Apnea, apnoea, or apnœa (from the Greek, meaning to breathe) is a term for suspension of external breathing. During an obstructive apnea event, the upper airway closes, usually at the pharynx, as the muscles of the throat and tongue relax. There is strong respiratory effort, but inspiration is blocked and very little or no air can enter the lungs. During a central apnea (CSA) there is no respiratory effort from expansion of the chest and diaphragm respiration and the volume of the lungs initially remains unchanged. For more details, see also Obstructive sleep apnea (OSA) and Central sleep apnea (CSA), and Hypopnea.