Thoracic diaphragm
Overview
 
In the anatomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...

 of mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, the thoracic diaphragm, or simply the diaphragm , is a sheet of internal skeletal muscle that extends across the bottom of the rib cage
Rib cage
The rib cage is an arrangement of bones in the thorax of animals. It is formed by the vertebral column, ribs and sternum and encloses the heart and lungs....

. The diaphragm separates the thoracic cavity
Thoracic cavity
The thoracic cavity is the chamber of the human body that is protected by the thoracic wall ....

 (heart, lungs & ribs) from the abdominal cavity
Abdominal cavity
The abdominal cavity is the body cavity of the human body that holds the bulk of the viscera. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the thoracic diaphragm , and its oblique floor is the pelvic inlet...

 and performs an important function in respiration
Respiration (physiology)
'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...

. A diaphragm
Diaphragm
-Optics and photography:* Diaphragm , a stop in the light path of a lens, having an aperture that regulates the amount of light that passes* Diaphragm shutter, a type of leaf shutter consisting of a number of thin blades in a camera-Acoustics:...

 in anatomy can refer to other flat structures such as the urogenital diaphragm
Urogenital diaphragm
Older texts have asserted the existence of an urogenital diaphragm, also called the triangular ligament, which was described as a layer of the pelvis that separates the deep perineal sac from the upper pelvis, lying between the inferior fascia of the urogenital diaphragm and superior fascia of the...

 or pelvic diaphragm, but "the diaphragm" generally refers to the thoracic diaphragm.
Encyclopedia
In the anatomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...

 of mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, the thoracic diaphragm, or simply the diaphragm , is a sheet of internal skeletal muscle that extends across the bottom of the rib cage
Rib cage
The rib cage is an arrangement of bones in the thorax of animals. It is formed by the vertebral column, ribs and sternum and encloses the heart and lungs....

. The diaphragm separates the thoracic cavity
Thoracic cavity
The thoracic cavity is the chamber of the human body that is protected by the thoracic wall ....

 (heart, lungs & ribs) from the abdominal cavity
Abdominal cavity
The abdominal cavity is the body cavity of the human body that holds the bulk of the viscera. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the thoracic diaphragm , and its oblique floor is the pelvic inlet...

 and performs an important function in respiration
Respiration (physiology)
'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...

. A diaphragm
Diaphragm
-Optics and photography:* Diaphragm , a stop in the light path of a lens, having an aperture that regulates the amount of light that passes* Diaphragm shutter, a type of leaf shutter consisting of a number of thin blades in a camera-Acoustics:...

 in anatomy can refer to other flat structures such as the urogenital diaphragm
Urogenital diaphragm
Older texts have asserted the existence of an urogenital diaphragm, also called the triangular ligament, which was described as a layer of the pelvis that separates the deep perineal sac from the upper pelvis, lying between the inferior fascia of the urogenital diaphragm and superior fascia of the...

 or pelvic diaphragm, but "the diaphragm" generally refers to the thoracic diaphragm. Other vertebrates such as amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s and reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s have diaphragms or diaphragm-like structures, but important details of the anatomy vary, such as the position of lungs in the abdominal cavity.

Function

The diaphragm functions in breathing. During inhalation, the diaphragm contracts, thus enlarging the thoracic cavity (the external intercostal muscles
Intercostal muscle
Intercostal muscles are several groups of muscles that run between the ribs, and help form and move the chest wall. The intercostal muscles are mainly involved in the mechanical aspect of breathing...

 also participate in this enlargement). This reduces intra-thoracic pressure: In other words, enlarging the cavity creates suction that draws air into the lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

s.

Cavity expansion happens in two extremes, along with intermediary forms. When the lower ribs are stabilized and the central tendon of the diaphragm is mobile, a contraction brings the insertion (central tendon) towards the origins and pushes the lower cavity towards the pelvis, allowing the thoracic cavity to expand downward. This is often called belly breathing. When the central tendon is stabilized and the lower ribs are mobile, a contraction lifts the origins (ribs) up towards the insertion (central tendon) which works in conjunction with other muscles to allow the ribs to slide and the thoracic cavity to expand laterally and upwards.

