A hemidiaphragm is the right or left half of the diaphragm, the muscle bisecting the torso that separates the thoracic cavity, which houses the heart and lungs, from the abdominal cavity, which contains the stomach, intestines, and other organs of digestion and waste removal. As this muscle is shaped like an umbrella, hemi refers to one semicircular half. When it contacts, it facilitates breathing by pulling downward and allowing the lungs to expand, making inhalation possible. The right hemidiaphragm may be stronger than the left, which is more susceptible to injury like muscle rupture.
Like a domed ceiling to the abdominal cavity, the two hemidiaphragm halves fill the bottom of the ribcage, continuously attaching to several structures. Where the two halves meet at the topmost point, the diaphragm attaches to the xiphoid process of the sternum, which is the lowest point on the breastbone in the center of the chest. It has additional attachments on the lower six ribs along their inside surfaces and in the lower spine on the lumbar vertebrae. Here it affixes to the spine as well as the lumbocostal arches. Found immediately to either side of the spine, each lumbocostal arch is a fibrous rim encircling a hole through which the psoas major muscle passes.
The diaphragm is punctured at several points along its middle where structures such as the spine and esophagus pass through it, with the right and left hemidiaphragm lying to either side. Still, they function as a unit, contracting and expanding each time a breath is taken. During inhalation, the diaphragm contracts, pulling away from the thoracic cavity. The suction created by this contraction allows the lungs to fill with air, expanding in the thoracic cavity, whereas during exhalation the lungs deflate, the diaphragm relaxes, and the abdominals contract, pushing the air out.
Contractions of the diaphragm also place pressure on the abdominal cavity, and therefore this muscle also functions to enable vomiting, urination, and defecation. Due to the amount of pressure it can create, however, excessive straining by this muscle can lead to such injuries as a hernia or muscle rupture, in which the muscle tears away from a bone or other structure to which it attaches. These injuries are more likely to occur in the left hemidiaphragm than in the right, which has the protection of the liver and which has been shown to be stronger, likely as a result of the way the muscle fuses in utero.