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. 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.