Diaphragm Anatomy & Function

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### Location and Structure - **Location:** Dome-shaped musculofibrous septum separating the thoracic and abdominal cavities. - **Structure:** Consists of a peripheral muscular part and a central aponeurotic part (central tendon). - **Right Dome:** Usually higher, reaching the 5th intercostal space. - **Left Dome:** Usually lower, reaching the 5th rib. ### Origin The muscular part of the diaphragm originates from three main areas: - **Sternal Part:** Two fleshy slips from the posterior aspect of the xiphoid process. - **Costal Part:** From the inner surfaces of the lower six costal cartilages and adjacent ribs on each side, forming large muscular slips. - **Lumbar Part:** From two aponeurotic arches (arcuate ligaments) and the lumbar vertebrae by two crura (musculotendinous bundles). - **Medial Arcuate Ligament:** A thickening of the fascia covering the psoas major muscle, extending from the body of L1/L2 to the tip of the transverse process of L1. - **Lateral Arcuate Ligament:** A thickening of the fascia covering the quadratus lumborum muscle, extending from the tip of the transverse process of L1 to the 12th rib. - **Right Crus:** Arises from the anterior surfaces of the bodies of L1, L2, and L3, and the intervertebral discs between them. - **Left Crus:** Arises from the anterior surfaces of the bodies of L1 and L2, and the intervertebral disc between them. ### Insertion All muscular fibers converge radially to insert into the **central tendon**, a strong aponeurotic structure. - The central tendon is trifoliate (cloverleaf-shaped) and lies just inferior to the pericardium, to which it is partially fused. ### Nerve Supply - **Motor Supply:** - **Phrenic Nerves (C3, C4, C5):** The primary motor supply to the diaphragm. "C3, 4, 5 keeps the diaphragm alive." - Each phrenic nerve supplies the ipsilateral (same side) half of the diaphragm. - **Sensory Supply:** - **Central Part:** Phrenic nerves. - **Peripheral Part:** Intercostal nerves (T5-T11) and subcostal nerves (T12). ### Action The diaphragm is the primary muscle of respiration. - **Inspiration (Contraction):** - The diaphragm descends, flattening its domes. - This increases the vertical dimension of the thoracic cavity. - It pushes abdominal viscera inferiorly, increasing intra-abdominal pressure. - This action creates negative pressure in the thoracic cavity, drawing air into the lungs. - **Expiration (Relaxation):** - The diaphragm ascends passively due to its elasticity and the recoil of the lungs. - This decreases the vertical dimension of the thoracic cavity, expelling air. - **Other Actions:** - Aids in increasing intra-abdominal pressure for defecation, urination, and parturition (childbirth). - Acts as a sphincter for the esophagus. - Facilitates venous return to the heart. ### Openings (Hiatuses) The diaphragm has several important openings for structures passing between the thorax and abdomen. | Opening Name | Vertebral Level | Structures Passing Through | Notes | | :-------------------- | :-------------- | :--------------------------------------------------------- | :-------------------------------------------------------------- | | **Caval Hiatus** | T8 | Inferior Vena Cava (IVC), right phrenic nerve (terminal branches) | Located in the central tendon; IVC is adherent to its margins. | | **Esophageal Hiatus** | T10 | Esophagus, anterior and posterior vagal trunks, esophageal branches of left gastric artery/vein | Formed by muscular fibers of the right crus; acts as a physiological sphincter. | | **Aortic Hiatus** | T12 | Aorta, thoracic duct, azygos vein | Posterior to the diaphragm, between the crura; not a true opening *in* the diaphragm. | | **Lesser Openings** | Variable | | | | - Sternal (foramen of Morgagni) | Variable | Superior epigastric vessels, lymphatics | Small, anterior opening. | | - Crus openings | Variable | Splanchnic nerves, sympathetic trunks | Located lateral to the crura. |