Diaphragm

• ANATOMY OR SYSTEM AFFECTED: Chest, lungs, muscles, musculoskeletal system, respiratory system, spine

DEFINITION: The diaphragm, a dome-shaped muscle that is unique to mammals, is the major muscle of respiration and separates the thoracic and abdominal cavities. When the diaphragm contracts, it decreases the pressure within the lungs by expanding the rib cage and increasing the volume of the lungs as it flattens.

Structure and Functions

The diaphragm is attached to the spine, the ribs, and the sternum. It is pierced by the esophagus, the phrenic nerve, the aorta, and the vena cava. The human body has three types of muscles: cardiac, which is striated and under involuntary control; smooth, which is not striated and is under involuntary control; and skeletal, which is striated and under voluntary control. The diaphragm is composed of skeletal muscle and is under both voluntary and involuntary control. That is, one can hold the breath, take deeper breaths, or take faster breaths (panting), examples of voluntary control. However, a person normally breathes, allowing the diaphragm to contract and relax as the skeletal muscle does, involuntarily—that is, without the conscious effort that is required with holding one’s breath.

As skeletal muscle, the diaphragm must be innervated; that is, it must receive a nerve impulse before it will contract. The impulses that are sent to the diaphragm originate in the higher brain centers when one voluntarily controls breathing, but originate in the lower brain when low oxygen concentrations or high carbon dioxide concentrations are present. The diaphragm relies on the phrenic nerve for its innervation.

When innervated, the diaphragm contracts, as all skeletal muscle does, and flattens or pulls downward. This movement causes the ribs to pull outward, increasing the volume within the lungs. Air pressure inside the lungs is now lower than air pressure outside the lungs (the environment), so air rushes in. As the diaphragm muscle relaxes, it once again domes upward, allowing the ribs to move back to a resting position. The lung volume decreases, but since the lungs are filled with air, the pressure inside the lungs is now greater than the pressure outside the lungs. Air moves outward as one exhales. (This simple expansion and contraction of the lung volume is the premise behind the original iron lung machine.) This cycle of contraction and relaxation is repeated approximately twelve to fourteen times per minute; with heavy exercise, it may be repeated forty times per minute.

The diaphragm has a role in laughing, singing, crying, yawning, hiccupping, vomiting, coughing, sneezing, whistling, defecating, and urinating, as well as in childbirth.

Disorders and Diseases

The diaphragm may be affected by both neurological and anatomical processes. Common neurological problems are disorders of innervation as a result of trauma to the head or brain stem; nerve impulses to the diaphragm are disrupted, and the diaphragm cannot contract and relax. These injuries are often fatal. Poliomyelitis, demyelinating diseases, and other diseases may also impair the innervation of the diaphragm. Anatomical problems may include hernias (protrusion of the stomach through the diaphragm and into the thoracic cavity). Blunt trauma from car accidents and the like may rupture the diaphragm.

Bibliography

"Diaphragm and Lungs." MedlinePlus, 1 Apr. 2025, medlineplus.gov/ency/imagepages/19380.htm. Accessed 7 Sept. 2025. 

Koch, Wijnand F. R. M., and Enrico Marani. Early Development of the Human Pelvic Diaphragm. Scholars Portal, 2019.

Kohnle, Diana, Marcin Chwistek, and Brian Randall. "Diaphragmatic Hernia." Health Library, 18 Mar. 2013.

"Lungs and Breathing." MedlinePlus, medlineplus.gov/lungsandbreathing.html. Accessed 7 Sept. 2025.

Marieb, Elaine N. Essentials of Human Anatomy and Physiology. 13th ed., Pearson/Benjamin Cummings, 2022.

Sherwood, Lauralee. Human Physiology: From Cells to Systems. 5th Canadian ed., Nelson, 2021.