The diaphragm is a musculo-tendinous sheet that separates the abdominal cavity from thoracic contents. It is also an important muscle of respiration.
The diaphragm has a continuous, peripheral attachment on each side, from the back of the xiphoid process, along the lower six costal cartilages, lateral and medial arcuate ligaments and 12th rib, to the vertebral column, where it attaches by two crura. The left crus is short and reaches down to the 2nd lumbar vertebra, whereas the right crus is more prominent and reaches the 3rd or 4th lumbar vertebra. Its fibres surround the esophageal hiatus to reinforce the gastroesophageal sphincter. The medial fibres of the crura join each other across the midline, to form the median arcuate ligament, which bridges the aortic hiatus. From the extensive origin outlined above, the muscle fibres converge to insert into the central tendon, a strong aponeurosis situated immediately below the fibrous pericardium to which it is attached.
The main arterial supply of the diaphragm is from:
- Superiorly, branches of the internal thoracic artery
- Inferiorly, the inferior phrenic artery (branch of the aorta)
The venous drainage is through corresponding veins.
Innervation of the diaphragm happens mainly through the phrenic nerves (C3-C5), which are its sole motor supply and are sensory to its central portion. The lower intercostal nerves supply sensory fibres to the diaphragm's peripheral region, while the sympathetic trunks enter the abdomen behind the medial arcuate ligaments. The thoracic splanchnic nerves pierce the crura of the diaphragm.
Structures passing through the diaphragm do so either via its three main openings - caval, esophageal and aortic, though they can also pass through the diaphragm independently. The caval opening is the most anterior, located in the central tendon at about the T8 vertebral level, to the right of the midline. Through it pass the inferior vena cava and right phrenic nerve.
The esophageal hiatus lays at the level of the T10 vertebra, to the left of the midline, an opening in the right crus of the diaphragm. In addition to the esophagus, it gives passage to the vagal trunks and esophageal branches of the left gastric vessels.