Diaphragm, << DY uh fram, >> the large muscle attached to the lower ribs, separates the chest from the abdomen. Among vertebrates (animals with backbones), only mammals have complete diaphragms. The diaphragm is the chief muscle used in breathing. It is shaped like a dome.
When a person takes a breath, the diaphragm contracts and moves downward. This increases the space in the chest. At the same time, muscles attached to the ribs cause the ribs to move outward. This expands the chest, and together with the downward motion of the diaphragm, creates a slight vacuum in the chest. The vacuum causes air to enter the lungs through the windpipe. This action is called inspiration or inhalation.
During expiration, also called exhalation, air moves out of the lungs as the diaphragm and rib muscles relax. When a person breathes normally, expiration is passive and muscles do no work. The expanded lung contains elastic fibers that were stretched during inspiration. This elastic tissue behaves like stretched rubber bands, causing the lung to contract like a collapsing balloon. This forces air out of the chest. The lung gets smaller until it reaches the size at which the breath started. The lungs do not empty completely during expiration because the chest wall holds them in a partially expanded state. In hard breathing, as occurs during exercise, expiration is active. Another set of rib muscles helps to make the chest smaller. Muscles in the abdominal wall also contract to push the abdominal organs upward against the diaphragm, helping force air out of the lungs.
The phrenic nerve carries the electrical signals to the diaphragm that stimulate it to contract. This nerve arises from the spinal cord high in the neck and extends into the chest down to the diaphragm.
