The diaphragm is a muscle that separates the chest and abdominal cavities; it is controlled by the phrenic nerves.
Diaphragmatic paralysis is uncommon. In patients where one side of the diaphragm is paralyzed, people usually have no symptoms unless they have another reason for shortness of breath (asthma, emphysema, etc.). Because a paralyzed diaphragm is higher than usual, it compresses the lung and prevents the patient from taking a normal breath. Also, when a patient breaths, the diaphragm usually moves down to pull air in to the lung. Paradoxically, a paralyzed diaphragm moves up and further compresses the lung