Diaphragm

Diaphragma

Also known as: thoracoabdominal diaphragm, hemidiaphragm, crura of diaphragm, central tendon

The dome-shaped musculotendinous partition between thorax and abdomen and the principal muscle of inspiration, supplied by the phrenic nerves (C3–C5).

Show on body map Respiratory Tract

Overview

Caval openingT8 — IVC, right phrenic nerve

Oesophageal hiatusT10 — oesophagus, vagal trunks

Aortic hiatusT12 — aorta, thoracic duct, azygos

Excursion≈1–2 cm quiet; up to ≈10 cm deep breathing

Contribution to quiet inspiration≈75%

The diaphragm consists of a peripheral muscular part converging on a trefoil-shaped central tendon, fused above with the fibrous pericardium. It forms right and left domes (cupulae); the right dome is higher (reaching the 5th rib in full expiration, over the liver) than the left (5th intercostal space). Its muscle fibres arise in three parts — sternal, costal and lumbar (crura and arcuate ligaments) — and all insert into the central tendon.

When it contracts the domes descend ≈1–2 cm in quiet breathing (up to ≈10 cm in deep breathing), increasing the vertical dimension of the thorax; with the abdominal viscera acting as a fulcrum, it also lifts and everts the lower ribs. It produces ≈75% of the volume change in quiet inspiration. Expiration is passive recoil. Contraction with a closed glottis raises intra-abdominal pressure for defecation, micturition, vomiting and childbirth, and its rhythmic movements aid venous and lymphatic return.

Structures pass between thorax and abdomen through three major apertures — the caval opening (T8), the oesophageal hiatus (T10) and the aortic hiatus (T12) — plus several smaller openings.

Location: Forms the floor of the thoracic cavity and roof of the abdominal cavity; domes reach the 5th rib (right) and 5th intercostal space (left) in expiration; attached peripherally to the xiphoid, lower six costal cartilages/ribs and upper lumbar vertebrae.

Parts & subdivisions

Sternal part
Two slips from the posterior surface of the xiphoid process.
Costal part
From the inner surfaces of the lower six costal cartilages and adjacent ribs (7–12), interdigitating with transversus abdominis.
Lumbar part — crura
Right crus from the bodies of L1–L3 (its fibres encircle the oesophageal hiatus); left crus from L1–L2. They unite over the aorta as the median arcuate ligament.
Arcuate ligaments
Medial (thickened psoas fascia, L1/L2 body to L1 transverse process) and lateral (thickened quadratus lumborum fascia, L1 transverse process to the 12th rib).
Caval opening — T8
In the central tendon, right of midline: IVC, terminal branches of the right phrenic nerve, lymphatics. Stretched open during inspiration, aiding venous return.
Oesophageal hiatus — T10
In the right crus, left of midline: oesophagus, anterior and posterior vagal trunks, oesophageal branches of the left gastric vessels, lymphatics. Constricted during inspiration — a physiological lower oesophageal sphincter.
Aortic hiatus — T12
Behind the median arcuate ligament (strictly behind the diaphragm): aorta, thoracic duct, often the azygos vein. Not compressed by contraction.
Smaller openings
Splanchnic nerves pierce the crura; the sympathetic trunks pass behind the medial arcuate ligaments; the hemiazygos vein pierces the left crus; the left phrenic nerve pierces the left dome; superior epigastric vessels pass through the sternocostal triangles (foramina of Morgagni); subcostal nerves and vessels pass behind the lateral arcuate ligaments.

