The termdiaphragm in anatomy, created byGerard of Cremona,[5] can refer to other flat structures such as theurogenital diaphragm orpelvic diaphragm, but "the diaphragm" generally refers to the thoracic diaphragm. In humans, the diaphragm is slightly asymmetric—its right half is higher up (superior) to the left half, since the large liver rests beneath the right half of the diaphragm. There is also speculation that the diaphragm is lower on the other side due to heart's presence.
Othermammals have diaphragms, and othervertebrates such asamphibians andreptiles have diaphragm-like structures, but important details of the anatomy may vary, such as the position of the lungs in the thoracic cavity.
Definition ofdiaphragm in Blount's 1707Glossographia Anglicana Nova
The diaphragm is an upward curved, c-shaped structure ofmuscle andfibrous tissue that separates thethoracic cavity from the abdomen.[6] The superior surface of the dome forms the floor of the thoracic cavity, and the inferior surface the roof of the abdominal cavity.[7]
As a dome, the diaphragm has peripheral attachments to structures that make up the abdominal and chest walls. The muscle fibres from these attachments converge in acentral tendon, which forms the crest of the dome.[7] Its peripheral part consists of muscular fibers that take origin from the circumference of theinferior thoracic aperture and converge to be inserted into a central tendon.
The muscle fibres of the diaphragm radiate outward from the central tendon. While the diaphragm is one muscle, it is composed of two distinct muscle regions: the costal, which serves as the driver in the work of breathing, and crural diaphragm, which serves as an "anchor;" attaching the muscle to the lower ribs and lumbar vertebrae. The costal diaphragm is further divided into ventral, medial, and dorsal costal portions.[8][9]
The vertebral part of the diaphragm arises from the crura and arcuate ligaments. Right crus arises from L1-L3 vertebral bodies and their intervertebral discs. Smaller left crus arises from L1, L2 vertebral bodies and their intervertebral discs.[8][7][10] Medial arcuate ligament arises from the fascia thickening from body of L2 vertebrae to transverse process of L1 vertebrae, crossing over the body of thepsoas major muscle. Thelateral arcuate ligament arises from the transverse process of L1 vertebrae and is attached laterally to the 12th rib. The lateral arcuate ligament also arises from fascia thickening that covers thequadratus lumborum muscle. Themedian arcuate ligament arises from the fibrous parts of right and left crura where descendingthoracic aorta passes behind it. No diaphragmatic muscle arises from the median arcuate ligament.[8] Both adrenal glands lie near the diaphragmatic crus and arcuate ligament.[11]
The costal part of diaphragm arises from the lower four ribs (7 to 10) costal cartilages.[8]
Thecentral tendon of the diaphragm is a thin but strongaponeurosis near the center of the vault formed by the muscle, closer to the front than to the back of thethorax. The central part of the tendon is attached above topericardium. The both sides of the posterior fibres are attached to paracolic gutters (the curving of ribs before attaching to both sides of the vertebral bodies).[8]
The inferior vena cava passes through the caval opening, a quadrilateral opening at the junction of the right and middle leaflets of thecentral tendon, so that its margins are tendinous. Surrounded by tendons, the opening is stretched open every time inspiration occurs. However, there has been argument that the caval opening actually constricts during inspiration. Since thoracic pressure decreases upon inspiration and draws the caval blood upwards toward the right atrium, increasing the size of the opening allows more blood to return to the heart, maximizing the efficacy of lowered thoracic pressure returning blood to the heart. Theaorta does not pierce the diaphragm but rather passes behind it in between the left and right crus.[citation needed]
There are several structures that pierce through the diaphragm, including:left phrenic nerve pierces through the central tendon, greater, lesser, and leastthoracic splanchnic nerves pierces through bilateral crura, and lymphatic vessels that pierce throughout the diaphragm, especially behind the diaphragm.[8]
Openings through the diaphragm and their content[7]
Thecaval opening passes through the central tendon of the diaphragm. It contains theinferior vena cava,[7] and some branches of the rightphrenic nerve.[14] The outermost wall of inferior vena cava is fused with the central tendon.[8]
Theesophageal hiatus is situated in the posterior part of the diaphragm, located slightly left of the west central tendon through the muscular sling of the right crus of the diaphragm. It contains theesophagus, and anterior and posteriorvagal trunks,[7]left gastric artery and veins, and lymphatics.[8]
Theaortic hiatus is in the posterior part of the diaphragm, between the left and right crus. It contains theaorta, thethoracic duct andAzygous vein.[14]
The diaphragm is primarily innervated by thephrenic nerve which is formed from thecervical nerves C3, C4 and C5.[7] While the central portion of the diaphragm sends sensory afferents via the phrenic nerve, the peripheral portions of the diaphragm send sensory afferents via theintercostal (T5–T11)[8] andsubcostal nerves (T12).[citation needed]
The sternal portion of the muscle is sometimes wanting and more rarely defects occur in the lateral part of thecentral tendon or adjoining muscle fibers.
The thoracic diaphragm develops duringembryogenesis, beginning in the third week after fertilization with two processes known as transverse folding and longitudinal folding. Theseptum transversum, the primitive central tendon of the diaphragm, originates at the rostral pole of theembryo 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 platemesoderm, 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, dorsalmesenchyme 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, thephrenic nerve 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 thepericardium, finally to innervate the diaphragm.
