Attachments
The diaphragm arises from the circumference of the thoracic outlet in three parts, sternal, costal and lumbar, whose fibres converge on a central tendon.
- Sternal part: two small slips from the back of the xiphoid process.
- Costal part: from the inner surfaces of the lower six costal cartilages and adjacent ribs, interdigitating with transversus abdominis.
- Lumbar part: from the two crura and the arcuate ligaments.
The right crus arises from the bodies and discs of L1–L3 and is larger; its fibres split to encircle the oesophagus. The left crus arises from L1–L2. The median arcuate ligament is a fibrous arch joining the two crura in front of the aorta. The medial arcuate ligament is thickened fascia over psoas, from the body of L1 or L2 to the transverse process of L1. The lateral arcuate ligament is thickened fascia over quadratus lumborum, from that transverse process to the twelfth rib.
The central tendon is a trefoil-shaped aponeurosis, fused above with the fibrous pericardium. In expiration the right dome rises to about the fifth rib and the left dome to about the fifth intercostal space, the right higher because of the liver.
Openings and structures passing through
The three main openings lie at T8, T10 and T12, and each level reflects where the structure passes: through tendon, through muscle, or behind the diaphragm altogether.
| Opening | Level | Formed by | Transmits |
|---|---|---|---|
| Caval opening | T8 | Central tendon, right of midline | Inferior vena cava, branches of right phrenic nerve |
| Oesophageal hiatus | T10 | Muscle fibres of right crus | Oesophagus, anterior and posterior vagal trunks, oesophageal branches of left gastric vessels |
| Aortic hiatus | T12 | Behind median arcuate ligament, between the crura | Aorta, thoracic duct, often azygos vein |
Because the caval opening is in tendon, contraction pulls it open and aids venous return. The oesophageal hiatus is squeezed by the crus on inspiration, helping prevent reflux. The aorta passes behind the diaphragm, so breathing does not compress it.
Other routes
- Greater and lesser splanchnic nerves pierce the crura.
- Sympathetic trunks pass behind the medial arcuate ligaments.
- Subcostal nerves and vessels pass behind the lateral arcuate ligaments.
- Superior epigastric vessels pass between the sternal and costal parts.
- The left phrenic nerve pierces the left dome.
Relations
The diaphragm is covered by pleura and pericardium above and by peritoneum below, except where organs lie directly on it. Above are the bases of both lungs and, on the central tendon, the heart within the fibrous pericardium. At the periphery the costodiaphragmatic recesses of the pleura lie between the diaphragm and chest wall.
Below, the right dome covers the liver and the left dome covers the stomach and spleen. Posteriorly the kidneys and suprarenal glands lie against the lumbar part, and the bare area of the liver is in direct contact with the muscle without peritoneum.
Blood supply and innervation
The diaphragm is supplied by vessels on both surfaces and by the phrenic nerves, which carry all its motor fibres.
- Above: pericardiophrenic and musculophrenic arteries from the internal thoracic artery, and superior phrenic arteries from the thoracic aorta.
- Below: inferior phrenic arteries, usually the first branches of the abdominal aorta at T12, which also give the superior suprarenal arteries.
- Periphery: lower intercostal and subcostal arteries.
Each phrenic nerve (C3–C5, mainly C4) supplies motor fibres to its own half and sensory fibres to the central pleura and peritoneum. The periphery receives sensory fibres from the lower intercostal nerves. Irritation of the central part is therefore felt at the shoulder tip, in the C4 dermatome, while irritation of the periphery is felt over the lower chest wall and upper abdomen.
Development and clinical relevance
The diaphragm forms from four components, and failures of fusion produce the congenital hernias. The septum transversum forms the central tendon; the pleuroperitoneal membranes close the posterolateral canals; the dorsal mesentery of the oesophagus forms the crura; and the body wall adds the peripheral muscle. The septum transversum starts in the cervical region, which is why the phrenic nerve arises from C3–C5.
- Bochdalek hernia: posterolateral, usually left-sided, from failure of the pleuroperitoneal membrane. It presents at birth with respiratory distress from pulmonary hypoplasia.
- Morgagni hernia: anterior, retrosternal, rarer and often found later in life.
- Hiatus hernia: acquired, sliding or rolling (para-oesophageal), through a widened oesophageal hiatus.
- Phrenic nerve palsy: a raised hemidiaphragm that moves paradoxically upwards on sniffing.
- Median arcuate ligament syndrome: a low ligament compressing the coeliac trunk, a rare cause of post-prandial pain.
On the specimen
This station views the diaphragm from below with the abdominal viscera removed, so the crura, openings and upper aortic branches are all visible.
- Central tendon: the glistening white, trefoil-shaped sheet in the middle; the caval opening lies in it to the right, containing the inferior vena cava.
- Right or left crus: the right crus is larger and longer and its fibres loop around the oesophageal hiatus; the left is smaller.
- Median arcuate ligament: the fibrous arch joining the crura in front of the aorta at the aortic hiatus.
- Coeliac trunk and superior mesenteric artery: the coeliac trunk emerges just below the median arcuate ligament; the superior mesenteric artery arises about a centimetre lower.
- Inferior phrenic arteries: small vessels running up and laterally over the crura.
- Costal and sternal fibres: radiate from the ribs and xiphoid into the tendon.
- Pericardiophrenic arteries: on the thoracic surface, accompanying the phrenic nerves.
The renal arteries and right kidney are labelled lower on the same specimen.