The diaphragm is the primary muscle of breathing, a domed musculotendinous partition separating the thorax from the abdomen. Its descent during contraction increases thoracic volume, creating the negative pressure that drives airflow into the lungs. Three major structures pass through it: the aorta at T12 via the aortic hiatus, the oesophagus at T10 via the oesophageal hiatus, and the inferior vena cava at T8 via the caval foramen. The diaphragm contributes 70 percent of the inspiratory effort during quiet breathing.
| Origin | Posterior surface of the xiphoid process; Inner surfaces of the lower six costal cartilages and their ribs; Medial and lateral arcuate ligaments and the lumbar vertebrae via the crura |
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| Insertion | Central tendon — a trefoil-shaped fibrous structure into which all diaphragm fibres converge |
| Nerve Supply | Phrenic nerve (C3, C4, C5) — sole motor supply |
| Blood Supply | Pericardiacophrenic artery; Musculophrenic artery; Superior and inferior phrenic arteries |
| Actions | The primary muscle of inspiration — contracts to descend the domes and increase thoracic volume; Stabilises the thoracolumbar junction during trunk loading via the crura; Increases intra-abdominal pressure during Valsalva manoeuvre and forced expiration |
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During normal quiet breathing it descends approximately 1 to 2 centimetres, but during maximal inspiration it can descend 8 to 10 centimetres, dramatically increasing thoracic volume. Its crural fibres also participate in intersegmental lumbar stabilisation alongside the transversus abdominis.
Diaphragm paralysis from phrenic nerve injury produces an elevated hemidiaphragm visible on chest radiograph with paradoxical diaphragm movement (rising with inspiration) on fluoroscopy. Hiatus hernia from oesophageal hiatus laxity allows gastric cardia to herniate into the chest and causes gastro-oesophageal reflux. Diaphragm rupture from high-energy abdominal trauma is a surgical emergency with delayed presentation sometimes occurring months after injury when abdominal contents herniate through the defect.
The diaphragm is not directly palpable but its descent can be appreciated as the lower rib cage expansion that occurs during deep diaphragmatic breathing, assessed clinically by placing hands on the lower rib cage and feeling the lateral expansion during deep inspiration.
Traumatic diaphragm tear from high-energy blunt abdominal trauma, producing immediate or delayed herniation of abdominal contents into the chest with respiratory compromise, requiring urgent surgical repair.
Herniation of the stomach through the oesophageal hiatus producing gastro-oesophageal reflux symptoms managed with proton pump inhibitors and surgical fundoplication in refractory cases.
