Diaphragm

Parts of the diaphragm and their origins

The muscle bundles of the diaphragm originate from the bony and cartilaginous structures along the perimeter of the inferior thoracic aperture and converge toward the center. Centrally, they weave into a triangular pale plate — the central tendon (centrum tendineum), which anteriorly adheres tightly to the pericardium.

Depending on the site of origin, three muscular parts are distinguished:

1. Sternal part (pars sternalis) — the narrowest part. It originates from the posterior surface of the xiphoid process.
2. Costal part (pars costalis) — the most massive part, forming the lateral walls of the diaphragmatic domes. It originates from the inner surface of the six lower ribs and their cartilages.
3. Lumbar part (pars lumbalis) — attaches to the lumbar vertebrae. Its structure includes crura (crus dextrum et sinistrum) and arcuate ligaments:
4. Right crus — longer and more massive, originates from the anterior surface of the bodies of vertebrae L1–L3 (sometimes up to L4) and the intervertebral discs.
5. Left crus — shorter, originates from L1–L2 (sometimes up to L3).
6. Median arcuate ligament (lig. arcuatum medianum) — a tendinous arch connecting the right and left crura over the aorta.
7. Medial arcuate ligament (lig. arcuatum mediale) — spans from the body of vertebra L1 to its transverse process, overlying the psoas major muscle (m. psoas major).
8. Lateral arcuate ligament (lig. arcuatum laterale) — runs from the transverse process of L1 to the 12th rib, overlying the quadratus lumborum muscle (m. quadratus lumborum).

Openings and weak spots

Large vessels, nerves, and the digestive tube pass through the diaphragm. The three main openings project onto different levels of the thoracic vertebrae.

In addition to the main openings, there are clefts between the muscular bundles of the crura in the diaphragm. Through these pass the splanchnic nerves (nn. splanchnici major et minor), sympathetic trunks, and the v. azygos and v. hemiazygos veins.

Weak spots (triangles):

• Sternocostal triangles (Larrey's clefts / Morgagni's spaces) — paired regions lateral to the sternal part. They serve as sites for retrosternal hernias.
• Lumbocostal triangle (Bochdalek's triangle) — located in the posterolateral region between the lumbar and costal parts. This is a typical site for congenital diaphragmatic hernias in newborns.

Topography and serous coverings

Anatomically, the diaphragm forms two domes. The right dome sits higher than the left because it is supported from below by the massive liver. The left dome is located above the stomach and spleen.

• Thoracic surface is lined by the endothoracic fascia (fascia endothoracica) and covered by the parietal pleura. The lungs lie superior to the diaphragm, and the heart with the pericardium rests centrally.
• Abdominal surface is lined by the subserous fascia and covered by the peritoneum. Exceptions include the so-called bare area of the liver (area nuda), as well as areas where the kidneys, suprarenal glands, and esophagus directly contact the muscle. Inferiorly, the neighboring organs of the muscle are the liver, stomach, spleen, kidneys, suprarenal glands, and colic flexures.

Blood supply, venous drainage, and innervation

Blood supply is provided by systems from the thoracic and abdominal aorta:

• The internal thoracic artery (a. thoracica interna) gives rise to 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 gives rise to the inferior phrenic arteries (aa. phrenicae inferiores).
• The posterior intercostal arteries also contribute.

Blood drainage flows via correspondingly named veins into the inferior vena cava system, the internal thoracic vein, and the azygos vein system (azygos).

Innervation:

• The primary motor nerve is the phrenic nerve (n. phrenicus), formed from branches of the cervical plexus (C3–C5).
• Additional innervation (primarily sensory and partially motor at the periphery) is provided by the intercostal nerves (Th6–Th11) and the subcostal nerve (n. subcostalis, Th12).

Functions and clinical aspects

Main functions:

• Respiratory: upon contraction of the muscle fibers, the domes of the diaphragm descend and flatten. This increases the volume of the thoracic cavity and ensures active inspiration. Upon relaxation, the domes ascend passively — resulting in expiration.
• Hemodynamic (piston pump): the descent of the diaphragm increases intra-abdominal pressure and decreases intrathoracic pressure, which promotes venous return to the inferior vena cava and lymph flow through the thoracic duct.
• Generation of abdominal press: participation in increasing intra-abdominal pressure during coughing, defecation, urination, and childbirth.
• Antireflux (sphincteric): the right crus, forming a loop around the esophagus, acts as a sphincter, preventing gastroesophageal reflux.

Clinical presentation: Lesion of the n. phrenicus leads to diaphragmatic paralysis. On a radiograph, this appears as an elevation of one of the domes; during respiration, paradoxical movements are observed (the dome rises during inspiration). Weakening of the muscular ring around the hiatus oesophageus leads to hiatal hernias.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Anatomy of the arcuate ligaments and their topographic relations with the muscles of the posterior abdominal wall
• Structure of the anti-reflux mechanism in the region of the hiatus oesophageus and the development of hiatal hernias
• Pathogenesis of congenital hernias through the Bochdalek triangle in newborns
• Topographic features of the vagus nerve trunks as they pass through the esophageal hiatus
• Radiological signs of diaphragmatic paralysis and paradoxical breathing

Other topics in the "Myology" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.