Diaphragm (Diaphragma) is a dome-shaped sheet of muscle and tendon that separates the thoracic cavity, containing the lungs and heart, from the abdominal cavity below. It forms the floor of the thorax, or chest, and is the principal muscle of inspiration, the movement that draws air into the lungs.
Where is the diaphragm located?
The diaphragm lies across the lower opening of the chest, attaching around its margins and rising centrally into the thoracic cavity. Its position changes during breathing: it descends during inspiration and rises during expiration, the movement that allows air to leave the lungs.
Its right dome usually lies higher than its left dome, largely because the liver lies beneath it. The stomach and spleen lie beneath the left dome, while the kidneys and adrenal glands are related to its posterior abdominal surface.
Above the diaphragm, the bases of the lungs rest against its domes. The pleura, the membrane surrounding each lung, covers much of its upper surface. The fibrous pericardium, the tough outer sac surrounding the heart, attaches to its central tendon.
What are the parts and surfaces of the diaphragm?
The diaphragm has a peripheral muscular portion and a central tendon, a broad sheet of strong connective tissue. Muscle fibres converge towards this tendon from attachments around the lower chest and upper spine.
The muscular portion is divided according to its attachments:
- Sternal part: arises from the back of the xiphoid process, the small lower projection of the breastbone.
- Costal part: arises from the inner surfaces of the lower six ribs and their costal cartilages, the cartilage at their anterior ends.
- Lumbar part: arises from the upper lumbar spine and arched ligamentous attachments across the posterior abdominal wall.
Its thoracic surface faces the lungs and heart. Its abdominal surface faces the abdominal organs and is largely covered by peritoneum, the membrane lining the abdominal cavity and covering many of its organs.
Where does the diaphragm originate and insert?
For a muscle, the origin and insertion are its attachment sites. The diaphragm originates around the lower chest and upper lumbar spine and inserts into the central tendon. The central tendon does not attach directly to bone.
At the chest wall, attachments include the inner surface of a lower rib and the adjacent costal cartilage. The costal cartilages of ribs 7–10 form the costal margin, the curved lower border of the front of the chest.
Posteriorly, the crura are muscular and tendinous pillars attached to the upper lumbar vertebral bodies and intervening discs. The body of a lumbar vertebra is its large, weight-bearing front portion; a vertebra is a bone of the spine. The right crus is longer than the left.
The arcuate ligaments are arched bands that provide additional attachment. The medial arcuate ligament arches over psoas major, a muscle beside the lumbar spine, and the lateral arcuate ligament arches over quadratus lumborum, a muscle of the posterior abdominal wall.
How does the diaphragm work during breathing?
When its muscle fibres contract, the diaphragm flattens and the central tendon descends. This increases the vertical dimension of the thoracic cavity. Pressure within the lungs falls relative to atmospheric pressure, allowing air to enter through the airways.
During quiet expiration, the diaphragm relaxes. Elastic recoil of the lungs and chest wall helps reduce thoracic volume, and the diaphragm returns towards its domed resting shape.
The diaphragm works with other breathing muscles rather than in isolation. Its movement also changes abdominal pressure, helping coordinate the chest and abdomen during actions such as coughing and straining.
What passes through the diaphragm?
Structures travelling between the chest and abdomen pass through openings in, or behind, the diaphragm. The main openings have characteristic positions relative to a thoracic vertebra, a vertebra in the chest region of the spine. The labels T8, T10 and T12 identify these spinal levels.
| Opening | Usual vertebral level | Main structures passing through |
|---|---|---|
| Caval opening | T8 | Inferior vena cava, the large vein returning blood from below the diaphragm |
| Oesophageal hiatus | T10 | Oesophagus, the tube carrying swallowed material to the stomach, and vagal trunks |
| Aortic hiatus | T12 | Aorta, the main artery carrying blood from the heart, and thoracic duct |
A hiatus is an opening or gap. The caval opening lies in the central tendon, whereas the oesophageal hiatus lies mainly within fibres of the right crus. The vagal trunks are bundles of vagus nerve fibres accompanying the oesophagus.
The aortic hiatus lies behind the diaphragm beneath the median arcuate ligament, an arch joining the crura. The thoracic duct is the main channel returning lymph, fluid collected from tissues, to the venous circulation.
What nerve supplies the diaphragm?
The phrenic nerve supplies motor innervation, meaning the nerve signals that cause the diaphragm to contract. It arises from spinal nerves C3–C5 in the neck and descends through the chest to reach the diaphragm.
The phrenic nerve also carries sensation from the central diaphragm and its associated membranes. Lower intercostal nerves, which run between the ribs, and the subcostal nerve, which runs below the lowest rib, supply sensation to the peripheral diaphragm.
What is the diaphragm’s blood supply?
The inferior phrenic arteries provide much of the diaphragm’s arterial supply from below. Superior phrenic arteries supply its posterior upper surface, while the pericardiacophrenic artery and musculophrenic artery contribute from above and along its margins.
Venous drainage broadly accompanies the arteries. The superior phrenic vein drains the upper surface, while inferior phrenic veins drain towards major abdominal veins.
How can you find the diaphragm in the 3D model?
Start at the lower chest and identify the boundary between the thoracic and abdominal regions. Use the lower ribs, xiphoid process and upper lumbar spine as landmarks for its peripheral attachments.
In the 3D model beside this article, select the diaphragm and isolate it, then rotate and zoom to examine its overall shape. Switch layers on and off to compare its position with surrounding structures.