The diaphragm is the primary muscle of respiration. It is a thin, dome-shaped muscle located at the base of the thoracic cavity and is responsible for creating the negative pressure needed to inhale air. During inhalation, the diaphragm contracts and flattens, creating a vacuum that causes air to rush into the lungs. During exhalation, the diaphragm relaxes, allowing air to be expelled from the lungs. The diaphragm is essential for normal breathing and is the most important muscle associated with respiration.

Muscles of respiration

breathing muscles
respiratory system.svg

Breathing muscles of the body
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anatomical terminology

The breathing muscles are the muscles that contribute to inhalation and exhalationassisting in the expansion and contraction of the thoracic cavity. The diaphragm and, to a lesser extent, the intercostal muscles direct the breath during quiet breath. The elasticity of these muscles is crucial for the health of the respiratory system and for maximizing its functional capabilities.


Main article: thoracic diaphragm

The diaphragm is the main muscle responsible breathing. It is a thin dome-shaped muscle that separates the abdominal cavity of the thoracic cavity. During inspiration, the diaphragm contracts so that its center moves caudally (down) and its edges move cranially (up). This compresses the abdominal cavity, lifts the ribs up and out, and thereby expands the chest cavity. This expansion pulls air into the lungs. When the diaphragm relaxes, the elastic recoil of the lungs causes the chest cavity to contract, forcing air out of the lungs and returning to its dome shape. The diaphragm is also involved in non-respiratory functions, helping to expel to vomit, fecesand urine of the body by increasing intra-abdominal pressure and preventing acid reflux putting pressure on the esophagus as it passes through esophageal hiatus.

intercostal muscles

Along with the diaphragm, the intercostal muscles are one of the most important groups of respiratory muscles. These muscles are trapped between the ribs and are important in manipulating the width of the ribcage. There are three layers of intercostal muscles. The external intercostal muscles are most important in breathing. These have fibers that are obliquely angled downwards and forwards from one rib to the other. Contraction of these fibers elevates each rib toward the upper rib, with the overall effect of raising the ribcageaiding in inhalation.

Accessory muscles of breathing

Accessory muscles of respiration are muscles that assist but do not play a primary role in breathing. The use of these while at rest is often interpreted as a sign of respiratory discomfort. There is no definitive list of accessory muscles, but the sternocleidomastoid and the scalenes (anterior, middle, and posterior) are usually included as they help lift the ribcage. 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. As respiratory volume increases, the sternocleidomastoids also become active. Both muscles are activated simultaneously when breathing at maximum flow rate.

In addition to the neck muscles above, the following muscles have also been observed to contribute to breathing: serratus anterior, pectoralis major and pectoralis minor, trapeze, latissimus dorsi, erector spinae, iliocostalis, quadratus lumborum, serratus posterior superior, serratus posterior inferior, costal elevators, transversus thorax, subclavius (Kendall et al., 2005). The levator labii superioris muscle raise the sides of nostrils.

muscles of exhalation

During silent breathing, there is little or no muscle contraction involved in exhaling; this process is simply driven by the elastic recoil of the lungs. When forced expiration is necessary, or when the elasticity of the lungs is reduced (as in emphysema), active expiration can be achieved by contracting the muscles of the abdominal wall (rectus abdominis, transverse abdominis, external oblique muscle and internal oblique muscle). These press the abdominal organs cranially (upwards) on the diaphragm, reducing the volume of the thoracic cavity.

The internal intercostal muscles have fibers that are angled obliquely downwards and backwards from rib to rib. These muscles can therefore help to lower the ribcage, adding force to exhalation.


Further reading

  • Kendall F, McCreary E, Provance P, Rodgers M, Romai W (2005). Muscle testing and function with posture and pain (5th ed.). PA, USA: Lippincott Williams & Wilkins.

Source: Muscles of respiration

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1. Diaphragm
2. Intercostal muscles
3. Abdominal muscles
4. Accessory muscles
5. Sternocleidomastoid
6. Scalenes
7. Pectoralis major
8. Serratus anterior
9. External intercostals
10. Internal intercostals
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