- Movement of air into and out of the lungs is achieved by cyclical changes to the lung's volume. These volume changes, responsible for inspiration and expiration, are actuated by motion of the diaphragm and chest wall which together serve to vary the anterior-posterior and superior-inferior dimensions of the thorax. As discussed below, a variety of muscle groups are required for this motion and are recruited in distinct ways during inspiration and expiration.
- Inspiration is achieved by expanding the superior-inferior and anterior-posterior dimensions of the thorax. The principal muscle that actuates these motions is the diaphragm whose contraction upon inspiration pushes the abdominal contents downward, thus increasing the superior-inferior dimension, as well as pushes the abdominal contents outward, dragging the ribs outward and thus increasing the anterior-posterior dimension of the thorax. In addition to the diaphragm the external intercostal muscles pull the ribs upward. Because the ribs are attached to the vertebrae similar to a downward sloping bucket-handle, this upward pulling force serves to drag the rib-cage outward, thus increasing the anterior-posterior dimension of the thorax. The action of the diaphragm appears to be sufficient for normal quite inspiration; however, during contexts of exercise or forceful inspiration, the additional force of the external intercostals is recruited.
- Expiration is achieved by contraction of the superior-inferior and anterior-posterior dimensions of the thorax. During expiration, the muscles of inspiration, the diaphragm and external intercostals, relax. Because the lung is naturally highly elastic, as discussed in lung compliance, the natural recoil of the lung actuates the contraction of the thoracic space. This elastic recoil of the lung is largely sufficient to actuate contraction during quiet breathing and thus expiration during quiet breathing require no muscle activity. However, during scenarios of exercise or forceful expiration a variety of other muscle groups are recruited to increase the rate or power of expiration. The most important muscles are those of the abdominal cavity, particularly the rectus abdominus, which increase intra-abdominal pressure and thus force the diaphragm upward yielding a decreased vertical dimension to the thorax. Additionally, the internal intercostal muscles pull the ribs down, which given the bucket-handle connections of the ribs to the vertebrae, results in a decrease in the anterior-posterior dimension of the thorax.