The diaphragm is a dome-shaped sheet of skeletal muscle that divides the thoracic cavity from the abdominal cavity.
That may sound simple, but in fact, the diaphragm is so much more than just a sheet of muscle. In fact, every breath you take is thanks to your diaphragm!
Ok, so let’s begin by looking at the general structure of the diaphragm. It curves superiorly into right and left domes.
It has a mobile, central portion known as the central tendon and a peripheral muscular portion that is fixed to the bones, cartilages, and ligaments of the thoracic cage.
The periphery of the diaphragm can be divided into three parts depending on its specific attachment points.
The sternal part, attaches to the posterior aspect of the xiphoid process; the costal part attaches to the internal surfaces of the 7th to the 12th ribs and their costal cartilages; while the lumbar part attaches to the medial and lateral arcuate ligaments, the L1 to L3 vertebral bodies and the intervertebral discs in between.
Now, the diaphragm is the chief muscle for inspiration; meaning that when it contracts, it helps us breathe in. During contraction, the central portion of the diaphragm depresses, increasing the volume of the thoracic cavity, which, in turn, increases the volume in the lungs.
This makes the intrapulmonary pressure fall below the atmospheric pressure, creating a partial vacuum that allows fresh air to be sucked in!
On the other hand, expiration is largely passive, meaning the diaphragm relaxes and rises, which decreases the volume of the thoracic cavity, and subsequently increases the intrapulmonary pressure to be above the atmospheric pressure, forcing the air in the lungs to be expelled out.
The diaphragm also helps with circulation. During contraction, the increased intra-abdominal pressure and decreased intrathoracic pressure helps venous return of blood towards the heart through the inferior vena cava.
In order to properly perform its functions, the diaphragm is attached to surrounding structures by various ligaments and muscular crura. First up, the medial and lateral arcuate ligaments!
The medial arcuate ligament is a thickening of fascia that covers the psoas major muscle which then attaches to the lumbar vertebral bodies and the transverse process of L1; whereas the lateral arcuate ligament covers the quadratus lumborum muscles, and attaches from the transverse process of L1 to the 12th rib.
Next are the crura, which are musculotendinous bands that attach to the anterior portion of the vertebral bodies.
The right crus is larger and longer, and it typically arises from the L1 to L3 vertebrae; while the shorter left crus typically arises from the L1 to L2 vertebrae.
Lastly, the fibrous median arcuate ligament is formed where the right and left crura join in the midline approximately at the level of the T12 vertebra.
The diaphragm also has three major apertures, which give passage to the structures running between the thoracic and the abdominal cavities.
These include the caval opening, which is located in the central tendon of the diaphragm, slightly to the right of the median plane.
The caval opening is located at the level of the T8 vertebra. It serves as a passageway for the inferior vena cava, plus the terminal branches of the right phrenic nerve.
Next is the esophageal hiatus, which is located in the right crus of the diaphragm at the level of the T10 vertebra.