When the diaphragm relaxes, air is exhaled by elastic recoil of the lung and the tissues lining the thoracic cavity. Assisting this function with muscular effort (called forced exhalation
Exhalation
Exhalation is the movement of air out of the bronchial tubes, through the airways, to the external environment during breathing....

) involves the internal intercostal muscles used in conjunction with the abdominal muscles, which act as an antagonist
Antagonist (muscle)
Most muscles work in pairs, and when a muscle works it needs to have an agonist and an antagonist, unless the muscle's natural state is opposite to that which is produced by the muscle, example Sphincter ani externus muscle....

 paired with the diaphragm's contraction.

The diaphragm is also involved in non-respiratory functions, helping to expel vomit, feces
Feces
Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...

, and urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

 from the body by increasing intra-abdominal pressure, and preventing acid reflux by exerting pressure on the esophagus
Esophagus
The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

 as it passes through the esophageal hiatus
Esophageal hiatus
In human anatomy, the esophageal hiatus is a hole in the diaphragm through which the esophagus passes. It is located in the right crus of the diaphragm.It is located approximately at level of the tenth thoracic vertebra ....

.

In some non-human animals, the diaphragm is not crucial for breathing; a cow, for instance, can survive fairly asymptomatically with diaphragmatic paralysis as long as no massive aerobic metabolic demands are made of it.

Anatomy

The diaphragm is a dome-shaped musculofibrous septum that separates the thoracic from the abdominal cavity, its convex upper surface forming the floor of the former, and its concave under surface forming the roof of the latter. Its peripheral part consists of muscular fibers that take origin from the circumference of the inferior thoracic aperture
Inferior thoracic aperture
-Anatomy:The inferior thoracic aperture is much larger than the superior thoracic aperture. In layman's terms, it is the round opening whose edges are the lowest ribs...

 and converge to be inserted into a central tendon.

The muscular fibers may be grouped according to their origins into three parts:
Part Origin
>-
| sternal
xiphoid process
Xiphoid process
The xiphoid process, or xiphisternum or metasternum, is a small cartilaginous process of the lower part of the sternum which is usually ossified in the adult human. By age 15 to 29, the xiphoid usually fuses to the body of the sternum with a fibrous joint. Unlike the synovial articulation of major...

.
>-
| costal
>-
| lumbar
aponeurotic arches, named the lumbocostal arches, and from the lumbar vertebrae
Lumbar vertebrae
The lumbar vertebrae are the largest segments of the movable part of the vertebral column, and are characterized by the absence of the foramen transversarium within the transverse process, and by the absence of facets on the sides of the body...

 by two pillars or crura.


There are two lumbocostal arches, a medial
Medial arcuate ligament
The medial arcuate ligament is a tendinous fascia that arches over the psoas major muscle as it passes through the diaphragm.-Structure:...

 and a lateral
Lateral arcuate ligament
The lateral arcuate ligament is a ligament under the diaphragm that arches across the upper part of the quadratus lumborum...

, on either side.

Innervation

The diaphragm is primarily innervated by the phrenic nerve
Phrenic nerve
The phrenic nerve originates mainly from the 4th cervical nerve, but also receives contributions from the 5th and 3rd cervical nerves in humans....

 which is formed from the cervical nerves
Cervical nerves
The cervical nerves are the spinal nerves from the cervical vertebrae.Although there are seven cervical vertebrae , there are eight cervical nerves . All nerves except C8 emerge above their corresponding vertebrae, while the C8 nerve emerges below the C7 vertebra...

 C3
Cervical spinal nerve 3
The cervical spinal nerve 3 is a spinal nerve of the cervical segment..It originates from the spinal column from above the cervical vertebra 3 ....

