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The respiratory muscles are the skeletal muscles that enable inhalation and exhalation by manipulating the volume of the thoracic cavity. These muscles work in a coordinated manner to pump air in and out of the lungs. The diaphragm, rib cage muscles, and abdominal muscles are the three primary groups of respiratory muscles. The diaphragm is the most crucial muscle for respiration, and it works in conjunction with other respiratory muscles such as the intercostals and sternomastoid muscles.
| 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, abdominal muscles, intercostal muscles, sternocleidomastoid, scalenes, serratus anterior, pectoralis major and pectoralis minor, trapezius, latissimus dorsi, erector spinae, iliocostalis, quadratus lumborum, serratus posterior superior, serratus posterior inferior, levatores costarum, transversus thoracis |
| Functions | Pump air in and out of the lungs, help expel vomit, faeces, and urine from the body, prevent acid reflux |
| Features | 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 |
| Mechanics | During inhalation, the diaphragm contracts, moving its center downward and its edges upward, compressing the abdominal cavity and expanding the thoracic cavity to draw air into the lungs |
| Mechanics | During exhalation, the diaphragm relaxes, and the elastic recoil of the lungs causes the thoracic cavity to contract, forcing air out of the lungs |
| Mechanics | During exercise, expiratory muscles play an active role in breathing, working in coordination with inspiratory rib cage muscles to prevent rib cage distortion and optimise breathing mechanics |
| Mechanics | At moderate exercise levels, metabolic requirements increase alongside alveolar ventilation, maintaining arterial blood-gas tensions and acid-base balance close to rest levels |
| Mechanics | At maximal exercise, the oxygen consumed by the respiratory muscles is only ~10% of the total for healthy individuals, but this can increase for trained athletes |
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The diaphragm
Several nerves, soft tissues, and blood vessels pass through the diaphragm, including the aorta, esophagus, inferior vena cava, phrenic nerve, and thoracic duct. Phrenic nerve damage from trauma or disease is the most common cause of diaphragm problems, which can lead to symptoms such as acid reflux, heartburn, cough, difficulty swallowing, chest pain, and trouble breathing. Diaphragmatic breathing exercises can help strengthen the diaphragm and improve its function.
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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 are accessory respiratory muscles that participate in the process of forced breathing. They help form and move the chest wall by helping to expand and shrink the size of the chest cavity.
The intercostal muscles are divided into three groups, from superficial to deep: external, internal, and innermost intercostals. The external intercostals are the outermost layer and lie directly under the skin. They originate from the lower border of the rib above and run obliquely to insert into the upper border of the rib below. They are responsible for the elevation of the ribs and bending them more open, thus expanding the transverse dimensions of the thoracic cavity during inhalation. The internal intercostals are the intermediate layer, originating from the costal groove near the inferior border of the rib above and inserting into the upper border of the rib below. They are responsible for the depression of the ribs and bending them inward, thus decreasing the transverse dimensions of the thoracic cavity during forced expiration. The innermost intercostals are the deepest layer, crossing more than one intercostal space and assisting the internal and external intercostals in their function. They originate from the costal groove of one rib and insert into the superior border of the immediate rib below.
The external and internal intercostals are innervated by the intercostal nerves (the ventral rami of thoracic spinal nerves T1-T11), are supplied by the intercostal arteries, and are drained by the intercostal veins. Their fibres run in opposite directions. The innermost intercostals follow the same pattern of vascularization and innervation as the internal intercostals.
Strained intercostal muscles can cause pain in the upper back or rib cage, muscle tension and stiffness, and difficulty breathing due to a short, shallow breathing pattern to avoid pain. This may lead to reduced blood oxygenation.
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Rib cage muscles
The rib cage muscles, also known as the thoracic wall, are a group of muscles that act on the upper part of the rib cage and aid in respiration. They include the intercostal muscles, the subcostalis, and the transversus thoracis.