The diaphragm is the mainmuscle of respiration and functions inbreathing. During inhalation, the diaphragm contracts and moves in the inferior direction, enlarging the volume of the thoracic cavity and reducing intra-thoracic pressure (theexternal intercostal muscles also participate in this enlargement), forcing the lungs to expand. In other words, the diaphragm's movement downwards creates a partialvacuum in the thoracic cavity, which forces the lungs to expand to fill the void, drawing air in the process.
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 calledbelly 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 (moves in the superior direction), air is exhaled by elastic recoil process of the lung and the tissues lining the thoracic cavity. Assisting this function with muscular effort (called forcedexhalation) involves theinternal intercostal muscles used in conjunction with theabdominal muscles, which act as anantagonist paired with the diaphragm's contraction. Diaphragm dysfunction is a well-known factor associated with various complications in patients, such as prolonged respiratory failure, difficulties in weaning from mechanical ventilation, extended hospitalization, increased morbidity, and mortality.[15] Studies have reported that a thin diaphragm leads to greater lung compliance, which can contribute to respiratory failure. Furthermore, reduction in diaphragm thickness during the early stages of disease can serve as a prognostic marker in sepsis patients, and COVID-19 patients.[16][17]
The diaphragm is also involved in non-respiratory functions. It helps to expelvomit,feces, andurine from the body by increasing intra-abdominal pressure, aids in childbirth,[18] and preventsacid reflux by exerting pressure on theesophagus as it passes through theesophageal hiatus.
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.[citation needed]
Ahiatus hernia is ahernia in which parts of the lower esophagus or stomach that are normally in the abdomen pass abnormally through the diaphragm and are present in the thorax. Hernias are described asrolling, in which the hernia is beside the oesophagus, orsliding, in which the hernia directly involves the esophagus. These hernias are implicated in the development of reflux, as the different pressures between the thorax and abdomen normally act to keep pressure on theesophageal hiatus. With herniation, this pressure is no longer present, and the angle between thecardia of thestomach and the oesophagus disappears. Not all hiatus hernias cause symptoms, although almost all people withBarrett's oesophagus oroesophagitis have a hiatus hernia.[19]
Hernias may also occur as a result of congenital malformation, acongenital diaphragmatic hernia. When thepleuroperitoneal membranes fail to fuse, the diaphragm does not act as an effective barrier between the abdomen and thorax. Herniation is usually of the left, and commonly through the posteriorlumbocostal triangle, although rarely through the anteriorforamen of Morgagni. The contents of the abdomen, including theintestines, may be present in the thorax, which may impact development of the growing lungs and lead tohypoplasia.[20] This condition is present in 0.8 - 5/10,000 births.[21] A large herniation has high mortality rate, and requires immediate surgical repair.[22]
Due to its position separating thethorax andabdomen, fluid abnormally present in the thorax, or air abnormally present in the abdomen, may collect on one side of the diaphragm. AnX-ray may reveal this.Pleural effusion, in which there is fluid abnormally present between the twopleurae of thelungs, is detected by anX-ray of the chest, showing fluid collecting in theangle between the ribs and diaphragm.[19] An X-ray may also be used to reveal apneumoperitoneum, in which there is gas in the abdomen.
An X-ray may also be used to check for herniation.[20]
The adoption of a deeper breathing pattern typically occurs during physical exercise in order to facilitate greater oxygen absorption. During this process the diaphragm more consistently adopts a lower position within the body's core. In addition to its primary role in breathing, the diaphragm also plays a secondary role in strengthening the posture of the core. This is especially evident during deep breathing where its generally lower position increases intra-abdominal pressure, which serves to strengthen the lumbar spine.[23][better source needed]
The key to real core stabilization is to maintain the increased IAP while going through normal breathing cycles. [...] The diaphragm then performs its breathing function at a lower position to facilitate a higher IAP.[23]
Therefore, if a person's diaphragm position is lower in general, through deep breathing, then this assists the strengthening of their core during that period. This can be an aid in strength training and other forms of athletic endeavour. For this reason, taking a deep breath or adopting a deeper breathing pattern is typically recommended when lifting heavy weights.
The existence of a membrane separating the pharynx from the stomach can be traced widely among thechordates. Thus themodel organism, the marine chordatelancelet, possesses an atriopore by which water exits the pharynx, which has been claimed (and disputed) to be homologous to structures inascidians andhagfishes.[25] Thetunicate 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 nothomologous. For instance, the alligator diaphragmaticus muscle does not insert on theesophagus and does not affect pressure of the lower esophageal sphincter.[26] 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 ofSinosauropteryx, 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.[citation needed] An explanation for this (put forward in 1905), 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 theparallel evolution of the herniation of the lungs from the abdominal cavity in both lineages.[24]
However, birds lack 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.[27]
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^Corradi F, Isirdi A, Malacarne P, Santori G, Barbieri G, Romei C, et al. (April 2021). "Low diaphragm muscle mass predicts adverse outcome in patients hospitalized for COVID-19 pneumonia: an exploratory pilot study".Minerva Anestesiologica.87 (4):432–438.doi:10.23736/S0375-9393.21.15129-6.PMID33594871.S2CID263501203.
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