CHAPTER 6 Structure and Ventilation Function of the Thorax
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Figure 6.6 (A) At rest, the diaphragm is a dome-shaped muscle. (B) The diaphragm has three sites of attachment within the rib cage: a sternal attachment, a costal attachment, and a vertebral attachment.
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Figure 6.7 (A) Anterior view of the diaphragm descending during inspiration. (B) Lateral view of the diaphragm showing the excursion of the diaphragm during inspiration and expiration.
The fi rst rib moves very little during ventilation, but ribs 2 through 7 are mobile, and their movement during inspiration pushes the sternum in a ventral and superior direction in a “pump-handle” motion. The motion of the upper ribs and sternum increases the anterior-posterior diameter of the thorax. The movement is mainly in the sagittal plane (Fig. 6.9). The lower six ribs elevate during inspiration in a motion that occurs more in the frontal plane around a sagittal axis of motion, producing the lateral increase in diameter of the thorax (the “bucket- handle” motion described above).
As the thoracic volume increases, the intrapulmonary pressure decreases below atmospheric pressure, and the air moves from the higher pressure in the external envi- ronment into the lower pressure in the lungs. As the diaphragm relaxes, it returns to its dome-shaped position. The return to this position decreases the thoracic volume, resulting in an increase in intrapulmonary pressure; the air moves from the higher lung pressure to the lower atmospheric pressure, and exhalation occurs. Essentially, exhalation during regular breathing is a passive phenom- enon in healthy people.
