The muscles of respiration are the

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

s that contribute to

inhalation Inhalation (or inspiration) happens when air or other gases enter the lungs. Inhalation of air Inhalation of air, as part of the cycle of breathing, is a vital process for all human life. The process is autonomic (though there are exceptions ...

and

exhalation Exhalation (or expiration) is the flow of the breathing, breath out of an organism. In animals, it is the movement of air from the lungs out of the airways, to the external environment during breathing. This happens due to elastic properties of ...

, by aiding in the expansion and contraction of the

thoracic cavity The thoracic cavity (or chest cavity) is the chamber of the body of vertebrates that is protected by the thoracic wall (rib cage and associated skin, muscle, and fascia). The central compartment of the thoracic cavity is the mediastinum. Ther ...

. The diaphragm and, to a lesser extent, the

intercostal muscle The intercostal muscles comprise many different groups of muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle ...

s drive respiration during quiet breathing. The elasticity of these muscles is crucial to the health of the respiratory system and to maximize its functional capabilities.

Diaphragm

The diaphragm is the major muscle responsible for

breathing Breathing (spiration or ventilation) is the rhythmical process of moving air into ( inhalation) and out of ( exhalation) the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxy ...

. It is a thin, dome-shaped muscle that separates the

abdominal cavity The abdominal cavity is a large body cavity in humans and many other animals that contain Organ (anatomy), organs. It is a part of the abdominopelvic cavity. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roo ...

from the thoracic cavity. During inhalation, the diaphragm contracts, so that its center 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

lung The lungs are the primary Organ (biology), organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the Vertebral column, backbone on either side of the heart. Their ...

s. When the diaphragm relaxes, elastic recoil of the lungs causes the thoracic 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

vomit Vomiting (also known as emesis, puking and throwing up) is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose. Vomiting can be the result of ailments like food poisoning, gastroenteritis, pregna ...

faeces Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the ...

, and

urine Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...

from the body by increasing intra-abdominal pressure, and preventing acid reflux by exerting pressure on the

esophagus The esophagus (American English), oesophagus (British English), or œsophagus (Œ, archaic spelling) (American and British English spelling differences#ae and oe, see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), c ...

as it passes through the esophageal hiatus.

Intercostal muscles

Along with the diaphragm, the intercostal muscles are one of the most important groups of respiratory muscles. These muscles are attached between the ribs and are important in manipulating the width of the rib cage. There are three layers of intercostal muscles. The external intercostal muscles are most important in respiration. These have fibres that are angled obliquely downward and forward from rib to rib. The contraction of these fibres raises each rib toward the rib above, with the overall effect of raising the

rib cage The rib cage or thoracic cage is an endoskeletal enclosure in the thorax of most vertebrates that comprises the ribs, vertebral column and sternum, which protect the vital organs of the thoracic cavity, such as the heart, lungs and great ve ...

, assisting in inhalation.

Accessory muscles of respiration

Accessory muscles of respiration are muscles that assist, but do not play a primary role, in breathing. Use of these while at rest is often interpreted as a sign of

respiratory distress Shortness of breath (SOB), known as dyspnea (in AmE) or dyspnoea (in BrE), is an uncomfortable feeling of not being able to breathe well enough. The American Thoracic Society defines it as "a subjective experience of breathing discomfort that c ...

. There is no definitive list of accessory muscles, but the

sternocleidomastoid The sternocleidomastoid muscle is one of the largest and most superficial cervical muscles. The primary actions of the muscle are rotation of the head to the opposite side and flexion of the neck. The sternocleidomastoid is innervated by the acces ...

and the

scalenes The scalene muscles are a group of three muscles on each side of the neck, identified as the anterior, the middle, and the posterior. They are innervated by the third to the eighth cervical spinal nerves (C3-C8). The anterior and middle scale ...

(anterior, middle, and posterior) are typically included, as they assist in elevating the rib cage.Netter FH. Atlas of Human Anatomy 3rd ed. Icon Learning Systems. Teterboro, New Jersey 2003 - plate 191 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.Raper, A. J., Thompson, W. T., Shapiro, W., & Patterson, J. L. (1966). Scalene and sternomastoid muscle function. Journal of Applied Physiology, 21, 497-502. With an increase in the respiratory volume, sternocleidomastoids also become active.Campbell, E. J. M. (1955). The role of the scalene and sternomastoid muscles in breathing in normal subjects. An electromyographic study. Journal of Anatomy, 89, 378. Both muscles are simultaneously activated when one breathes in at the maximal flow rate. Apart from the above neck muscles, the following muscles have also been observed contributing to respiration:

serratus anterior The serratus anterior is a muscle of the chest. It originates at the side of the chest from the upper 8 or 9 ribs; it inserts along the entire length of the anterior aspect of the medial border of the scapula. It is innervated by the long thor ...

, pectoralis major and

pectoralis minor Pectoralis minor muscle () is a thin, triangular muscle, situated at the upper part of the chest, beneath the pectoralis major in the human body. It arises from ribs III-V; it inserts onto the coracoid process of the scapula. It is innervated by ...

trapezius The trapezius is a large paired trapezoid-shaped surface muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae of the human spine, spine and laterally to the spine of the scapula. It moves the scapula and ...

latissimus dorsi The latissimus dorsi () is a large, flat muscle on the back that stretches to the sides, behind the arm, and is partly covered by the trapezius on the back near the midline. The word latissimus dorsi (plural: ''latissimi dorsi'') comes from L ...

, erector spinae,

iliocostalis Iliocostalis muscle is the muscle immediately lateral to the longissimus that is the nearest to the furrow that separates the epaxial muscles from the hypaxial. It lies very deep to the fleshy portion of the serratus posterior muscle. It late ...

, quadratus lumborum, serratus posterior superior, serratus posterior inferior, levatores costarum, transversus thoracis, subclavius (Kendall et al., 2005). The

levator labii superioris alaeque nasi muscle The levator labii superioris alaeque nasi muscle (occasionally shortened alaeque nasi muscle) is, translated from Latin, the "lifter of both the upper lip and of the wing of the nose". The muscle is attached to the upper frontal process of the m ...

lifts the sides of the nostrils.

Muscles of exhalation

During quiet breathing, there is little or no muscle contraction involved in exhalation; this process is simply driven by the elastic recoil of the lungs. When forceful exhalation is required, or when the elasticity of the lungs is reduced (as in

emphysema Emphysema is any air-filled enlargement in the body's tissues. Most commonly emphysema refers to the permanent enlargement of air spaces (alveoli) in the lungs, and is also known as pulmonary emphysema. Emphysema is a lower respiratory tract di ...

), active exhalation can be achieved by contraction of the abdominal wall muscles (

rectus abdominis The rectus abdominis muscle, () also known as the "abdominal muscle" or simply better known as the "abs", is a pair of segmented skeletal muscle on the ventral aspect of a person, person's abdomen. The paired muscle is separated at the midline b ...

, transverse abdominis, external oblique muscle and internal oblique muscle). These press the abdominal organs cranially (upward) into the diaphragm, reducing the volume of the thoracic cavity. The internal intercostal muscles have fibres that are angled obliquely downward and backward from rib to rib. These muscles can therefore assist in lowering the rib cage, adding force to exhalation.

Diaphragm

Intercostal muscles

Accessory muscles of respiration

Muscles of exhalation

References

Further reading