Apnea or apnoea is the cessation of breathing. During apnea, there is no movement of the muscles of inhalation, and the volume of the lungs initially remains unchanged. Depending on how blocked the airways are (patency), there may or may not be a flow of gas between the lungs and the environment; gas exchange within the lungs and cellular respiration is not affected. Voluntarily doing this is called holding one's breath.