Analysis of volume displacement and length changes of the diaphragm during breathing.

Diaphragmatic volume displacements cause equal displacements of abdominal contents. Since the rib cage forms a variable part of the abdominal container (the variable part being its area of apposition with the diaphragm) the rib cage and ventral abdominal wall share in abdominal displacements. The fraction of total rib cage volume displacement (delta Vrc) contributing to abdominal displacement is predicted from anatomic considerations and measurements. During quiet inspirations it is estimated that more than half of delta Vrc goes into abdominal expansion. This displacement plus the outward displacement of the anterolateral abdominal wall constitute the diaphragmatic displacement. In a typical inspiration in which delta Vrc accounts for 3/4 of the lung volume change, the diaphragm displaces nearly the same volume. Associated changes in diaphragm length are estimated with a model. Diaphragm shortening during an inspiration in which only the rib cage expands is estimated to be only 11% less than during a normal inspiration. We conclude that the direct rib cage contribution to lung volume change is much less, and that diaphragmatic contribution is much more than was previously thought.