, C4
Cervical spinal nerve 4
The cervical spinal nerve 4 is a spinal nerve of the cervical segment..It originates from the spinal column from above the cervical vertebra 4 .Its control of the thoracic diaphragm has inspired a medical mnemonic: "Cut C4, breathe no more."...

, and C5
Cervical spinal nerve 5
The cervical spinal nerve 5 is a spinal nerve of the cervical segment..It originates from the spinal column from above the cervical vertebra 5 ....

. A useful mnemonic to remember this is, "C-3, 4, 5 keep the diaphragm alive." The peripheral portions of the diaphragm send sensory afferents via the intercostal (T5-T11) and subcostal nerves (T12).

Crura and central tendon

At their origins the crura
Crus of diaphragm
The crura of the diaphragm are tendinous structures that extend inferiorly from the diaphragm to attach to the vertebral column...

 are tendinous in structure, and blend with the anterior longitudinal ligament
Anterior longitudinal ligament
The anterior longitudinal ligament is a ligament that runs down the anterior surface of the spine. It traverses all of the vertebral bodies and intervertebral discs....

 of the vertebral column
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

.

The central tendon of the diaphragm is a thin but strong aponeurosis situated near the center of the vault formed by the muscle, but somewhat closer to the front than to the back of the thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...

, so that the posterior muscular fibers are the longer.

Openings

The diaphragm is pierced by a series of apertures to permit the passage of structures between the thorax and abdomen. Three large openings — the aortic
Aorta
The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...

, the esophageal, and the vena cava
Inferior vena cava
The inferior vena cava , also known as the posterior vena cava, is the large vein that carries de-oxygenated blood from the lower half of the body into the right atrium of the heart....

 — and a series of smaller ones are described.
opening level structures
>-
| caval opening
Caval opening
The caval opening is a hiatus in the diaphragm of humans through which passes the inferior vena cava, the wall of which is adherent to the margins of the opening, and some branches of the right phrenic nerve....

 
T8 inferior vena cava
Inferior vena cava
The inferior vena cava , also known as the posterior vena cava, is the large vein that carries de-oxygenated blood from the lower half of the body into the right atrium of the heart....

, and some branches of the right phrenic nerve
Phrenic nerve
The phrenic nerve originates mainly from the 4th cervical nerve, but also receives contributions from the 5th and 3rd cervical nerves in humans....


>-
| esophageal hiatus
Esophageal hiatus
In human anatomy, the esophageal hiatus is a hole in the diaphragm through which the esophagus passes. It is located in the right crus of the diaphragm.It is located approximately at level of the tenth thoracic vertebra ....

 
T10 esophagus
Esophagus
The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

, the anterior and posterior vagal trunks, and some small esophageal arteries
Esophageal arteries
The esophageal arteries four or five in number, arise from the front of the aorta, and pass obliquely downward to the esophagus, forming a chain of anastomoses along that tube, anastomosing with the esophageal branches of the inferior thyroid arteries above, and with ascending branches from the...


>-
| aortic hiatus
Aortic hiatus
The aortic hiatus is a hole in the human diaphragm. It is the lowest and most posterior of the large apertures.It is located approximately at the level of the twelfth thoracic vertebra .-Structure:...

 
T12 aorta
Aorta
The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...

, the azygos vein
Azygos vein
The azygos vein is a vein running up the right side of the thoracic vertebral column. It can also provide an alternate path for blood to the right atrium by allowing the blood to flow between the venae cavae when one vena cava is blocked.-Structure:...

, and the thoracic duct
Thoracic duct
In human anatomy, the thoracic duct of the lymphatic system is the largest lymphatic vessel in the body. It is also known as the left lymphatic duct, alimentary duct, chyliferous duct, and Van Hoorne's canal....


>-
| two lesser aperture of right crus 
>-
| three lesser aperture of left crus 
hemiazygos vein
Hemiazygos vein
The hemiazygos vein is a vein running superiorly in the lower thoracic region, just to the left side of the vertebral column.-Structure:...


