Chapter 55: Respiratory Diaphragm & Phrenic Nerves

The diaphragm is composed of a strong, central tendinous portion (aponeurosis), classically described as trifoliate (having three folia), surrounded by a peripheral muscular section. This muscular area is divided into three parts—sternal, costal, and lumbar—which originate from the circumference of the inferior thoracic aperture, including the lower six costal cartilages, the xiphoid process, and the lumbar vertebrae via the crura and arcuate ligaments. As the primary muscle of inspiration, the diaphragm is responsible for roughly two-thirds of quiet air intake volume. During contraction, it descends, causing a complementary descent of the pelvic floor and viscera while minimizing curvature change; this action fixes the central tendon, allowing the muscle fibers to elevate the inferior ribs in a manner described as 'bucket handle' and 'pump handle' movements, thereby increasing the volume of the thoracic cavity. The diaphragm's position is highly dynamic, varying with posture, ventilatory phase, and body mass, although the right dome is generally situated superior to the left. Three significant openings allow structures to pass between the thorax and abdomen: the most superior caval foramen (for the inferior vena cava and right phrenic nerve branches, located near T11), the oesophageal hiatus (for the esophagus and vagal trunks, typically T11), and the most posteroinferior aortic hiatus (for the aorta and thoracic duct, located at T12). Pathological defects include the posterolateral lumbocostal triangle (of Bochdalek), a frequent site for congenital herniation, and the acquired sliding or para-oesophageal types of hiatus hernia, which compromise the physiological sphincter action provided by the crural muscle fibers. The diaphragm receives its motor and central sensory supply entirely from the phrenic nerves (C3, C4, C5), while the periphery is innervated by the lower intercostal nerves. Arterial supply is rich, derived predominantly from the inferior phrenic arteries, supplemented by the superior phrenic, pericardiacophrenic, musculophrenic, and lower intercostal/subcostal arteries. Venous drainage occurs through corresponding phrenic veins, with the right inferior phrenic vein often serving as an essential surgical landmark for identifying the confluence of the hepatic veins into the inferior vena cava. Irritation of the diaphragm causes referred pain perceived at the shoulder tip, reflecting the shared spinal nerve roots of the phrenic and supraclavicular nerves.