Diaphragm is a muscular and tendinous sheath that closes the opening between thorax and abdomen and is pierced by structures that pass between these two regions of the body.
The diaphragm is the primary muscle of respiration. It is dome shaped and consists of a peripheral muscular part and central tendinous part. The muscular part arises from the margins of the thoracic opening and gets inserted into the central tendon.



Origin of diaphragm:

The origin of the diaphragm can be divided into three parts.

Sternal part: It consists of small left and right strips that arise prom the posterior surface of the xiphoid process.
Costal part: It consists of six slips that arise from the lower six ribs (rib 7 to rib 12) and their costal cartilages.
Vertebral part: It arises by means of vertical columns, also known as crura, and from the arcuate ligaments.

  • Crura: The right crus arises from the bodies of first three lumbar vertebrae and their intervertebral discs. The left crus, on the other hand, arises from the bodies of first two lumbar vertebrae.
  • From arcuate ligaments: Lateral to the crura on both sides, the diaphragm arises from the medial and lateral arcuate ligaments. Medial arcuate ligament is thickened upper margin of fascia that covers the psoas muscle on it anterior surface and the lateral arcuate ligament is thickened upper margin of the fascia covering the quadratus lumborum muscle. The medial arcuate ligament is extended from the side of the body of first lumber vertebra to the tip of the transverse process of second lumber vertebra. The lateral arcuate ligament, in contrast to the medial, extends from the tip of the transverse process of first lumbar vertebra and is inserted into the lower border of the 12th rib.

Insertion of diaphragm:

The diaphragm is inserted into the central tendon that is shaped like three leaves. On the superior side, the surface of the tendon is partially fused with the pericardium.

Shape of diaphragm:

diaphragm position

diaphragm position

In anatomic position, the diaphragm curves into right and left domes (also known as cupolae). The right dome is slightly higher than the left dome and reaches as high as the upper border of the fifth rib. The left dome may reach up to the lower border of the fifth rib. The reason behind the right dome being higher than the left one is probably the larger size of the right lobe of liver. The central tendon, which lies between the two domes, remains at the level of xiphisternal joint. The domes of diaphragm support the right and left lungs while the central tendon provides support to heart.
The position and shape of diaphragm is not absolute but vary with the phase of respiration. During inspiration, the diaphragm lies at a lower level and during expiration, it rises to a higher level. The position also depends upon the posture of body (lower when a person is sitting or standing and higher in supine position) and degree of distention of the abdominal viscera.

Openings in the diaphragm:

All the structures that pass from the thorax into the abdomen or rise from abdomen into the thorax pass through the diaphragm. For this reason, diaphragm contains numerous openings. Besides many small openings, there are three major ones as described below.
Aortic opening: This opening transmits aorta, thoracic duct and azygous vein and lies anterior to the body of 12th thoracic vertebra between the crura.
Esophageal opening: It transmits esophagus, left and right vagus nerves, esophageal branches of the left gastric vessels and lymphatics from lower third of the esophagus. It lies at the level of 12th thoracic vertebra in a sling of muscle fibers derived from the right crus.
Caval opening: It transmits the inferior vena cava and terminal branches of right phrenic nerve. It lies at the level of 8th thoracic vertebra.
Other minor openings: Besides the three major openings, the diaphragm contains numerous small openings for various structures. Some of the structures that have their own small openings in the diaphragm are:

  • Sympathetic trunk (pass posterior to the medial arcuate ligament on both sides)
  • Superior epigastric vessels (pass between the sterna and costal origins of the diaphragm on each side)
  • Left phrenic nerve (pierces the left dome of diaphragm)
  • Neurovascular bundles of lower six intercostal spaces (pass between the muscular slips of costal origin of diaphragm)

Action of diaphragm:

Diaphragm is the primary respiratory muscle of the body. It, on contraction, increases the vertical diameter of the chest cavity by pulling its central tendon downwards. In this way, the lungs get extra space to expand and air rushes into them.

Nerve supply of diaphragm:

The motor nerve supply of diaphragm is only from the phrenic nerve. The sensory supply of the central tendon of diaphragm that is covered by parietal and peritoneal pleura is from phrenic nerve. Sensory supply to the periphery of diaphragm is from lower six intercostal nerves.

Functions of diaphragm:

Muscle of inspiration: Diaphragm is the primary muscle of inspiration and holds the most important value for the overall process of exchange of gases.
Muscle of abdominal straining:
With the aid of a contracted diaphragm, muscles of the anterior abdominal wall can raise the intra-abdominal pressure to evacuate pelvic contents, which is important for micturition, defectaion and parturition. The role of diaphragm is further aided when the person takes a deep breath and closes the glottis of the larynx. Consequently, the diaphragm is unable to rise because of the air trapped in the respiratory tract and the abdominal muscles can create increased abdominal pressure. During the process, whenever air escapes, a grunting sound is produced.
Muscle of weight-lifting:
By fixing the diaphragm in the same way as described above, it is possible to raise the intra-abdominal pressure to such an extent that it helps support the vertebral column and prevent flexion. This greatly assists postvertebral muscles in lifting of heavy weights.
Thoraco-abdominal pump of blood flow:
The descent of diaphragm into the abdomen decreases the intra-throacic pressure while increasing the intra-abdominal pressure at the same time. This pressure gradient causes the blood to flow upward into the heart. Lymph within the lymph vessels is also forced to move in a similar way.