Diaphragm

The diaphragm is a double-domed, musculotendinous partition separating the thoracic and abdominal cavities. Its mainly convex superior surface faces the thoracic cavity, and its concave inferior surface faces the abdominal cavity. The diaphragm is the chief muscle of inspiration (actually, of respiration altogether, because expiration is largely passive). It descends during inspiration; however, only its central part moves because its periphery, as the fixed origin of the muscle, attaches to the inferior margin of the thoracic cage and the superior lumbar vertebrae.

The pericardium, containing the heart, lies on the central part of the diaphragm, depressing it slightly. The diaphragm curves superiorly into right and left domes; normally the right dome is higher than the left dome owing to the presence of the liver. During expiration, the right dome reaches as high as the 5^th rib and the left dome ascends to the 5^th intercostal space. The level of the domes of the diaphragm varies according to the:

• Phase of respiration (inspiration or expiration).

• Posture (e.g., supine or standing).

• Size and degree of distension of the abdominal viscera.

The muscular part of the diaphragm is situated peripherally with fibers that converge radially on the trifoliate central aponeurotic part, the central tendon. The central tendon has no bony attachments and is incompletely divided into three leaves, resembling a wide cloverleaf. Although it lies near the center of the diaphragm, the central tendon is closer to the anterior part of the thorax.

The caval opening (vena caval foramen), through which the terminal part of the IVC passes to enter the heart, perforates the central tendon. The surrounding muscular part of the diaphragm forms a continuous sheet; however, for descriptive purposes it is divided into three parts, based on the peripheral attachments:

• Sternal part: consisting of two muscular slips that attach to the posterior aspect of the xiphoid process; this part is not always present.

• Costal part: consisting of wide muscular slips that attach to the internal surfaces of the inferior six costal cartilages and their adjoining ribs on each side; the costal parts form the right and left domes.

• Lumbar part: arising from two aponeurotic arches, the medial and lateral arcuate ligaments, and the three superior lumbar vertebrae; the lumbar part forms right and left muscular crura that ascend to the central tendon.

The crura of the diaphragm are musculotendinous bands that arise from the anterior surfaces of the bodies of the superior three lumbar vertebrae, the anterior longitudinal ligament, and the IV discs. The right crus, larger and longer than the left crus, arises from the first three or four lumbar vertebrae. The left crus arises from the first two or three lumbar vertebrae. Because it lies to the left of the midline, it is surprising to find that the esophageal hiatus is a formation in the right crus; however, if the muscular fibers bounding each side of the hiatus are traced inferiorly, it will be seen that they pass to the right of the aortic hiatus.

The right and left crura and the fibrous median arcuate ligament, which unites them as it arches over the anterior aspect of the aorta, form the aortic hiatus. The diaphragm is also attached on each side to the medial and lateral arcuate ligaments. The medial arcuate ligament is a thickening of the fascia covering the psoas major, spanning between the lumbar vertebral bodies and the tip of the transverse process of L₁. The lateral arcuate ligament covers the quadratus lumborum muscles, continuing from the L₁₂ transverse process to the tip of the 12^th rib.

The superior aspect of the central tendon of the diaphragm is fused with the inferior surface of the fibrous pericardium, the strong, external part of the fibroserous pericardial sac that encloses the heart.

Vessels and Nerves of Diaphragm

The arteries of the diaphragm form a branch-like pattern on both its superior (thoracic) and inferior (abdominal) surfaces. The arteries supplying the superior surface of the diaphragm are the pericardiacophrenic and musculophrenic arteries, branches of the internal thoracic artery, and the superior phrenic arteries, arising from the thoracic aorta. The arteries supplying the inferior surface of the diaphragm are the inferior phrenic arteries, which typically are the first branches of the abdominal aorta; however, they may arise from the celiac trunk.

The veins draining the superior surface of the diaphragm are the pericardiacophrenic and musculophrenic veins, which empty into the internal thoracic veins and, on the right side, a superior phrenic vein, which drains into the IVC. Some veins from the posterior curvature of the diaphragm drain into the azygos and hemi-azygos veins. The veins draining the inferior surface of the diaphragm are the inferior phrenic veins. The right inferior phrenic vein usually opens into the IVC, whereas the left inferior phrenic vein is usually double, with one branch passing anterior to the esophageal hiatus to end in the IVC and the other, more posterior branch usually joining the left suprarenal vein. The right and left phrenic veins may anastomose with each other.

