Diaphragm
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Direct answer
"C3, 4, 5 keeps the diaphragm alive" — the mnemonic survives every exam cycle because it encodes the muscle's clinical significance: the phrenic nerve from C3, C4 and C5 supplies every motor fibre, which is why a high cervical cord injury stops breathing while a lower one does not. The diaphragm is a musculotendinous dome between thorax and abdomen, muscle at the periphery and central tendon above, and it is the chief muscle of inspiration. Its three major openings transmit the inferior vena cava at T8, the oesophagus at T10 and the aorta at T12 — "I 8 (ate) 10 eggs at 12".
What you must remember
- Openings: T8 — inferior vena cava with the right phrenic nerve; T10 — oesophagus in the right crus with the vagal trunks and left gastric oesophageal vessels; T12 — aorta behind the median arcuate ligament with the thoracic duct and azygos vein.
- Origin, three parts: sternal — back of the xiphoid; costal — inner surfaces of the lower six ribs and their cartilages; lumbar — right and left crura and the medial, lateral and median arcuate ligaments; all converge on the central tendon.
- Nerve supply: phrenic (C3–C5) for all motor fibres and central sensation; lower six intercostal and subcostal nerves for the peripheral rim.
- Minor apertures: sternocostal triangle (Larrey's gap) for the superior epigastric vessels; behind the medial arcuate ligament, the sympathetic chain; the splanchnic nerves pierce the crura.
- Actions: the domes descend on contraction, providing most of resting inspiration, and raise intra-abdominal pressure for micturition, defaecation, vomiting and parturition.
- Referred pain: central irritation refers to the shoulder tip, the C4 dermatome shared with the supraclavicular nerves — the sign of splenic rupture (Kehr's sign) and subphrenic abscess.
- Radiology: the right dome stands about half an interspace higher than the left (liver beneath); paradoxical ascent on inspiration suggests phrenic palsy.
- Development: septum transversum (central tendon), pleuroperitoneal membranes (dorsolateral), body wall (rim), oesophageal mesenchyme (crura).
Two cases that test the openings
A 60-year-old with postprandial burning and regurgitation: the T10 opening explains the disease. The oesophagus passes through a hiatus formed by the right crus, whose splitting fibres act as the lower oesophageal sphincter; when the hiatus laxes, the junction slides up — sliding hiatus hernia — while the rarer rolling hernia pushes the fundus alongside an intact junction and risks strangulation. The left gastric vein's oesophageal branches traverse the same hiatus, so portal hypertension dilates them into varices.
A newborn in respiratory distress with a scaphoid abdomen and bowel sounds in the left hemithorax: a posterolateral Bochdalek defect — the commonest congenital diaphragmatic hernia, usually left-sided, through the pleuroperitoneal canal that failed to close in the eighth to tenth week — has let gut herniate and the lung become hypoplastic. Morgagni hernias through Larrey's gap are anterior, rare, and usually present in adulthood with incarcerated omentum.
Where students slip
The oesophageal opening is assigned to the left crus on the logic that the oesophagus is left-sided; it is the right crus, whose fibres decussate around it. The azygos vein and thoracic duct are filed with the oesophagus instead of the aortic opening — all three pass behind the median arcuate ligament at T12, and none traverses muscle, which is why the aorta is not compressed by every breath. The inferior vena cava's bond to its foramen needs its reason: the central tendon's fibres fuse with the adventitia, so inspiration widens the opening and encourages venous return. And shoulder-tip pain is explained as "touching the diaphragm"; say instead that blood or pus irritates the central parietal peritoneum whose C3–C5 phrenic afferents converge with the supraclavicular nerves in the cord, and the examiner hears the sentence they were fishing for.
Frequently asked questions
What passes through the three openings, and at what levels?
At T8 the inferior vena cava with the right phrenic nerve; at T10 the oesophagus with the vagal trunks and oesophageal branches of the left gastric artery and vein; at T12 the aorta with the thoracic duct and azygos vein.
Why does the oesophagus pass through the right crus, and why does that matter?
The right crus fibres split around the oesophagus and behave as a sphincter against reflux; laxity of this mechanism predisposes to sliding hiatus hernia, and an enlarged hiatus permits the para-oesophageal type.
Why does diaphragmatic irritation cause shoulder-tip pain?
Central parietal peritoneum sends phrenic sensory fibres from C3–C5, the segments of the supraclavicular nerves; blood or pus under the diaphragm is referred to the shoulder tip — Kehr's sign in splenic rupture.
Why is the right dome of the diaphragm higher than the left?
The liver's bulk beneath elevates it by about half an intercostal space; the left dome is depressed by the stomach and heart — knowing this normal asymmetry prevents misreading chest films. A Bochdalek hernia, for contrast, is the posterolateral congenital defect of the pleuroperitoneal membrane, usually left-sided, presenting with respiratory distress and bowel loops in the thorax of a newborn.