Lung function 1 — Normal respiratory movements —

Normal respiratory movements – Respiratory movements introduce oxygen into the alveoli and expel carbon dioxide. The lungs are situated inside the pleural cavity of the thorax and are wrapped in the pleural membrane. The lungs are unable to extend by themselves. Instead, they are inflated by the expansion and contraction movements of the diaphragm and the ribs that give the thorax its shape.

The muscles used for quiet breathing are not the same as those used during heavy breathing.

Respiratory movements during quiet breathing are described first. The muscles that contribute to quiet breathing are the external intercostal muscles and the diaphragm. (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. This causes the thorax to expand and inflate the lungs by creating negative pressure inside the thoracic cavity. During expiration, the contraction of these muscles ceases, causing them to relax.
The lungs can contract in a manner similar to a deflating balloon. When the muscles that expand the thorax are relaxed, the lungs contract by their own elastic recoil forces, so that breath is expired. In other words, no muscles are used for expiration in quiet breathing.

The movement of the ribs during quiet breathing is described below. 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 ribs are attached by joints to the thoracic vertebrae, from which they are tilted forward and downward. 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. As a result, the volume of the thorax increases. Additional muscles called internal intercostal muscles are situated inside the external intercostal muscles, but these make no contribution to quiet breathing. (Quiet breathing is called thoracic respiration. A bucket handle offers a useful illustration of how this mechanism works. When the handle is lifted away from the side of a bucket, it occupies a larger area as seen from above. Similarly, when the ribs expand, the cross-sectional area of the rib cage increases.)

The motion of the diaphragm during quiet breathing is described next. Why does the thorax expand when the diaphragm contracts? The diaphragm is a sheet of muscle that is domed upwards towards the head. 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. As a result, the thorax expands and negative pressure is formed inside the thoracic cavity, causing the lungs to inflate. (At this time, intra-abdominal pressure increases, causing the abdomen to bulge. This is called abdominal breathing.) The diaphragm and intercostal muscles both relax when exhaling. 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. The diaphragm does more work than the external intercostal muscles and is responsible for 70-80% of the effort in quiet breathing.

Next, respiratory movements during heavy or labored breathing are described. During deep breathing, the external intercostal muscles and diaphragm work as hard as possible, while a number of other muscles called accessory respiratory muscles assist. The accessory respiratory muscles that assist the work of the external intercostal muscles include the scalenus, sternocleidomastoid, levatores costarum, pectoralis major and pectoralis minor. These contract during inspiration to help expand the ribs. The elevation of the ribs is also assisted by the spinal erector muscles, which flex the spine in the posterior direction. The accessory respiratory muscles that act during respiration include the internal intercostal and abdominal muscles. The action of the abdominal muscles is much more important than that of the internal intercostal muscles.

Respiration is generally divided into thoracic breathing and abdominal breathing. Thoracic deep breathing is one of the deep breathing methods. Thoracic breathing is a respiration method that mainly involves movement of the ribs, where inspiration involves the external intercostal muscles and the accessory muscles of inspiration, and expiration involves the internal intercostal muscles. During inspiration, the external intercostal muscles and the accessory muscles of inspiration contract strongly to assist the drawing of breath by applying a large upward lift to the ribs. During expiration, the internal intercostal muscles relax and the thorax contracts, causing air to be exhaled.

Abdominal deep breathing is one form of abdominal breathing. The muscles used in abdominal breathing are the diaphragm for inspiration and the abdominal muscles for expiration. During inspiration, the diaphragm contracts strongly, causing a large expansion of the thorax vertically. At the same time, intra-abdominal pressure increases and causes the abdomen to bulge. During expiration, the diaphragm relaxes and the abdominal muscles contract, causing the abdomen to flatten out. This increases intra-abdominal pressure and pushes strongly against the diaphragm, which assists with expiration. (As a result, the pressure inside the thorax increases and breath is expired. One of the differences between deep breathing and quiet breathing is that in deep breathing, the abdominal muscles play a major role in expiration.)

Next, a respiration method that uses both thoracic and abdominal breathing is described. The muscles that contract during inspiration are the external intercostal muscles, the accessory muscles of inspiration, and the diaphragm. The muscles used during expiration are the internal intercostal muscles and abdominal muscles, with the latter doing most of the work.

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