General structure and parts
The diaphragmatic dome is divided into two vaults during expiration due to the lower position of its central part. The right vault sits higher than the left because the liver lies snugly against its inferior surface. The thickness of the muscular plate varies: in the costal region, it is 0.3 to 0.5 cm, while in the tendinous center, it is about 0.3 cm.
Anatomically, the diaphragm consists of a central connective tissue part and a peripheral muscular part. The tendinous center (centrum tendineum diaphragmatis) lacks muscle fibers, resembles a trefoil in shape, and occupies the middle of the dome.
The muscular part (pars muscularis) is divided into three sections depending on their skeletal attachment points:
- Sternal part (pars sternalis): originates from the posterior surface of the lower margin of the xiphoid process and partially from the aponeurosis of the rectus abdominis muscle.
- Costal part (pars costalis): forms the lateral slopes of the dome. It originates on the inner surface of the cartilages of the six lower ribs and partially from the bone of the 3rd–4th lower ribs.
- Lumbar part (pars lumbalis): consists of right and left muscular crura (crura diaphragmatis). It originates from the anterior surface of the bodies of the I–IV lumbar vertebrae and dense tendinous cords — the medial and lateral arcuate ligaments. The medial ligament (lig. arcuatum mediale) arches over the psoas major muscle, and the lateral ligament (lig. arcuatum laterale) arches over the quadratus lumborum muscle.
Main openings of the diaphragm
Vital blood vessels, nerves, and digestive organs pass through the diaphragm. Specialized openings with distinct edge structures are provided for them:
- Aortic hiatus (hiatus aorticus). Located at the level of the first lumbar vertebra, slightly to the left of the midline. Formed by the internal tendinous margins of the diaphragmatic crura. Its structural feature is that the margins are tendinous, which prevents compression of the aorta during muscle contraction. The aorta itself and the thoracic duct (ductus thoracicus) pass through it.
- Esophageal hiatus (hiatus esophageus). Located superiorly and to the left of the aortic hiatus. Unlike the former, its margins are formed by muscular bundles of the crura. These bundles cross each other, acting as a sphincter (valve) for the esophagus. The esophagus and vagus nerves (trunci vagales) pass through here. This opening is a frequent site for diaphragmatic hernias (protrusion of the cardiac part of the stomach).
- Vena caval opening (foramen venae cavae). Located directly within the tendinous center. The adventitia of the inferior vena cava is firmly fused with the margins of the opening by connective tissue strands. This fixation keeps the lumen of the vein permanently open, which is critical for ensuring normal venous return.
"Weak points" (anatomical triangles)
There are areas in the structure of the diaphragm where muscle bundles are completely absent. In these zones, only the endothoracic fascia (fascia endothoracica) and the extraperitoneal/endoabdominal fascia are in contact. These areas are called "weak points" because diaphragmatic hernias most frequently occur here, and abscesses can potentially break through from one cavity into another.
- Sternocostal triangle (Larrey's slit or Morgagni's triangle). Located between the sternal and costal parts of the muscle, and is more clearly pronounced on the left side. The internal thoracic vessels pass through this space.
- Lumbocostal triangle (Bochdalek's triangle). Located between the lumbar and costal parts. Its outer boundary is the lumbar part, and its upper boundary is the 12th rib (the length of the triangle's base along the rib is about 3–4 cm).
Topography and attachment line
The attachment line of the diaphragm to the thoracic cage runs along the lower margin of the xiphoid process, then proceeds 1–2 cm above and strictly parallel to the lower margin of the costal arch, continuing along the 12th rib and the bodies of the III–IV lumbar vertebrae.
The projection of the domes onto the chest wall shows asymmetry:
- Right dome: anteriorly projected at the level of the 4th intercostal space.
- Left dome: anteriorly located at the level of the upper border of the 5th rib (lower than the right).
- Posteriorly: both domes are projected at the level of the 9th intercostal space.
The height of the diaphragmatic standing position is variable and depends on several factors. First, on the shape of the chest: with a short and wide chest, the dome is located high; with a long and narrow chest, it is low. Second, flattening of the dome occurs with age. Third, topography is influenced by pathological processes — for instance, the accumulation of exudate in the pleural or abdominal cavities leads to significant flattening of the muscular vault.
