The Thoracic Diaphragm

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The Thoracic Diaphragm

• The thoracic diaphragm is a dome-shaped musculotendinous partition between the thoracic and abdominal cavities.
• The convexity of its dome bulges into the thoracic cavity during expiration.
• The diaphragm is the principal muscle of respiration and forms the floor of the thoracic cavity and the roof of the abdominal cavity.
• During respiration, it descends as it contracts and ascends as it relaxes.

Structure of the Thoracic Diaphragm (pp. 224-7)

• The diaphragm is composed of two portions: a peripheral muscular part, and a central aponeurotic part, the central tendon.

The Muscular Part of the Diaphragm (pp. 224-5)

• The fibres forming this part converge radially to the central tendon. Because they have distinct attachments, the muscular part is descriptively divided in the sternal, costal, and lumbar parts.

The Sternal Part (p. 224)

• This portion consists of two small muscular slips that are attached to the posterior aspect of the xiphoid process. These slips converge radially to the central tendon.
• On each side of these muscular slips, there is a small anterolateral gap known as the sternocostal hiatus.

The Costal Part (pp. 224-5)

• This portion consists of wide muscular slips that arise from the internal surfaces of the inferior six ribs and their costal cartilages on each side.
• These slips interdigitate with slips of the transversus abdominis muscles.
• The costal part forms the left and right hemidiaphragms or domes that move during respiration.

The Lumbar Part (p. 225)

• This portion arises from the lumbar vertebrae by two musculotendinous crura (L. legs), which are attached on each side of the aorta to the anterolateral surfaces of the superior two (left) or three (right) lumbar vertebrae and their intervertebral discs.
• The crura of the diaphragm blend with the anterior longitudinal ligament of the vertebral column.
• The right crus is broader and longer than the left crus.

• The crura are united opposite the disc between T12 and L1 vertebrae by a tendinous band or narrow arch called the median arcuate ligament
• It passes over the anterior surface of the aorta and provides attachment for some fibres of the right crus.

• The medial arcuate ligaments are thickenings of the anterior layer of the thoracolumbar fascia over the superior parts of the psoas major muscles.
• Each ligament forms a fibrous arch that runs from the crus of the diaphragm, anterior to the psoas major muscle, and attaches to the transverse process of L1 vertebra.

• The lateral arcuate ligaments are thickenings of the anterior layer of the thoracolumbar fascia over the superior parts of the quadratus lumborum muscles.
• Each ligament forms a fibrous arch that runs from the transverse process of L1 to the 12^th rib.

The Central Part of the Diaphragm (pp. 225, 227)

• The muscular fibres of the diaphragm converge radially to a strong, sheet-like aponeurosis called the central tendon, which is fused with the inferior surface of the fibrous pericardium.
• The central tendon has no bony attachments and is incompletely divided into three leaves, which resemble a cloverleaf. This gives it a C-shape.
• The right lateral leaf is the largest; the anterior (middle) leaf is intermediate in size, and the left one is the smallest.
• The lateral leaves curve posteriorly as they blend with the corresponding halves of the diaphragm.
• The anterior leaf lies just inferior to the heart.

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The Diaphragmatic Apertures

• There are several apertures in the diaphragm that permits structures to pass between the thorax and abdomen. The major orifices are the venal caval foramen, the oesophageal hiatus, and the aortic hiatus.

The Vena Caval Foramen (p. 227)

• The foramen for the inferior vena cava is at the posterior junction of the right and anterior leaves of the central tendon.
• It is located at the level of T8 vertebra, 2 to 3 cm to the right of the medial plane.
• It is the most superior of the three large apertures of the diaphragm.

• The inferior vena cava is adherent to the margin of the vena caval foramen; consequently, when the diaphragm contracts during inspiration, it widens the foramen and stretches and dilates the inferior vena cava.
• These changes facilitate the blood flow through the inferior vena cava.

The Oesophageal Hiatus (p. 227)

• The oesophagus passes obliquely through this oval aperture in the muscular part of the diaphragm, posterior and to the left of the vena caval foramen.
• The hiatus is usually in the right crus of the diaphragm, 2 to 3 cm left of the medial plane and approximately at the level of T10 vertebra.

• The fleshy fibres of the right crus form the oesophageal sphincter, which constricts the distal end of the oesophagus during inspiration, helping to prevent reflux of gastric contents into the oesophagus.

The Aortic Hiatus (p. 277)

• The aorta does not pierce the diaphragm because this aperture is posterior to it.
• It passes posterior to the median arcuate ligament, which arches between the crura, anterior to T12 vertebra and to the left of the median plane.
• The aorta is unaffected by the contraction of the diaphragm because it does not pass through it.
• The aortic hiatus also transmits the thoracic duct and the azygos vein.

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Vessels and Nerves of the Diaphragm

Arterial Supply of the Diaphragm (p. 227)

• Superior surface: superior phrenic arteries (arise from the thoracic aorta), and the musculophrenic and pericardiophrenic arteries (branches of the internal thoracic).
• Inferior surface: inferior phrenic arteries (branches of the abdominal aorta).

Venous Drainage of the Diaphragm (pp. 227-8)

• Superior surface: pericardiophrenic and musculophrenic veins, which drain into the internal thoracic vein.
• Inferior surface: inferior phrenic veins.
• The right inferior phrenic vein usually opens into the inferior vena cava, whereas the left inferior phrenic vein usually joins the left suprarenal vein.

Innervation of the Diaphragm (p. 228)

• The entire motor supply to the diaphragm is from the phrenic nerves, which arise from the ventral rami of segments C3-5 of the spinal cord.
• The phrenic nerves also supply sensory fibres to most of the diaphragm.
• Peripheral parts of the diaphragm receive their sensory supply from the inferior six or seven intercostal nerves and subcostal nerve.

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Actions of the Diaphragm (p. 228)

• The diaphragm is the chief muscle of inspiration.
• When it contracts its right and left domes move inferiorly so that its convexity is flattened. The descent of the domes increases the vertical diameter of the thoracic cavity.
• As the diaphragm descends, the intra-thoracic pressure is decreased and the intra-abdominal pressure is increased.

• Diaphragmatic movements are also important in blood circulation because of the changes in pressure in the thoracic and abdominal cavities accompanying the contraction of the diaphragm. Blood from the inferior vena cava is forced superiorly into the heart.

• The diaphragm is also an important muscle for abdominal straining. It assists the anterior abdominal muscles in raising intra-abdominal pressure during micturition (urination), defecation (bowel movements), and parturition (childbirth).

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