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The breathing muscles, also known as the muscles of respiration, are the muscles that enable inhalation and exhalation by facilitating the expansion and contraction of the thoracic cavity. The diaphragm, a dome-shaped muscle that separates the abdominal cavity from the thoracic cavity, is the primary muscle responsible for breathing. It contracts during inhalation, compressing the abdominal cavity and raising the ribs outward and upward, thereby expanding the thoracic cavity and drawing air into the lungs. During exhalation, the diaphragm relaxes, allowing the elastic recoil of the lungs to cause the thoracic cavity to contract and force air out of the lungs. In addition to the diaphragm, various other muscles, including the intercostal and abdominal muscles, also play a role in the breathing process.
| Characteristics | Values |
|---|---|
| Definition | Muscles that contribute to inhalation and exhalation by aiding in the expansion and contraction of the thoracic cavity |
| Types | Diaphragm, rib cage muscles, and abdominal muscles |
| Diaphragm | A thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity |
| Intercostal muscles | External, internal, and innermost intercostal muscles |
| Accessory muscles | Sternocleidomastoid and scalenes (anterior, middle, and posterior) |
| Other muscles | Serratus anterior, pectoralis major and pectoralis minor, trapezius, latissimus dorsi, erector spinae, iliocostalis, quadratus lumborum |
| Function | Contraction and relaxation, altering the volume of the thoracic cavity and the lungs, producing inspiration and expiration |
| Control | Automatic, controlled subconsciously by the respiratory center at the base of the brain |
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The diaphragm
During inhalation, the diaphragm contracts, so that its centre moves caudally (downward) and its edges move cranially (upward). This compresses the abdominal cavity, raises the ribs upward and outward, and thus expands the thoracic cavity. This expansion draws air into the lungs.
When the diaphragm relaxes, the elastic recoil of the lungs causes the thoracic cavity to contract, forcing air out of the lungs, and returning to its dome shape. During quiet breathing, there is little or no muscle contraction involved in exhalation; this process is driven by the elastic recoil of the lungs. When forceful exhalation is required, or when lung elasticity is reduced, active exhalation can be achieved by contraction of the abdominal wall muscles.
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Intercostal muscles
The intercostal muscles are a group of intrinsic rib cage muscles that occupy the 11 intercostal spaces between the ribs. They help form and move the chest wall, and are mainly involved in the mechanical aspect of breathing by helping to expand and shrink the size of the chest cavity.
There are three types of intercostal muscles: external, internal, and innermost. The external intercostal muscles are the most superficial muscle of the intercostal muscles. They originate on ribs 1–11 and have their insertion on ribs 2–12. The external intercostals are responsible for the elevation of the ribs and bending them more open, thus expanding the transverse dimensions of the thoracic cavity. The internal intercostal muscles form the intermediate layer. They originate from the lateral aspect of the costal groove of the rib above and insert into the superior aspect of the rib below. The internal intercostal muscles are responsible for the depression of the ribs and bending them inward, thus decreasing the transverse dimensions of the thoracic cavity. The innermost intercostal muscles are the deepest layer of the thorax wall. They originate from the medial aspect of the costal groove of the rib above and insert onto the internal aspect of the rib below.
The external intercostals facilitate forced inspiration, while the internal and innermost intercostals aid forced expiration. All three groups of muscles support the rib cage and are accessory respiratory muscles that participate in the process of forced breathing. The function of the external intercostal muscles is to elevate the ribs during forced inspiration, while the function of the internal intercostal muscles is to depress the ribs during forced expiration. The innermost intercostals originate from the costal groove of one rib, posteriorly to the origin of the internal intercostals, and insert into the superior border of the immediate rib below.
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Abdominal muscles
The abdominal muscles are one of the three groups of respiratory muscles, the other two being the diaphragm and the rib cage muscles. The diaphragm is the major muscle responsible for breathing. It is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. The diaphragm contracts during inhalation, compressing the abdominal cavity, raising the ribs outward and upward, and expanding the thoracic cavity. This expansion draws air into the lungs.
The abdominal muscles act on the abdomen and the abdominal rib cage and are expiratory. They are activated by increases in end-expiratory lung volume (EELV). During forceful exhalation, the abdominal muscles contract, pressing the abdominal organs upward into the diaphragm, reducing the volume of the thoracic cavity, and forcing air out of the lungs.
