The diaphragm is an unpaired, dome shaped skeletal muscle that is located in the trunk which separates the thoracic and abdominal cavities from each other by closing the inferior thoracic aperture. The diaphragm is the primary muscle that is active in inspiration. Contraction of the muscle facilitates expansion of the thoracic cavity. This increases volume of the the cavity, which in turn decreases the intrathoracic pressure allowing the lungs to expand and inspiration to occur.
The diaphragm is a musculotendinous structure with a peripheral attachment to a number of bony structures. It is attached anteriorly to the xiphoid process and costal margin, laterally to the 11th and 12th ribs, and posteriorly to the lumbar vertebrae. The posterior attachment to the vertebrae is by tendinous bands called crura. The crura are attached to the anterior aspect of the bodies of the 1st, 2nd and 3rd lumbar vertebrae. The muscle fibres, extending from their bony attachments, converge on a central tendon.
The diaphragm is innervated by the phrenic nerve, which arises in the neck from the 3rd, 4th and 5th cervical spinal nerves (to help us remember, we use C3,4,5 to keep the diaphragm alive). The phrenic nerves pass through the diaphragm and innervate it from its abdominal surface.
The diaphragm is described as having a right and a left dome either side of the central tendon. In the thorax, the right and left pleural cavities are immediately superior to their respective domes of the diaphragm. The pericardial sac, containing the heart and roots of the great blood vessels, sits on the central tendon of the diaphragm.
In the abdomen, the liver, the right kidney and right suprarenal gland are just below the right dome of the diaphragm. The stomach, spleen , left kidney and left suprarenal gland are just below the left dome.
Structures passing between the thoracic and abdominal cavities pass through the diaphragm, or through openings between some of the peripheral attachments.
- Aortic hiatus: The thoracic aorta passes through this opening between the crura of the diaphragm. The azygos vein and the thoracic duct also pass through here.
- Caval opening: The inferior vena cava passes through the central tendon of the diaphragm at the level of the 8th thoracic vertebra.
- Esophageal hiatus: The esophagus passes through muscular fibres of the diaphragm slightly to the left of the midline at the level of the T10 vertebra. The vagus nerves also pass through this opening.
An easy way to remember the location and structures passing through the diaphragm is by using this mnemonic: 'I ate ten eggs at 12.' - I (IVC) ate (T8) ten (T10) eggs (esophagus + vagus) at (azygos + thoracic duct) 12 (T12).
The greater and lesser splanchnic nerves pass through the crura on either side of the diaphragm.
The sympathetic and least splanchnic nerves pass under the median arcuate ligament that extends between the crura of the diaphragm.
The superior epigastric vessels pass anterior to the diaphragm, just deep to the ribs.
A very common disorder of the diaphragm that affects most people at some point is hiccups. Hiccups occur due to involuntary, intermittent contraction of the muscle. They are usually caused by consumption of large volumes of food over a short amount of time.
Herniation may occur through the diaphragm. At the level of the esophageal hiatus, the stomach may herniate into the posterior mediastinum. A diaphragmatic hernia can be congenital. These occur as a result of abnormalities in the development of the diaphragm in the fetus. Abdominal organs may protrude into the thoracic cavity and disrupt the development of the lungs, causing problems in lung development and the functioning of the lungs after birth.
Diaphragmatic hernias can also be acquired. These usually occur as a result of blunt force trauma, such as in a road traffic accident or a severe fall. Surgery is required to repair a diaphragmatic hernia.