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RespirationRespiration is the process of exchanging O2 with CO2. In order to get the oxygen into the lungs, all of the structures that you have just learned act together to increase the area of the thoracic cavity. The ribs and diaphragm move in such a way that three dimensions of the thoracic cavity are increased:
During inspiration, the lateral dimensions of the thoracic cavity are increased by the 7-10th ribs moving laterally (similar to bucket handles). The anteroposterior dimension is increased by the sternum being pushed forward by the true ribs (1-6). The superoinferior dimension is increased by the diaphragm contracting and becoming lower. During restful breathing, the diaphragm probably does most of the work, although small movement in all directions probably occur. During increased need for oxygen (exercise, pathology), the lateral and anterioposterior movements will be increased. When the thoracic muscles can no longer do the job, other muscles attaching to the thorax will be called into action (pectoralis major and minor, sternomastoid, etc.) During expiration, the intercostal muscles and the diaphragm relax and the elastic fibers of the lung and the costal cartilages recoil to their original state before inspiration. The automatic nature of the respiratory cycle is controlled in the respiratory centers of the brain stem. These various actions are demonstrated here: Respiratory movements of thorax If we take a look at the thoracic wall in more detail, we can see just how the lung enlarges to draw in air.
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