The inferior surface is drained by the inferior phrenic veins (to IVC and suprarenal vein). Venous drainage: the superior surface is drained by the superior phrenic vein, musculophrenic and pericardiacophrenic veins.The inferior surface is supplied by the inferior phrenic arteries (from the abdominal aorta). Arterial supply: the superior surface is supplied by the superior phrenic arteries (from the thoracic aorta), musculophrenic and pericardiacophrenic arteries (from the internal thoracic arteries).This can be remembered using the phrase ‘C3, 4, 5 keep the diaphragm alive’. The diaphragm is innervated by the phrenic nerve, which arises from the spinal nerve roots C3, C4 and C5. The right dome is typically higher due to the presence of the liver below. Lumbar: arises from lumbar vertebrae 1-3 and the medial and lateral arcuate ligaments.Costal: attach to the inferior six ribs and costal cartilages.Sternal: two muscular slips which attach to the posterior aspect of the xiphoid process.The surrounding muscular component is divided into three parts: The diaphragm is a double-domed structure with the muscular fibres blending into a central tendon. The adduc-tor muscles contract reflexively and close the glottis during swallowing or gagging, to prevent food from entering the larynx.During normal breathing, the diaphragm may only move by 1.5cm, during deep breathing this may increase to 7cm. The abductor muscles in the larynx contract early in inspiration, pulling the vocal cords apart. The abdominal muscles, which include the external obliques, internal obliques, transversus abdominis, and the rectus abdominis, help compress the ab-domen and force the diaphragm upward. In forced respiration, expiration becomes active and is assisted by the in-ternal intercostals and transversus thoracis, which help depress the ribs (Figure 10.10 and Table 10.1). Expiratory Musclesĭuring normal respiration, the expiration process is passive and caused by the relaxation of the inspira-tory muscles described above. The accessory muscles come into play when respiration is forced, as in individuals with asthma. Muscles, such as the serratus anterior, pectoralis major and minor, act as muscles of inspiration by el-evating the ribs or pulling the ribs toward the arms by reverse muscle action (i.e., the insertion is fixed while the origin moves) when the upper limb is fixed in po-sition. The sternocleidomastoid muscles help ele-vate the sternum, and the scalene muscles elevate the upper few ribs, increasing the anteroposterior diame-ter. In addition to the diaphragm and external inter-costals, other accessory muscles assist inspiration, in-cluding the sternocleidomastoid, scalenes, serratus anterior, pectoralis minor, pectoralis major, and upper trapezius. The activity of the external intercostals is responsible for bringing approximately 25% of the volume of air into the lungs. The ribs articulate with the vertebrae and sternum in a way that the antero-posterior and transverse diameters are increased when they are lifted up by this muscle. They are innervated by nerves arising from thoracic segments 1 to 12. The external intercostal muscles that originate in the upper rib, with insertion in the lower rib, with the fibers running anteriorly and inferiorly, help elevate the ribs. The diaphragm is responsible for 75% of air movement into the lungs. When the di-aphragm contracts, the central tendon is pulled down-ward into the abdominal cavity, increasing the diame-ter of the thoracic cavity in the superior-inferior direction.
The di-aphragm is innervated by the phrenic nerve that arises from cervical segments 3, 4, and 5. Its origin is along the walls of the thorax and in-sertion is into a centrally placed tendon. The diaphragm is a dome-shaped, skeletal muscle, projecting into the thoracic cavity. The major muscle of in- spiration is the diaphragm. Inspiration is an active process where the muscles of respiration (see Figure 10.10 and Table 1) contract to increase the thoracic volume.
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