The intercostal muscles are the primary muscles of the thoracic wall and are responsible for changing the volume of the thoracic cavity during respiration. There are three layers of intercostal muscles: external, internal, and innermost. These muscles fill the space between the ribs and help to stabilise their position during normal respiration. During inspiration, the external intercostals contract and raise the lateral part of the ribs, increasing the transverse diameter of the thorax. In contrast, the internal intercostals help collapse the lungs during expiration. The innermost intercostals assist the internal and external intercostals in their function.
The subcostalis and transversus thoracis muscles are also important for respiration. They originate from the lower posterior sternum and insert onto the ribs, depressing the ribs during forced expiration. The subcostalis is present in abundance in the lower regions of the posterior thoracic wall, while the transversus thoracis appears in the same space as the innermost intercostal muscle.
In addition to these primary respiratory muscles, other muscles attach to the thoracic wall and may assist in respiratory efforts. These include the pectoralis major and minor, subclavius, and serratus anterior. The pectoralis minor, for example, may assist as an accessory muscle during inspiration by lifting the third, fourth, and fifth ribs.
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Abdominal muscles
The abdominal muscles are one of the three groups of respiratory muscles, along with the diaphragm and the rib cage muscles. The diaphragm is the main muscle used for breathing and is responsible for most of the inhalation and exhalation processes. However, the abdominal muscles also play a crucial role in respiration, particularly during physical activity and exercise.
The abdominal muscles are composed of two internal muscles: the internal oblique and the transverse abdominis. There are also two external muscles: the rectus abdominis and the external oblique. During inhalation, the diaphragm contracts, moving downwards and compressing the abdominal cavity, while the abdominal muscles relax. This contraction of the diaphragm increases the volume of the abdomen and the lower part of the rib cage, allowing air to flow into the lungs.
During exhalation, the diaphragm relaxes, and the elastic recoil of the lungs causes the thoracic cavity to contract, forcing air out of the lungs. While the diaphragm is the primary muscle responsible for exhalation, the abdominal muscles also play a role, especially during forceful exhalation. The abdominal muscles contract, pressing the abdominal organs upwards into the diaphragm and further reducing the volume of the thoracic cavity. This mechanism helps to optimise the mechanics of breathing by preventing rib cage distortion and allowing the diaphragm to act as a flow generator.
The abdominal muscles are also important in maintaining the stability and integrity of the core, which is essential for balance and posture. Additionally, they play a role in protecting the internal organs and maintaining intra-abdominal pressure. During exercise, the abdominal muscles, along with the diaphragm, may function as an "auxiliary heart," contributing to increased cardiac output by shifting blood between the trunk and the extremities.
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Accessory muscles
The diaphragm is the only muscle used in normal breathing. It contracts during inspiration, while normal expiration is a passive process that relies on the elastic recoil of the lungs. Accessory muscle use refers to the contraction of muscles other than the diaphragm during inspiration or expiration.
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 accessory expiratory muscles.
Accessory muscle use is a common finding in patients with chronic obstructive lung disease or respiratory muscle fatigue. In patients with obstructed airways, the abdominal muscles help expel air across the obstruction. Contraction of the sternocleidomastoid and scalene muscles lifts the clavicles and first ribs, helping to expand the thorax of distressed patients, especially those with chronic obstructive lung disease.
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Frequently asked questions
Respiratory muscles are skeletal muscles that pump air in and out of the lungs. They are also known as inspiratory and expiratory muscles, depending on whether they increase or decrease the capacity of the thoracic cage.
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 intercostal muscles are another important group of respiratory muscles, attached between the ribs and manipulating the width of the rib cage.
The rib cage and abdominal muscles are primarily "pressure generators", developing the pressure required to move the rib cage and abdomen respectively. Other muscles that have been observed contributing to respiration include the serratus anterior, pectoralis major and pectoralis minor, trapezius, latissimus dorsi, and erector spinae.
