band (lungs) and the salad tongs (chest wall). The spread of the salad tong arms is increased at the new balance point (compare Figs. 2 and 3). This can be used to demonstrate why people with emphysema may appear "barrel chested" or "overinflated". The decrease in elasticity of the lungs results in a new balance point between the normal, outwardly directed recoil force of the chest wall and the diminished, inwardly directed recoil force of the lungs. The change in recoil forces results in an increase in the functional residual capacity.
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Footnotes |
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As educators we are continually designing new methods and procedures to enhance learning. During this process good ideas are frequently generated and tested, but the extent of such activities may not be adequate for a full manuscript. Nonetheless, the ideas may be quite beneficial in improving the teaching and learning of physiology. Illuminations is a column designed to facilitate the sharing of these ideas (illuminations). The format of submissions is quite simple: a succinct description of about one or two double-spaced pages (less title and authorship) of something you have used for the classroom, teaching, lab, conference room, etc. You may include one or two simple figures or references. Submit ideas for inclusion in Illuminations directly to the Associate Editor in charge, Stephen DiCarlo (sdicarlo{at}med.wayne.edu). 
This article has been cited by other articles:
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  P. McCulloch A SIMPLE MODEL ILLUSTRATING THE BALANCING FORCES OF LUNG AND CHEST WALL RECOIL Advan Physiol Educ, September 1, 2004; 28(3): 125 - 127. [Full Text] [PDF]
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