Diaphragm

During inhalation, the diaphragm contracts and flattens its dome-shaped resting configuration, descending toward the abdomen. This increases the vertical dimension of the thoracic cavity, reducing intrathoracic pressure below atmospheric pressure and causing air to rush into the lungs. During exhalation at rest, the diaphragm relaxes and passively returns to its dome shape, reducing thoracic volume and expelling air.

The diaphragm is controlled almost entirely by the phrenic nerve, which originates from cervical spinal cord levels C3, C4, and C5—remembered by the mnemonic 'C3, 4, and 5 keep the diaphragm alive.' Each hemidiaphragm receives its own phrenic nerve, one from each side. This is why a high cervical spinal cord injury (above C3) results in loss of spontaneous breathing.

A diaphragmatic hernia occurs when abdominal organs—such as the stomach, intestines, or spleen—herniate through a defect in the diaphragm into the thoracic cavity. Congenital diaphragmatic hernias are a serious neonatal emergency causing lung compression and respiratory failure. Acquired hernias can occur through the esophageal hiatus (hiatal hernia) or from traumatic rupture.

Hiccups result from involuntary, repetitive spasms of the diaphragm. Each spasm causes a sudden inhalation that is abruptly stopped by closure of the vocal cords, producing the characteristic 'hic' sound. Common triggers include rapid eating, carbonated drinks, alcohol, and gastric distension. Persistent hiccups lasting more than 48 hours may indicate an underlying medical condition.

Three major structures pass through openings in the diaphragm: the inferior vena cava passes through at the T8 level (caval opening), the esophagus passes through at T10 (esophageal hiatus), and the aorta passes through at T12 (aortic hiatus). A common mnemonic is 'I ate (8) ten (10) eggs (esophagus) at (aorta) twelve (12).'