The diaphragm is a double dome-shaped skeletal muscle covering the inferior thoracic aperture, separating the thorax from the abdomen. It is one of the most important muscles of respiration and the main muscle of inspiration.

The diaphragm has two parts - the outer is called the muscular part, while the inner is known as the central tendon.

The muscular part of the diaphragm is divided into smaller subdivisions - sternal, costal, and lumbar parts.

The sternal part originates from the posterior surface of the xiphoid process of the sternum.
The costal part originates from the inner surfaces of the costal cartilages and the 7th to 12th ribs
The lumbar part has two crura, and their origin is described below.

Muscle fibers of the diaphragm arise from the margins of the inferior thoracic aperture (from the walls of the thoracic and abdominal cavities), pass radially, and converge into a large central tendon. The posterior attachment of the diaphragm is lower than the anterior attachment.

Two tendinous structures called crura (singular: crus) extend from the muscular part below the diaphragm to the vertebral column.

The right crus arises from the anterior surface of the first to third lumbar vertebrae (L1-L3), and their intervertebral discs and some of its fibers surround the esophageal hiatus and act as a sphincter. The left crus arises from the first to second lumbar vertebrae (L1-L2) and their intervertebral discs. Also, both crura originate from two ligaments named the medial and lateral arcuate ligaments.

The medial arcuate ligament stretches between the body and transverse process of the first lumbar vertebra (L1), and arches across the psoas major muscle.
The lateral arcuate ligament extends between the transverse process of the first lumbar vertebra and the tip or the lower margin of the 12th rib, arching across the quadratus lumborum muscle.

The diaphragm forms two domes. The right dome lies higher than the left because of the liver located below it. The right dome reaches as far as the 4th rib. The highest point of the left dome is located at the level of the 5th rib.

As the muscle contracts, the domes descend, and the volume of the thoracic cavity increases. The contraction and relaxation of the diaphragm thus increase the volume of the lungs, providing inspiration and expiration movements.

The diaphragm has two surfaces - thoracic and abdominal - as it faces the thoracic and abdominal cavities.

The thoracic surface is covered by the diaphragmatic part of the parietal pleura, as well as it contacts with the fibrous layer of the pericardium.
The abdominal surface has a connection with the liver, spleen, as well as with the peritoneum and stomach.

Structures that travel between the thorax and abdomen pierce the diaphragm. There are three openings that serve as passages for such structures:

Caval foramen (also known as the caval hiatus) - it serves for the passage of the inferior vena cava; also it serves as a conduit for terminal branches of the right phrenic nerve;
Esophageal hiatus - for the esophagus, right and left vagus nerves (CN X), and esophageal branches of the left gastric artery together with its corresponding veins;
Aortic hiatus - it is an opening for the descending aorta, thoracic duct, and azygos vein.

Neurovascular supply

The diaphragm receives arterial blood supply from the:

Subcostal artery - arises from the thoracic aorta;
Lower five intercostal arteries
Inferior phrenic arteries - from the abdominal aorta or coeliac trunk;
Superior phrenic arteries - from the thoracic aorta;
Musculophrenic artery - from the internal thoracic artery.

The venous drainage is provided by the:

Brachiocephalic veins
Azygos vein
Tributaries of the inferior vena cava
Tributaries of the left suprarenal vein

The motor and sensory innervation for the diaphragm is mainly provided by phrenic nerves on each side. They are branches of the cervical plexus - a network of nerves in the neck. The phrenic nerves originate from the anterior rami of cervical nerves C3, C4, and C5, with the majority of the fibers come from the C4.

Additionally, the muscular part of the diaphragm receives its sensory innervation from the 6th to 11th intercostal nerves.

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Diaphragm