The thoracic diaphragm is a structure found in the bodies of all mammals, including human beings. It is a sheet of muscle in the lower part of the chest with a dome-like shape that spreads across the lower rib cage and divides the thoracic and abdominal cavities of the body. It is very important to respiration in humans and many other animals and is also involved in other functions, such as excretion and vomiting.
When the thoracic diaphragm contracts as an organism inhales, this causes the thoracic cavity to expand. This takes pressure off of the lungs and allows them to expand, creating an empty space that in turn produces suction, drawing air from outside of the body through the esophagus and into the lungs. Once the thoracic diaphragm relaxes, this causes the thoracic cavity and lungs to return to their original size. If there is gas in the lungs, the sudden reduction in available space pushes some of the gas out through the esophagus, causing exhalation.
The importance of the thoracic diaphragm to normal human breathing can be seen by what happens when its normal functioning is disrupted. The reason people often struggle to breathe, or “have the wind knocked out of them,” after being knocked or thrown to the ground or being struck in the torso is that the impact causes a muscle spasm in the thoracic diaphragm that temporarily paralyzes it, forcing the person to struggle to breathe without its assistance until it recovers. This is why being punched in the upper abdomen just below the rib cage, a region commonly called the solar plexus that lies in front of the tendons that connect the thoracic diaphragm to the spinal column, can be so incapacitating. Hiccups are caused by sudden involuntary contractions of the diaphragm, which produces a “hic” sound when the vocal cords close due to the sudden surge of air into the expanded lungs.
The thoracic diaphragm also assists with other bodily functions, most of which involve expelling things from the body. Contraction of the diaphragm puts pressure on the abdominal cavity. This helps to push waste out of the body by putting pressure on the gastrointestinal tract while defecating and on the bladder while urinating. Vomiting is caused by the combined, extended contractions of the thoracic diaphragm and abdominal muscles, which puts pressure on the stomach that forces its contents upwards into the esophagus.