The diaphragm is a double-dome shaped skeletal muscle that covers the inferior thoracic aperture, separating the thorax from the abdomen. It is the main respiratory muscle in the human body.

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

The diaphragm forms two domes. The right dome is higher than the left, it can reach as far as 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 increases the volume of the lungs, providing inspiration and expiration movements.

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

Caval foramen - for the inferior vena cava, also 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 their corresponding veins;
Aortic hiatus - for the descending thoracic aorta, thoracic duct and azygos vein.

The diaphragm is innervated by two phrenic nerves on each side that arise as branches from the cervical plexus in the neck. They originate from the anterior rami of cervical nerves C3, C4 and C5, with the majority of the fibers arising from C4.

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Diaphragm