Parts of the Diaphragm and Their Origins
The muscular bundles of the diaphragm originate from the bony and cartilaginous structures along the perimeter of the inferior thoracic aperture and converge centrally. Centrally, they blend into a pale, triangular tendinous plate known as the central tendon (centrum tendineum), which is intimately related anteriorly to the pericardium.
Depending on their site of origin, three muscular parts are distinguished:
- Sternal part (pars sternalis) — the narrowest portion, originating from the posterior surface of the xiphoid process.
- Costal part (pars costalis) — the most massive part, forming the lateral walls of the diaphragmatic domes. It arises from the inner surface of the lower six ribs and their costal cartilages.
- Lumbar part (pars lumbalis) — anchors the diaphragm to the lumbar vertebrae. It consists of crura (crus dextrum et sinistrum) and arcuate ligaments:
- Right crus is longer and bulkier, arising from the anterior surface of the bodies of L1–L3 vertebrae (sometimes extending to L4) and their intervertebral discs.
- Left crus is shorter, arising from L1–L2 vertebrae (sometimes extending to L3).
- Median arcuate ligament (lig. arcuatum medianum) — a tendinous arch spanning the left and right crura anterior to the aorta.
- Medial arcuate ligament (lig. arcuatum mediale) — arches from the body of the L1 vertebra to its transverse process, overlying the psoas major muscle (m. psoas major).
- Lateral arcuate ligament (lig. arcuatum laterale) — extends from the transverse process of the L1 vertebra to the 12th rib, overlying the quadratus lumborum muscle (m. quadratus lumborum).
Openings and Weak Points
Major blood vessels, nerves, and the alimentary canal pass through the diaphragm. The three primary apertures project onto different thoracic vertebral levels.
| Structure | Level | Description and Contents |
|---|---|---|
| Opening for the inferior vena cava (foramen venae cavae) | Th8 | Located within the central tendon. The fibrous margins do not constrict during inspiration, ensuring unobstructed venous return. Transmits the v. cava inferior and (frequently) branches of the right n. phrenicus. |
| Esophageal hiatus (hiatus oesophageus) | Th10 | Formed by muscular bundles of the right crus (providing a sphincteric function). Transmits the esophagus (oesophagus), vagal trunks (trunci vagales), branches of the a. gastrica sinistra, veins, and lymphatic vessels. |
| Aortic hiatus (hiatus aorticus) | Th12 | Located posterior to the diaphragm, deep to the lig. arcuatum medianum. Transmits the aorta, thoracic duct (ductus thoracicus), and occasionally the azygos or hemiazygos veins. |
In addition to the main apertures, clefts exist between the muscular bundles of the crura. These transmit the splanchnic nerves (nn. splanchnici major et minor), sympathetic trunks, and the azygos and hemiazygos veins.
Weak Points (Triangles):
- Sternocostal triangles (Larrey’s cleft / spaces of Morgagni) — paired regions lateral to the sternal part. These serve as potential sites for retrosternal hernias.
- Lumbocostal triangle (Bochdalek’s triangle) — located posterolaterally between the lumbar and costal parts. This is the classic site for congenital diaphragmatic hernias in neonates.
Topography and Serous Coverings
Anatomically, the diaphragm forms two domes. The right dome sits higher than the left because it is supported inferiorly by the massive liver. The left dome lies superior to the stomach and spleen.
- Thoracic surface is lined by the endothoracic fascia (fascia endothoracica) and covered by the parietal pleura. Superiorly, the lungs relate to the diaphragm, while the heart and its pericardium rest centrally.
- Abdominal surface is lined by subserous fascia and covered by the peritoneum, with notable exceptions such as the bare area of the liver (area nuda), as well as regions where the kidneys, suprarenal glands, and esophagus directly abut the muscle. Inferiorly, the visceral neighbors include the liver, stomach, spleen, kidneys, suprarenal glands, and colonic flexures.
Blood Supply, Venous Drainage, and Innervation
Blood supply is derived from branches of both the thoracic and abdominal aorta:
- The internal thoracic artery (a. thoracica interna) gives off the pericardiacophrenic (a. pericardiacophrenica) and musculophrenic (a. musculophrenica) arteries.
- The thoracic aorta gives rise to the superior phrenic arteries (aa. phrenicae superiores), while the abdominal aorta supplies the inferior phrenic arteries (aa. phrenicae inferiores).
- Posterior intercostal arteries also contribute collateral branches.
Venous blood drains via corresponding veins into the inferior vena cava, internal thoracic vein, and the azygos venous system.
Innervation:
- The primary motor nerve is the phrenic nerve (n. phrenicus), arising from the cervical plexus (segments C3–C5).
- Additional innervation (primarily sensory and partially peripheral motor) is provided by the intercostal nerves (Th6–Th11) and the subcostal nerve (n. subcostalis, Th12).
Functions and Clinical Aspects
Main Functions:
- Respiratory: Contraction of the muscular fibers causes the domes of the diaphragm to descend and flatten, increasing the volume of the thoracic cavity and driving active inspiration. Relaxation allows the domes to passively recoil upward, producing expiration.
- Hemodynamic (piston action): Descent of the diaphragm increases intra-abdominal pressure while decreasing intrathoracic pressure, facilitating venous return to the inferior vena cava and lymph flow through the thoracic duct.
- Abdominal press: Contributes to the generation of intra-abdominal pressure during coughing, defecation, micturition, and childbirth.
- Antireflux (sphincteric): The right crus forms a muscular loop around the esophagus that functions as a physiological sphincter, preventing gastroesophageal reflux.
- Clinical Aspects:
- Injury to the n. phrenicus results in diaphragmatic paralysis. On radiography, this appears as an abnormally high elevation of the affected dome, exhibiting paradoxical movement during respiration (the paralyzed dome moves upward during inspiration). Weakening of the muscular ring around the hiatus oesophageus leads to hiatal hernias.