The diaphragm, also known as the thoracic diaphragm as a nod to the fact that humans also have another muscle with a similar name, is a layer of muscle positioned within and across the bottom of the rib cage. Shaped similarly to an open parachute, it separates the thoracic cavity of the rib cage, in which the heart and lungs are situated, from the cavity in the abdomen that houses the stomach, liver, and other organs. It also functions to control breathing, as its contraction and relaxation are what allow air to be drawn into the lungs and then expelled, respectively.
Convex as the bottom wall of the thoracic cavity and concave as the ceiling of the abdominal cavity, this muscle originates in several locations. The top starts on the xiphoid process at the tip of the sternum, or breastbone. From the lower six ribs arises the middle portion, while the lower portion originates on both the lumbar vertebrae of the spine as well as on the lumbocostal arch, the rim of the opening in the diaphragm, through which the psoas major, a hip flexor muscle, passes. It should be noted that the this muscle has several holes in it at points where vertical structures, including the spine, the psoas major and quadratus lumborum muscles, the esophagus, and several major arteries pass through.
This muscle is most significant for its role in helping humans breathe. Whereas the abdominals, particularly the transverse abdominus muscle, assist in exhalation by placing pressure on the thoracic cavity, the diaphragm makes inhalation possible. To do this, it contracts, which reduces pressure on the thoracic cavity and allows the lungs to expand. This produces suction, which further allows air to be drawn in. This relationship between the two muscle groups, with the abdominals opposing the action of the diaphragm, is known in anatomy as antagonistic.
Another role of the muscle is related to its function in the abdominal cavity. As intra-thoracic pressure is reduced during diaphragmatic contraction, pressure in the abdominal cavity is subsequently increased. Therefore, this muscle contracts to increase this intra-abdominal pressure during activities like vomiting, urination, and defecation. Similarly, it places pressure on the esophagus during contraction, which helps to prevent stomach acid from traveling back upward into the thoracic region of the esophagus, a condition known as acid reflux. It is also responsible for hiccups, which are caused by spasms in the muscle, often the result of eating too quickly.