Inspiration and Expiration
Between the bony portions of the rib cage are two layers of intercostal muscles: the external intercostal muscles and the internal intercostal muscles (fig. 16.14). Between the costal cartilages, however, there is only one muscle layer, and its fibers are oriented in a manner similar to those of the internal inter-costals. These muscles are therefore called the interchondral part of the internal intercostals. Another name for them is the parasternal intercostals.
An unforced, or quiet, inspiration results primarily from contraction of the dome-shaped diaphragm, which lowers and flattens when it contracts. This increases thoracic volume in a vertical direction. Inspiration is aided by contraction of the parasternal and external intercostals, which raise the ribs when they contract and increase thoracic volume laterally. Other thoracic muscles become involved in forced (deep) inspiration. The most important of these are the scalenes, followed by the pec-toralis minor, and in extreme cases the sternocleidomastoid muscles. Contraction of these muscles elevates the ribs in an an-teroposterior direction; at the same time, the upper rib cage is stabilized so that the intercostals become more effective. The increase in thoracic volume produced by these muscle contractions decreases intrapulmonary (intra-alveolar) pressure, thereby causing air to flow into the lungs.
Quiet expiration is a passive process. After becoming stretched by contractions of the diaphragm and thoracic muscles, the thorax and lungs recoil as a result of their elastic tension when the respiratory muscles relax. The decrease in lung volume raises the pressure within the alveoli above the atmospheric pressure and pushes the air out. During forced expiration, the internal intercostal muscles (excluding the interchondral part) contract and depress the rib cage. The abdominal muscles also aid expiration because, when they contract, they force abdominal organs up against the diaphragm and further decrease the volume of the thorax. By this means, the intrapulmonary pressure can rise 20 or 30 mmHg above the atmospheric pressure. The events that occur during inspiration
Sternocleidomastoid Scalenes
Diaphragm
Muscles of inspiration
Sternocleidomastoid Scalenes
External intercostals

Internal intercostals
Internal abdominal oblique Transversus abdominis Rectus abdominis
Diaphragm
Internal intercostals
External abdominal oblique
Internal abdominal oblique Transversus abdominis Rectus abdominis
■ Figure 16.14 The muscles involved in breathing. The principal muscles of inspiration are shown on the left, and those of expiration are shown on the right.
Table Í6.2 |
Mechanisms Involved in Normal, Quiet Ventilation and Forced Ventilation | |
Inspiration |
Expiration | |
Normal, quiet breathing Contraction of the diaphragm and external intercostal |
Relaxation of the diaphragm and external intercostals, plus | |
muscles increases the thoracic and lung volume, |
elastic recoil of lungs, decreases lung volume and | |
decreasing intrapulmonary pressure to about -3 mmHg. |
increases intrapulmonary pressure to about +3 mmHg. | |
Forced ventilation |
Inspiration, aided by contraction of accessory muscles |
Expiration, aided by contraction of abdominal muscles and |
such as the scalenes and sternocleidomastoid, |
internal intercostal muscles, increases intrapulmonary | |
decreases intrapulmonary pressure to -20 mmHg or |
pressure to +30 mmHg or higher. | |
lower. |
and expiration are summarized in table 16.2 and shown in figure 16.15.

Fire Up Your Core
If you weaken the center of any freestanding structure it becomes unstable. Eventually, everyday wear-and-tear takes its toll, causing the structure to buckle under pressure. This is exactly what happens when the core muscles are weak – it compromises your body’s ability to support the frame properly. In recent years, there has been a lot of buzz about the importance of a strong core – and there is a valid reason for this. The core is where all of the powerful movements in the body originate – so it can essentially be thought of as your “center of power.”
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