Diaphragm (anatomy)

In the anatomy of mammals, the diaphragm is a shelf of muscle extending across the bottom of the ribcage. The diaphragm separates the thoracic cavity (with lung and heart) from the abdominal cavity (with liver, stomach, intestines, etc.). In its relaxed state, the diaphragm is shaped like a dome.

Function

It is critically important in respiration: in order to draw air into the lungs, the diaphragm contracts, thus enlarging the thoracic cavity and reducing intra-thoracic pressure (the external intercostals muscles also participate in this enlargement). When the diaphragm relaxes, air is exhaled by elastic recoil of the lung and the tissues lining the thoracic cavity. The diaphragm is also found in other vertebrates such as reptiles.

The diaphragm also helps to expel vomit, feces, and urine from the body by increasing intra-abdominal pressure.

Pathology

A hiatal hernia can result from a tear or weakness in the diaphragm near the gastroesophageal junction.

If the diaphragm is struck, or otherwise spasms, breathing will become difficult. This is called having the wind knocked out of you.

The diaphragm is sometimes deemed to consist of left and right hemidiaphragms. The two are visible as separate dome-like structures on chest X-ray. In addition, they are controlled separately by the left and right phrenic nerves; damage to one of these nerves leads to dysfunction or paralysis of the corresponding hemidiaphgram (and damage to both nerves can cause bilateral paralysis, severely impairing respiration).

A hiccup occurs when the diaphragm contracts periodically without voluntary control.

Anatomy

The Diaphragm is a dome-shaped musculofibrous septum which separates the thoracic from the abdominal cavity, its convex upper surface forming the floor of the former, and its concave under surface the roof of the latter. Its peripheral part consists of muscular fibers which take origin from the circumference of the thoracic outlet and converge to be inserted into a central tendon.

The muscular fibers may be grouped according to their origins into three parts—sternal, costal, and lumbar.

The sternal part arises by two fleshy slips from the back of the xiphoid process.
the costal part from the inner surfaces of the cartilages and adjacent portions of the lower six ribs on either side, interdigitating with the Transversus abdominis.
The lumbar part from aponeurotic arches, named the lumbocostal arches, and from the lumbar vertebræ by two pillars or crura.

There are two lumbocostal arches, a medial and a lateral, on either side.

The Medial Lumbocostal Arch (arcus lumbocostalis medialis; internal arcuate ligament) is a tendinous arch in the fascia covering the upper part of the Psoas major; medially, it is continuous with the lateral tendinous margin of the corresponding crus, and is attached to the side of the body of the first or second lumbar vertebra; laterally, it is fixed to the front of the transverse process of the first and, sometimes also, to that of the second lumbar vertebra.

The Lateral Lumbocostal Arch (arcus lumbocostalis lateralis ^*; external arcuate ligament) arches across the upper part of the Quadratus lumborum, and is attached, medially, to the front of the transverse process of the first lumbar vertebra, and, laterally, to the tip and lower margin of the twelfth rib.

The Crura

At their origins the crura are tendinous in structure, and blend with the anterior longitudinal ligament of the vertebral column.

The right crus, larger and longer than the left, arises from the anterior surfaces of the bodies and intervertebral fibrocartilages of the upper three lumbar vertebræ, while the left crus arises from the corresponding parts of the upper two only.

The medial tendinous margins of the crura pass forward and medialward, and meet in the middle line to form an arch across the front of the aorta; this arch is often poorly defined.

From this series of origins the fibers of the diaphragm converge to be inserted into the central tendon.

The fibers arising from the xiphoid process are very short, and occasionally aponeurotic; those from the medial and lateral lumbocostal arches, and more especially those from the ribs and their cartilages, are longer, and describe marked curves as they ascend and converge to their insertion. The fibers of the crura diverge as they ascend, the most lateral being directed upward and lateralward to the central tendon.

The medial fibers of the right crus ascend on the left side of the esophageal hiatus, and occasionally a fasciculus of the left crus crosses the aorta and runs obliquely through the fibers of the right crus toward the vena caval foramen.

The Central Tendon

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, so that the posterior muscular fibers are the longer.

It is situated immediately below the pericardium, with which it is partially blended.

It is shaped somewhat like a trefoil leaf, consisting of three divisions or leaflets separated from one another by slight indentations.

The right leaflet is the largest, the middle, directed toward the xiphoid process, the next in size, and the left the smallest.

In structure the tendon is composed of several planes of fibers, which intersect one another at various angles and unite into straight or curved bundles—an arrangement which gives it additional strength.

Openings in the Diaphragm

The diaphragm is pierced by a series of apertures to permit of the passage of structures between the thorax and abdomen. Three large openings—the aortic, the esophageal, and the vena caval—and a series of smaller ones are described.

The aortic hiatus (hiatus aorticus) is the lowest and most posterior of the large apertures; it lies at the level of the twelfth thoracic vertebra. Strictly speaking, it is not an aperture in the diaphragm but an osseoaponeurotic opening between it and the vertebral column, and therefore behind the diaphragm; occasionally some tendinous fibers prolonged across the bodies of the vertebræ from the medial parts of the lower ends of the crura pass behind the aorta, and thus convert the hiatus into a fibrous ring. The hiatus is situated slightly to the left of the middle line, and is bounded in front by the crura, and behind by the body of the first lumbar vertebra. Through it pass the aorta, the azygos vein, and the thoracic duct; occasionally the azygos vein is transmitted through the right crus.

The esophageal hiatus is situated in the muscular part of the diaphragm at the level of the tenth thoracic vertebra, and is elliptical in shape. It is placed above, in front, and a little to the left of the aortic hiatus, and transmits the esophagus, the vagus nerves, and some small esophageal arteries. The right crus of the diaphragm loops around forming a sling around the diaphragm. Upon inspiration, this sling would constrict the diaphragm, forming an anatomical sphincter that prevents stomach contents from refluxing up the oesophagus when intra-abdominal pressure rises during inspiration.

The caval opening (vena caval foramen) is the highest of the three, and is situated about the level of the fibrocartilage between the eighth and ninth thoracic vertebræ. It is quadrilateral in form, and is placed at the junction of the right and middle leaflets of the central tendon, so that its margins are tendinous. By being situated in the tendinous part of the diaphragm, it is stretched open every time inspiration occurs. 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. It transmits the inferior vena cava, the wall of which is adherent to the margins of the opening, and some branches of the right phrenic nerve.

Of the lesser apertures, two in the right crus transmit the greater and lesser right splanchnic nerves; three in the left crus give passage to the greater and lesser left splanchnic nerves and the hemiazygos vein. The gangliated trunks of the sympathetic usually enter the abdominal cavity behind the diaphragm, under the medial lumbocostal arches.

On either side two small intervals exist at which the muscular fibers of the diaphragm are deficient and are replaced by areolar tissue. One between the sternal and costal parts transmits the superior epigastric branch of the internal mammary artery and some lymphatics from the abdominal wall and convex surface of the liver. The other, between the fibers springing from the medial and lateral lumbocostal arches, is less constant; when this interval exists, the upper and back part of the kidney is separated from the pleura by areolar tissue only.

Variations

The sternal portion of the muscle is sometimes wanting and more rarely defects occur in the lateral part of the central tendon or adjoining muscle fibers.

External links

Thorax | Respiratory system | Muscles of the trunk

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