The diaphragm muscle is a sheet of muscle essential for humans and other mammals to breathe. It is a dome-shaped cavity near the lungs, between the thoracic cavity of the upper chest and the abdominal cavity of the stomach. The diaphragm muscle extends across the bottom of the ribcage and is the principle muscle of respiration. It is a convex muscle that contracts and relaxes as breathing takes place, and is often known as the thoracic diaphragm to differentiate it from two other diaphragms in the human body, located near the pelvis.

Composed of two main parts called the peripheral muscular part and the central aponeurotic part, the diaphragm muscle is a complex muscle consisting of fibers and tendons. The fibers of the peripheral muscular part converge to the central part. These fibers include the sternal, costal, and lumbar parts. The costal part descends from the ribs to compose the left and right parts of the dome that contract and expanding during respiration. The lumbar part attaches to the vertebrae in the back and connects to each side of the aorta.

The central part of the diaphragm muscle is a sheet-like tendon, void of bones and shaped like a cloverleaf. Also included in the diaphragm muscle are many vessels, nerves, and apertures. The vessels benefit from blood and oxygen supply from the arteries and drainage into other veins.

The diaphragm muscle in its natural state is a convex dome, rounded up toward the lungs. When a breath is inhaled the right and left sides of the dome contract from their convex shape, decreasing pressure from the upper thoracic diaphragm muscle. At this point the pressure in the stomach is increased until the breath is held and released again, relaxing the muscles and reforming the dome shape under the lungs and ribcage once again. The cavity is enlarged during breathing, and a suction is created to help draw breath from the mouth down into the diaphragm muscle.

Performing other essential actions of the body, the diaphragm muscle is also important to the processes ridding the body of vomit, feces, and urine. The diaphragm increases abdominal pressue on the muscles responsible for excretion, and allows for the action when it is necessary. It is also useful in preventing acid reflux. The diaphragm muscle also appears in other animals, like certain reptiles, though it does not follow the same basic structure that it does in mammals.