Next, respiratory movements during heavy or labored breathing are described. The diaphragm does more work than the external intercostal muscles and is responsible for 70-80% of the effort in quiet breathing. This allows the lungs to contract by means of their own elastic recoil forces so that the thorax returns to its original size and the air inside the lungs is gently exhaled. This is called abdominal breathing.) The diaphragm and intercostal muscles both relax when exhaling. (At this time, intra-abdominal pressure increases, causing the abdomen to bulge. As a result, the thorax expands and negative pressure is formed inside the thoracic cavity, causing the lungs to inflate. The base of this dome is fixed to the chest wall, so when the diaphragm muscle contracts, the top of the dome moves down and the diaphragm flattens out. Why does the thorax expand when the diaphragm contracts? The diaphragm is a sheet of muscle that is domed upwards towards the head. The motion of the diaphragm during quiet breathing is described next. Similarly, when the ribs expand, the cross-sectional area of the rib cage increases.) When the handle is lifted away from the side of a bucket, it occupies a larger area as seen from above. A bucket handle offers a useful illustration of how this mechanism works. (Quiet breathing is called thoracic respiration. Additional muscles called internal intercostal muscles are situated inside the external intercostal muscles, but these make no contribution to quiet breathing. As a result, the volume of the thorax increases. When the external intercostal muscles contract, the ribs are elevated with the thoracic vertebra joints acting as fulcrums, causing a large expansion in the anterior/posterior size of the thorax and a slight increase in its lateral size. The ribs are attached by joints to the thoracic vertebrae, from which they are tilted forward and downward. Why does the thorax expand when the external intercostal muscles contract? The external intercostal muscles fill the gaps between the ribs, but are only partially visible in this video. The movement of the ribs during quiet breathing is described below. In other words, no muscles are used for expiration in quiet breathing. When the muscles that expand the thorax are relaxed, the lungs contract by their own elastic recoil forces, so that breath is expired. The lungs can contract in a manner similar to a deflating balloon. During expiration, the contraction of these muscles ceases, causing them to relax. This causes the thorax to expand and inflate the lungs by creating negative pressure inside the thoracic cavity. (The external and internal intercostals are the muscles that fill the gaps between the ribs.) When drawing breath (i.e., during inspiration), the external intercostal muscles and diaphragm contract simultaneously. The muscles that contribute to quiet breathing are the external intercostal muscles and the diaphragm. Respiratory movements during quiet breathing are described first. The muscles used for quiet breathing are not the same as those used during heavy breathing. Instead, they are inflated by the expansion and contraction movements of the diaphragm and the ribs that give the thorax its shape. The lungs are unable to extend by themselves. The lungs are situated inside the pleural cavity of the thorax and are wrapped in the pleural membrane. Normal respiratory movements – Respiratory movements introduce oxygen into the alveoli and expel carbon dioxide.
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