The lymphatic plexuses on the superior and inferior surfaces of the diaphragm communicate freely. The anterior and posterior diaphragmatic lymph nodes are on the superior surface of the diaphragm. Lymph from these nodes drains into the parasternal, posterior mediastinal, and phrenic lymph nodes. Lymphatic vessels from the inferior surface of the diaphragm drain into the anterior diaphragmatic, phrenic, and superior lumbar (caval/aortic) lymph nodes. Lymphatic capillaries are dense on the inferior surface of the diaphragm, constituting the primary means for absorption of peritoneal fluid and substances introduced by intraperitoneal (I.P.) injection.

The entire motor supply to the diaphragm is from the right and left phrenic nerves, each of which arises from the anterior rami of C₃-C₅ segments of the spinal cord and is distributed to the ipsilateral half of the diaphragm from its inferior surface. Sensory innervation (pain and proprioception) to the diaphragm is also mostly from the phrenic nerves. Peripheral parts of the diaphragm receive their sensory nerve supply from the intercostal nerves (lower six or seven) and the subcostal nerves.

Diaphragmatic Apertures

The diaphragmatic apertures (openings, hiatus) permit structures (vessels, nerves, and lymphatics) to pass between the thorax and abdomen. There are three large apertures for the IVC, esophagus, and aorta and a number of small ones.

Caval Opening

The caval opening is an aperture in the central tendon primarily for the IVC. Also passing through the caval opening are terminal branches of the right phrenic nerve and a few lymphatic vessels on their way from the liver to the middle phrenic and mediastinal lymph nodes. The caval opening is located to the right of the median plane at the junction of the central tendon's right and middle leaves. The most superior of the three large diaphragmatic apertures, the caval opening lies at the level of the IV disc between the T₈ and T₉ vertebrae. The IVC is adherent to the margin of the opening; consequently, when the diaphragm contracts during inspiration, it widens the opening and dilates the IVC. These changes facilitate blood flow through this large vein to the heart.

Esophageal Hiatus

The esophageal hiatus is an oval opening for the esophagus in the muscle of the right crus of the diaphragm at the level of the T₁₀ vertebra. The esophageal hiatus also transmits the anterior and posterior vagal trunks, esophageal branches of the left gastric vessels, and a few lymphatic vessels. The fibers of the right crus of the diaphragm decussate (cross one another) inferior to the hiatus, forming a muscular sphincter for the esophagus that constricts it when the diaphragm contracts. The esophageal hiatus is superior to and to the left of the aortic hiatus. In most individuals (70%), both margins of the hiatus are formed by muscular bundles of the right crus. In others (30%), a superficial muscular bundle from the left crus contributes to the formation of the right margin of the hiatus.

Aortic Hiatus

The aortic hiatus is the opening posterior in the diaphragm for the descending aorta. Because the aorta does not pierce the diaphragm, movements of the diaphragm do not affect blood flow through the aorta during respiration. The aorta passes between the crura of the diaphragm posterior to the median arcuate ligament, which is at the level of the inferior border of the T₁₂ vertebra. The aortic hiatus also transmits the thoracic duct and sometimes the azygos and hemi-azygos veins.

Small Openings in Diaphragm

In addition to the three main apertures, there is a small opening, the sternocostal triangle (foramen), between the sternal and costal attachments of the diaphragm. This triangle transmits lymphatic vessels from the diaphragmatic surface of the liver and the superior epigastric vessels. The sympathetic trunks pass deep to the medial arcuate ligament, accompanied by the least splanchnic nerves. There are two small apertures in each crus of the diaphragm; one transmits the greater splanchnic nerve and the other the lesser splanchnic nerve.

Actions of Diaphragm

When the diaphragm contracts, its domes are pulled inferiorly so that the convexity of the diaphragm is somewhat flattened. Although this movement is often described as the "descent of the diaphragm," only the domes of the diaphragm descend. The diaphragm's periphery remains attached to the ribs and cartilages of the inferior six ribs. As the diaphragm descends, it pushes the abdominal viscera inferiorly. This increases the volume of the thoracic cavity and decreases the intrathoracic pressure, resulting in air being taken into the lungs. In addition, the volume of the abdominal cavity decreases slightly and intra-abdominal pressure increases somewhat.

Movements of the diaphragm are also important in circulation because the increased intra-abdominal pressure and decreased intrathoracic pressure help return venous blood to the heart. When the diaphragm contracts, compressing the abdominal viscera, blood in the IVC is forced superiorly into the heart.

The diaphragm is at its most superior level when a person is supine (with the upper body lowered, the Trendelenburg position). In this position, the abdominal viscera push the diaphragm superiorly in the thoracic cavity. When a person lies on one side, the hemidiaphragm rises to a more superior level because of the greater push of the viscera on that side. Conversely, the diaphragm assumes an inferior level when a person is sitting or standing. For this reason, people with dyspnea (difficult breathing) prefer to sit up, not lie down; non-tidal (reserve) lung volume is increased, and the diaphragm is working with gravity rather than opposing it.