>-
| behind the diaphragm, under the medial lumbocostal arches 
sympathetic trunk
Sympathetic trunk
The sympathetic trunks are a paired bundle of nerve fibers that run from the base of the skull to the coccyx.-Structure:...


>-
| areolar tissue between the sternal and costal parts (see also foramina of Morgagni
Foramina of Morgagni
The sternocostal triangle or foramina of Morgagni are small zones lying between the costal and sternal attachments of the thoracic diaphragm...

)
liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...


>-
| areolar tissue between the fibers springing from the medial and lateral lumbocostal arches
This interval is less constant; when this interval exists, the upper and back part of the kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

 is separated from the pleura by areolar tissue only.


A commonly used mnemonic to remember the level of the diaphragmatic apertures is this: Mnemonic
Mnemonic
A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...

  • Aortic hiatus
    Aortic hiatus
    The aortic hiatus is a hole in the human diaphragm. It is the lowest and most posterior of the large apertures.It is located approximately at the level of the twelfth thoracic vertebra .-Structure:...

     = 12 letters = T12
  • Oesophagus = 10 letters = T10
  • Vena cava = 8 letters = T8


Another common mnemonic is: "I ate ten eggs at twelve"
I (IVC) ate (TV8); ten (TV10) eggs (esophagus); at (aorta, azygos) twelve (TV12)

Comparative anatomy and evolution

The existence of some membrane separating the pharynx from the stomach can be traced widely among the chordate
Chordate
Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...

s. Thus amphioxus possesses an atrium by which water exits the pharynx, which has been argued (and disputed) to be homologous to structures in ascidians and hagfish
Hagfish
Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...

es. The urochordate epicardium separates digestive organs from the pharynx and heart, but the anus returns to the upper compartment to discharge wastes through an outgoing siphon.

Thus the diaphragm emerges in the context of a body plan that separated an upper feeding compartment from a lower digestive tract, but the point at which it originates is a matter of definition. Structures in fish, amphibians, reptiles, and birds have been called diaphragms, but it has been argued that these structures are not homologous. For instance, the alligator diaphragmaticus muscle does not insert on the esophagus
Esophagus
The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

 and does not affect pressure of the lower esophageal sphincter. The lungs are located in the abdominal compartment of amphibians and reptiles, so that contraction of the diaphragm expels air from the lungs rather than drawing it into them. In birds and mammals, lungs are located above the diaphragm. The presence of an exceptionally well-preserved fossil of Sinosauropteryx
Sinosauropteryx
Sinosauropteryx was the first genus of dinosaur outside of Avialae to be found with evidence of feathers. They were covered with "furry" coats of very simple filament-like feathers...

, with lungs located beneath the diaphragm as in crocodiles, has been used to argue that dinosaurs could not have sustained an active warm-blooded physiology, or that birds could not have evolved from dinosaurs. An explanation for this state of affairs is that lungs originated beneath the diaphragm, but as the demands for respiration increased in warm-blooded birds and mammals, natural selection came to favor the parallel evolution
Parallel evolution
Parallel evolution is the development of a similar trait in related, but distinct, species descending from the same ancestor, but from different clades.-Parallel vs...

 of the herniation of the lungs from the abdominal cavity in both lineages. However, birds do not have diaphragms. They do not breathe in the same way as mammals, and do not rely on creating a negative pressure in the thoracic cavity, at least not to the same extent. They rely on a rocking motion of the keel of the sternum to create local areas of reduced pressure to supply thin, membranous airsacs cranially and caudally to the fixed-volume, non-expansive lungs. A complicated system of valves and air sacs cycles air constantly over the absorption surfaces of the lungs so allowing maximal efficiency of gaseous exchange. Thus, birds do not have the reciprocal tidal breathing flow of mammals. On careful dissection, around eight air sacs can be clearly seen. They extend quite far caudally into the abdomen.

Variations

The sternal portion of the muscle is sometimes wanting and more rarely defects occur in the lateral part of the central tendon
Central tendon
The central tendon of the diaphragm is a thin but strong aponeurosis situated slightly anterior to the vault formed by the muscle, resulting in longer posterior muscle fibers....

 or adjoining muscle fibers.

