The diaphragm is the large dome-shaped muscle that rests right under the lungs and separates the abdominal cavity from the thoracic cavity. The contractions of the diaphragm are what truly facilitate the movement of air in and out of the lungs. The contraction of the diaphragm, or breathing in, leaves more room in the chest cavity for the lungs to expand as the diaphragm "tightens". The expanded lungs are filled with oxygen-rich air which is then is diffused through capillaries to different parts of the body. The process of breathing out occurs when the diaphragm relaxes, slowly resuming its position its original position in the chest cavity. When relaxing, carbon dioxide is forced out of the lungs.
During inhalation, the diaphragm contracts and moves downward, increasing the space in the chest cavity. Air is sucked in through the nose and mouth and eventually travels to the lungs, which expand.
During exhalation, the diaphragm relaxes and moves upward, reducing the space in the chest cavity. This forces carbon dioxide out of the lungs through the nose and mouth.
