The human diaphragm is an unpaired broad and flat muscle which divides the thorax from the abdominal cavity. Being dome-shaped, it isolates the thoracic viscera, such as the lungs and heart, from the digestive organs (stomach, intestines, liver, and pancreas). It plays a vital role in breathing as it is innervated by the phrenic nerve, whose center is located in the medulla oblongata (see breathing centers).
Both its thoracic and abdominal surface are covered by fasciae, which in turn are covered by connective tissue. A relaxed diaphragm has a beveled spherical convexity, which faces the thoracic cavity with two domes; the right and left. The apex of the dome rises on the medioclavicular line to the level of the fourth intercostal space on the right side, and to the level of the fifth intercostal space on the left. When the diaphragm contracts, its domes flatten out, increasing the thoracic space volume and the capacity of the lungs to expand during the inspiration phase of breathing.
The muscular fibers of the diaphragm arise from the inner border of the outlet of thorax, which can be divided into a sternal, costal, and lumbar portion. The sternal part is the smallest, arising from the posterior surface of the xiphoid and is continuous with the central tendon. The costal portion originates from the inner surface of the bony and cartilaginous parts of the lower six ribs, with its fibers running upwards and inwards, being continuous with the central tendon. The lumbar portion of diaphragm arises from the anterolateral surface of the body of the first three lumbar vertebrae and from the medial and lateral arcuate ligament; at this point, there is the aorta diaphragmatic foramen (opening).
The muscular fibers of the human diaphragm, which runs towards the center, continue as tendons from the central tendon. This area of the diaphragm is trifoliate (having three leaf-like parts) in shape, with one leaf directed forwards and on which the heart lies, and the other two directed to the sides, with the lungs being located there. In the posterior part of the central tendon, to the right of the midline, there is the vena-caval opening for the inferior vena cava. The medial muscular bundles of both crura opens up to form the esophageal opening for both the esophagus and the vagus nerve. There are other smaller orifices in it for the splanchnic nerve and veins.
The lungs and heart lie on the thoracic surface of the diaphragm, while the liver, stomach, and spleen are in contact with its abdominal surface. The pancreas, duodenum, kidneys, and adrenal glands touch areas of the abdominal surface of diaphragm which are not covered by the parietal peritoneum.
Action/Function
The human diaphragm takes part in inspiration (breath in) as it becomes flat upon contraction, increasing the thoracic cavity volume and decreasing its inner pressure. This unbalance between the external atmospheric pressure and the thoracic inner pressure, causes a stream of air to flow into the lungs through the nose to balance it out.
Blood Supply
It is supplied by the phrenic arteries, which originate straight from the thoracic aorta, and by the musculophrenic artery, which springs from the internal thoracic artery.
Innervation
It is innervated by the phrenic nerve, which arises from vertebral roots C3, C4, and C5 as it is part of the cervical plexus.
Below, an anterior aspect of the human diaphragm.