Anatomy

Origin: Xiphoid process (sternal part); inner surfaces of the lower six costal cartilages and ribs 7–12 (costal part); L1–L3 bodies via the right crus, L1–L2 via the left crus, and the medial and lateral arcuate ligaments (lumbar part).
Insertion: Central tendon.
Action: Contraction flattens and lowers the domes, enlarging the vertical (and, via the lower ribs, transverse) dimensions of the thorax for inspiration.
Innervation: Motor: phrenic nerves (C3–C5), each supplying its own half. Sensory: central part by the phrenic nerves; periphery by the lower six intercostal nerves (T7–T11) and subcostal nerve (T12).
Nerve roots: C3, C4, C5 (motor); T7–T12 (peripheral sensory).
Arterial supply: Superior surface: pericardiacophrenic and musculophrenic arteries (internal thoracic) and superior phrenic arteries (thoracic aorta). Inferior surface: inferior phrenic arteries (from the abdominal aorta — the largest supply). Lower intercostal and subcostal arteries supply the periphery.
Venous drainage: Pericardiacophrenic and musculophrenic veins → internal thoracic veins; inferior phrenic veins → IVC (right) and left suprarenal vein/IVC (left).
Lymphatic drainage: Anterior and posterior diaphragmatic nodes → parasternal, posterior mediastinal and upper lumbar nodes; the peritoneal surface absorbs peritoneal fluid via lymphatics.
Relations: Above: pleura and lung bases, pericardium and heart. Below: liver (right), stomach fundus and spleen (left), kidneys and suprarenal glands posteriorly.
Development: From four sources: septum transversum (central tendon), pleuroperitoneal membranes, dorsal mesentery of the oesophagus (crura) and the lateral body wall (peripheral muscle). Its cervical origin (septum transversum at C3–C5) explains the phrenic nerve supply.

Function

Principal muscle of inspiration (≈75% of quiet tidal volume).
Raises intra-abdominal pressure (with a closed glottis and abdominal muscles) for expulsive efforts.
Right crus acts as an external sphincter at the gastro-oesophageal junction.
Promotes venous return (thoracoabdominal pump) and lymph flow.
Separates the thoracic and abdominal cavities.

Clinical significance

Phrenic nerve palsy

Injury (cardiac surgery, cervical trauma, tumour invasion — e.g. lung cancer, neck surgery) paralyses the hemidiaphragm, which is raised on radiographs and moves paradoxically upward on sniffing (fluoroscopic sniff test). High cervical cord injury above C3 abolishes diaphragmatic breathing and requires ventilation.

Hernias

Hiatus hernia: sliding (≈95%; the gastro-oesophageal junction slides into the thorax — reflux) or para-oesophageal/rolling (fundus herniates beside the oesophagus — risk of strangulation). Congenital diaphragmatic hernia: usually posterolateral (Bochdalek, most often left) with pulmonary hypoplasia; anterior (Morgagni) hernias are rarer. Traumatic rupture is more often left-sided (the liver protects the right).

Referred shoulder-tip pain

Irritation of the diaphragm (subphrenic abscess, blood from splenic rupture or ectopic pregnancy, laparoscopic CO2) causes pain at the shoulder tip via C4 — Kehr's sign on the left.

Hiccups

Involuntary spasmodic contractions of the diaphragm followed by abrupt glottic closure; persistent hiccups may reflect irritation of the phrenic or vagus nerves, CNS lesions or metabolic disturbance.

Mnemonics

Apertures

"I 8 10 Eggs At 12" — IVC T8, oesophagus T10, aorta T12. (Also: "vena cava has 8 letters, oesophagus 10, aortic hiatus 12".)

Phrenic nerve

"C3, 4, 5 keeps the diaphragm alive."

Test yourself

Which structures pass through the diaphragm at the level of T10?

Inferior vena cava and right phrenic nerve
Oesophagus and anterior and posterior vagal trunks
Aorta, thoracic duct and azygos vein
Greater splanchnic nerves

Answer: Oesophagus and anterior and posterior vagal trunks

The oesophageal hiatus (T10) lies in the muscular right crus and transmits the oesophagus, the vagal trunks and oesophageal branches of the left gastric vessels. The IVC passes at T8 (caval opening in the central tendon) and the aorta at T12 (behind the median arcuate ligament). Mnemonic: "I 8 10 Eggs At 12".

Irritation of the pleura over the central part of the diaphragm typically produces pain referred to the:

Epigastrium
Tip of the shoulder
Lower intercostal spaces
Umbilicus

Answer: Tip of the shoulder

Mediastinal and central diaphragmatic parietal pleura are supplied by the phrenic nerve (C3–C5), so pain is referred to the C4 dermatome over the shoulder. Costal and peripheral diaphragmatic pleura are supplied by intercostal nerves, giving localised chest-wall or abdominal-wall pain.

In the erect patient, where does a small pleural effusion first collect?

Cardiac notch
Costomediastinal recess
Pleural cupula
Costodiaphragmatic recess

Answer: Costodiaphragmatic recess

The costodiaphragmatic recess is the most dependent part of the pleural cavity, lying between the inferior border of the lung (6–8–10) and the pleural reflection (8–10–12). Fluid there blunts the costophrenic angle on an erect radiograph; it is also the site for diagnostic aspiration.