The diaphragm is a thin double-domed musculotendinous sheet that separates the thoracic cavity from the abdominal cavity. Its convex superior surface faces the thorax, while its concave inferior surface faces towards the abdomen. The position of the domes varies because it is dependent on the individual's body build, phase of ventilation, and body position. The diaphragm is the main muscle of respiration. It descends during inspiration, and ascends during expiration.
The muscle fibers of the diaphragm originate from the oblique circumference of the thoracic outlet. Despite its continuous form, the muscle can be divided into three parts: sternal, costal, and lumbar. This is based according to the region of peripheral attachment. The sternal part arises as two fleshy slips from the xiphoid process of the sternum. The costal part arises from the lower six costal cartilages and their adjoining ribs on each side, and merges with the transversus abdominis muscle. The lumbar part arises from the medial and lateral arcuate ligaments, and the lumbar vertebrae. From these attachments, muscle fibers converge to join the central tendon, where part of the pericardium is attached. The central tendon is a thin yet strong aponeurosis of closely interwoven fibers located near the center of the muscle.
Structures that travel between the thorax and the abdomen pass through apertures in the diaphragm. There are three major openings for the aorta: the esophagus and the inferior vena cava, and a number of other smaller ones.
The aortic aperture contains the aorta, the thoracic duct, the lymphatic trunks and the occasional azygos and hemiazygos veins; together they pass behind the posterior attachment of the diaphragm.
The esophageal aperture transmits the esophagus, the vagus nerves and lymphatic vessels; they pierce the muscular component of the diaphragm, just left to the midline.
The caval aperture is transversed by the inferior vena cava and some branches of the right phrenic nerve; they pass through the central tendon of the diaphragm.
There are other two smaller apertures on each crus, one transmits the greater splanchnic nerve, and the other transmits the lesser splanchnic nerve. Moreover, minute openings for veins may also occur in the central tendon. The crura are two fibroelastic bands that together form the median arcuate ligament. The right crus is broader and longer. It arises from the anterolateral surface of the upper three lumbar vertebrae, while the left crus arises from upper two lumbar vertebrae only.
The arterial supply of the costal margin of the diaphragm is provided by the lower five intercostal and subcostal arteries, while the main central portion of the diaphragm is supplied by the phrenic arteries. These vessels anastomose to ensure a profuse blood supply. The phrenic veins are responsible for the venous drainage of the diaphragm. They follow their corresponding arteries on the inferior thoracic surface. The right phrenic vein empties into the inferior vena cava, while the left phrenic vein drains into either the left renal, suprarenal, or inferior vena cava. The diaphragm receives its motor supply from the phrenic nerves. However, its sensory innervation is provided by the lower six to seven intercostal nerves. The right crus of the diaphragm receives innervation from both the right and left phrenic nerves.