Pathology

The right crus of the diaphragm is part of the lower esophageal sphincter (LES), which separates the thoracic and abdominal parts of the esophagus. A hiatal hernia, in which the abdominal esophagus or even the fundus of the stomach rises up through the esophageal hiatus into the thoracic cavity, can result from a tear or weakness in the diaphragm. Weakness may also result from injury to the neck at levels C3, C4, C5 which can result in altered tone of either half of the diaphragm. Both general weakness of the LES and hiatal hernia can cause "acid reflux," also known as gastroesophageal reflux disease
Gastroesophageal reflux disease
Gastroesophageal reflux disease , gastro-oesophageal reflux disease , gastric reflux disease, or acid reflux disease is chronic symptoms or mucosal damage caused by stomach acid coming up from the stomach into the esophagus...

 u(GERD).

If the diaphragm is struck, or otherwise spasm
Spasm
In medicine a spasm is a sudden, involuntary contraction of a muscle, a group of muscles, or a hollow organ, or a similarly sudden contraction of an orifice. It is sometimes accompanied by a sudden burst of pain, but is usually harmless and ceases after a few minutes...

s, breathing will become difficult. This is called "having the wind knocked out of you."

Hiccoughs are involuntary intermittent sudden contractions of the diaphragm.

Gas inferior to the diaphragm in a human (or posterior in a four-footed creature) may be pneumoperitoneum
Pneumoperitoneum
Pneumoperitoneum is air or gas in the abdominal cavity. It is often seen on X-ray, but small amounts are often missed, and CT is nowadays regarded as a criterion standard in the assessment of a pneumoperitoneum. CT can visualize quantities as small as 5 cm³ of air or gas...

, which can be a sign of serious pathology.

Development

The thoracic diaphragm develops embryologically beginning in the third week after fertilization with two processes known as transverse folding and longitudinal folding. The septum transversum
Septum transversum
The septum transversum is a thick mass of cranial mesenchyme that gives rise to parts of the thoracic diaphragm and the ventral mesentery of the foregut in the developed human being.- Origins :...

, the primitive central tendon of the diaphragm, originates at the rostral pole of the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

 and is relocated during longitudinal folding to the ventral thoracic region. Transverse folding brings the body wall anteriorly to enclose the gut and body cavities. The pleuroperitoneal membrane and body wall myoblasts, from somatic lateral plate mesoderm, meet the septum transversum to close off the pericardio-peritoneal canals on either side of the presumptive esophagus, forming a barrier that separates the peritoneal and pleuropericardial cavities. Furthermore, dorsal mesenchyme surrounding the presumptive esophagus form the muscular crura of the diaphragm.

Because the earliest element of the embryological diaphragm, the septum transversum, forms in the cervical region, the phrenic nerve
Phrenic nerve
The phrenic nerve originates mainly from the 4th cervical nerve, but also receives contributions from the 5th and 3rd cervical nerves in humans....

 that innervates the diaphragm originates from the cervical spinal cord (C3,4, and 5). As the septum transversum descends inferiorly, the phrenic nerve follows, accounting for its circuitous route from the upper cervical vertebrae, around the pericardium
Pericardium
The pericardium is a double-walled sac that contains the heart and the roots of the great vessels.-Layers:...

, finally to innervate the diaphragm.

Clinical Relevance

Failure of one of the pleuroperitoneal membrane to form of a competent barrier between cavities can result in congenital diaphragmatic hernia
Congenital diaphragmatic hernia
Congenital diaphragmatic hernia is a congenital malformation of the diaphragm. The most common type of CDH is a Bochdalek hernia; other types include Morgagni's hernia, diaphragm eventration and central tendon defects of the diaphragm...

. This condition is present in 1 out of 2,000 births. A large herniation has a mortality rate of 3/4 and requires immediate surgical repair.
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