Briefly: answers to topical questions
At the level of which vertebra is the opening of the inferior vena cava located?
The opening of the inferior vena cava (foramen venae cavae inferioris) is located at the level of the eighth thoracic vertebra (Th VIII). It lies directly within the tendinous center of the diaphragm. Because the opening is localized in the tendinous part, the adventitia (wall) of the vein is tightly fused to its margins by connective tissue strands. Due to this anatomical feature, when the diaphragm contracts, the vein is not compressed but rather dilated, which keeps the lumen open and facilitates venous blood return to the heart.
What are the anatomical boundaries of the lumbocostal triangle (Bochdalek)?
The lumbocostal triangle (trigonum lumbocostale), also known as Bochdalek's triangle, is located between the costal and lumbar parts of the diaphragm. Its anatomical boundaries include:
- External boundary — lumbar part of the diaphragm.
- Upper boundary (base) — 12th rib.
The length of the triangle's base along the 12th rib is 3–4 cm. In this weak spot, muscle bundles are absent, and only the endothoracic and intra-abdominal fascias are in contact (the content consists of adipose tissue).
Which nerves provide innervation to the diaphragm?
The diaphragm is innervated by the phrenic and lower intercostal nerves.
- Phrenic nerve (n. phrenicus) is the main source of innervation. It is a mixed nerve formed from the cervical plexus (roots C3–C5). It contains motor fibers to the diaphragm muscle, as well as sensory and sympathetic fibers.
- Lower intercostal nerves (nn. intercostales VI–XII) provide additional sensory innervation to the peripheral parts of the diaphragm.
Irritation of the main nerve can manifest as pain radiating to the shoulder and neck ("phrenic sign").
Which arteries supply blood to the diaphragm?
The blood supply to the diaphragm is explicitly indicated in sources for the inferior phrenic arteries (aa. phrenicae inferiores): they supply the diaphragm from below.
Through which clefts in the lumbar part of the diaphragm do the sympathetic trunk and splanchnic nerves pass?
The sympathetic trunk and splanchnic nerves pass through the intermuscular clefts of the crura of the lumbar part of the diaphragm. Sources do not provide special eponymous names for these openings; they are described as follows:
- Intermuscular clefts of the lumbar part — transmit the sympathetic trunks (trunci sympathici), which pass lateral to the vertebral bodies, as well as the splanchnic nerves.
- Cleft of the diaphragmatic crura — through it, the splanchnic nerves (nn. splanchnici major et minor) travel from the thoracic ganglia to the celiac plexus ganglia.
Specifically, which vessels pass through the sternocostal triangle (Larrey's slit)?
The internal thoracic vessels pass through the sternocostal triangle (Larrey's slit or Morgagni's triangle).
- Internal thoracic vessels (vasa thoracica interna) are the only specified contents of this weak point.
This triangle is located between the sternal and costal parts of the diaphragm (it is more distinct on the left). In this zone, muscle bundles are absent, and only the endothoracic and intra-abdominal fascias are in contact, making it a potential site for the emergence of diaphragmatic hernias.
Class and exam questions
Why are the margins of the aortic hiatus tendinous while those of the esophageal hiatus are muscular?
The tendinous margins of the aortic hiatus do not contract, which protects the aorta from compression and ensures uninterrupted blood flow. Conversely, the muscular margins of the esophageal hiatus function as a sphincter, preventing the backflow of food from the stomach.
Where are diaphragmatic hernias most frequently localized?
The most vulnerable areas are the esophageal hiatus (through which the cardiac part of the gastroesophageal junction frequently herniates), as well as the "weak spots": the sternocostal triangle (Larrey's slit) and the lumbocostal triangle (Bochdalek's triangle).
Why is the caval opening located in the tendinous center?
The location within the dense tendinous center and the fusion of the venous adventitia with the edges of the opening prevent the vein from collapsing during respiratory excursions, guaranteeing a stable blood outflow to the heart.
What else to study on this topic
- Contents of the intermuscular clefts of the lumbar part (splanchnic nerves, sympathetic trunks, azygos and hemiazygos veins)
- Innervation and blood supply of the diaphragm (phrenic nerves and arteries)
- Topography of the thoracic cavity above the diaphragm (pleural sacs and mediastinum)
- Structure and functions of the endothoracic fascia (fascia endothoracica)
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