The abdominal muscles also contribute to inspiration by lengthening the diaphragm and reducing or preventing its shortening at increased lung volumes. This maintains the diaphragm closer to its optimal length for tension generation. Abdominal tone also reduces the compliance of the abdominal compartment, enabling the region in contact with the rib cage to act as a fulcrum for the expansion of the lower rib cage during inspiration.
During exercise, the diaphragm is primarily a "flow generator", meaning its mechanical power is expressed as velocity rather than pressure. On the other hand, the abdominal muscles, along with the rib cage muscles, are "pressure generators", developing the pressures required to move the abdomen and rib cage. During inspiration, the rib cage muscles contract while the abdominal muscles relax, and vice versa during expiration. This mechanism prevents rib cage distortion and allows the diaphragm to act as a flow generator.
In addition to their role in breathing, the abdominal muscles contribute to protective reflexes such as coughing, sneezing, and vomiting. They also generate the high intra-abdominal pressures necessary for defecation and parturition, are active during postural adjustments, and play a role in vocalization in many species.
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Accessory muscles
The breathing muscles, also known as the 'respiratory pump muscles', are the muscles that contribute to inhalation and exhalation by aiding in the expansion and contraction of the thoracic cavity. The diaphragm and, to a lesser extent, the intercostal muscles drive respiration during quiet breathing. The diaphragm is the major muscle responsible for breathing. It is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity.
The accessory inspiratory muscles include the sternocleidomastoid, the scalenus anterior, medius, and posterior, the pectoralis major and minor, the inferior fibres of serratus anterior and latissimus dorsi, the serratus posterior superior may help in inspiration, and the iliocostalis cervicis. Any muscle attached to the upper limb and the thoracic cage can act as an accessory muscle of inspiration through reverse muscle action.
The accessory expiratory muscles are the abdominal muscles: rectus abdominis, external oblique, internal oblique, and transversus abdominis. In the thoracolumbar region, the lowest fibres of iliocostalis and longissimus, the serratus posterior inferior, and quadratus lumborum are also accessory expiratory muscles.
The involvement of these muscles seems to depend on the degree of respiratory effort. During quiet breathing, the scalenes are consistently physically active, while the sternocleidomastoids are quiet. With an increase in respiratory volume, the sternocleidomastoids also become active. Both muscles are simultaneously activated when one breathes in at the maximal flow rate.
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Thoracic muscles
The muscles of respiration are the muscles that contribute to inhalation and exhalation by aiding in the expansion and contraction of the thoracic cavity. The diaphragm and, to a lesser extent, the intercostal muscles drive respiration during quiet breathing. The diaphragm is the major muscle responsible for breathing. It is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity.
The thoracic wall is made up of five muscles: the external intercostal muscles, internal intercostal muscles, innermost intercostal muscles, subcostalis, and transversus thoracis. These muscles are primarily responsible for changing the volume of the thoracic cavity during respiration. The intercostal muscles lie in the intercostal spaces between the ribs and are organised into three layers. There are 11 pairs of external intercostal muscles. They run from the rib above to the rib below and are continuous with the external oblique of the abdomen. The internal intercostal muscles have fibres that are angled obliquely downward and backward from rib to rib. The deepest layer of the thorax wall is made up of the innermost intercostal muscles. They originate from the medial aspect of the costal groove of the rib above and insert onto the internal aspect of the rib below.
The subcostal muscles are most developed in the inferior portion of the thoracic wall. The direction of the fibres is similar to that of the innermost intercostal muscles. The transversus thoracis is located on the internal surface of the thoracic wall and is continuous with the transversus abdominis inferiorly. The transversus thoracis originates from the posterior surface of the inferior sternum and costal cartilage of the lower ribs.
There are additional muscles that do not form the thoracic wall but attach to it. These are termed accessory muscles of respiration and include the pectoralis major, minor, serratus anterior and the scalene muscles. Accessory muscles assist in breathing but do not play a primary role. Use of these muscles while at rest is often interpreted as a sign of respiratory distress.
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Frequently asked questions
Breathing muscles, or the muscles of respiration, are the muscles that contribute to inhalation and exhalation by aiding in the expansion and contraction of the thoracic cavity.
The diaphragm is the major muscle responsible for breathing. The primary inspiratory muscles are the diaphragm and external intercostals.
Intercostal muscles are the muscles located in the intercostal spaces between the ribs. There are three types: external intercostals, internal intercostals, and innermost intercostals.
Accessory breathing muscles are muscles that assist, but do not play a primary role, in breathing. The sternocleidomastoid and the scalenes (anterior, middle, and posterior) are typically included as accessory muscles, as they assist in elevating the rib cage.
