The Merck Manual states that the diaphragm is the most important muscle involved in respiration. As a person breathes in and out, the diaphragm contracts and expands, forcing air to rush into or exit the lungs.
According to The Merck Manual, the lungs themselves do not have any muscular structures. Instead, they rely on the diaphragm and muscles in the neck, abdomen and between the ribs to power the breathing process. The diaphragm is a thick, dome-shaped sheet of muscle that serves as the dividing line between the chest and abdominal cavities.
The National Heart, Lung, and Blood Institute states that as a person inhales, the diaphragm contracts and moves down and away from the lungs. This movement creates greater space in the chest cavity, causing the lungs to expand. The suction created by this expansion causes air to flow into the lungs where it enters the alveoli, or air sacs, that serve as the point where oxygen can enter the bloodstream.
The National Heart, Lung, and Blood Institute describes exhalation as starting in the alveoli when they become filled with carbon dioxide that must be expelled from the bloodstream and body. During exhalation, the diaphragm expands and moves upwards, constricting the chest cavity. As a result, air laden with carbon dioxide is quickly forced out of the lungs.