Breathing Resistance Exercise The flow of air in and out of the alveoli is called ventilation and has two stages: inspiration (or inhalation) and expiration (or exhalation).  To accomplish this, the whole thorax moves and changes size, due to the action of two sets of muscles: the intercostal muscles and the diaphragm.  These are the muscles that are strengthened by using the Expand-A-Lung Breathing Resistance Exerciser.   Inspiration (Inhaling) with resistance:  The diaphragm contracts and flattens downwards The external intercostal muscles contract, pulling the ribs up and out The volume of the thorax increases More air is delivered deeper into the lungs The lung and alveoli volume increases More oxygen is absorbed  Expiration (Exhaling) with resistance:  The abdominal muscles contract, pushing the diaphragm upwards The internal intercostal muscles contract, pulling the ribs downward This gives a larger and faster expiration More CO2 is expulsed   In essence, you inhale more oxygen, and you exhale more carbon dioxide.  The result is a substantial improvement in shortness of breath and endurance through better breathing. All is done naturally, with the help of the Expand-A-Lung Breathing Resistance Exerciser.