Chapter 55 - Respiratory diaphragm and phrenic nerves

Abstract

The respiratory diaphragm is a domed septum, between the thoracic and abdominopelvic cavities, consisting of muscle surrounding a central tendon. The right respiratory diaphragm is higher than the left, while the anterior and medial parts are higher than the posterior and lateral parts. There are three separate diaphragmatic openings present, the foramen for the inferior vena cava located in the midportion of the central tendon, the oesophageal hiatus located within the right diaphragmatic crus, and the aortic hiatus between the diaphragmatic crura and median arcuate ligament. These openings allow structures to pass through the diaphragm and communicate between the thoracic and abdominopelvic cavities. The motor and sensory innervation of the diaphragm is mainly from the left and right phrenic nerves (C3–C5), with additional contributions from the lower six intercostal nerves. Contraction of the respiratory diaphragm, with assistance from the secondary respiratory muscles, causes a decrease in intrathoracic pressure, which allows air to move down a pressure gradient into the lungs. Relaxation of the respiratory diaphragm and contraction of the abdominal wall muscles causes exhalation because intrathoracic pressure now exceeds atmospheric air pressure. In addition to serving as the primary muscle of ventilation, the respiratory diaphragm also assists with regulation of intra-abdominal pressure, especially during normal bodily functions such as urination and defecation.