is the sensation
of the urge to breathe
. It is usually caused by the detection of high levels of carbon dioxide
in the blood by sensors in the carotid sinus
and is one of the body's homeostatic
mechanisms to ensure proper oxygenation. Natural chemicals in the blood such as epinephrine
(adrenaline) can also induce an urge to breathe by a separate pathway. Insufficient pulmonary minute ventilation
, a sustained breath-hold, constriction of the alveoli
of the lungs as in asthma
, or high ambient levels of carbon dioxide in the air breathed can cause air hunger resulting in a respiratory distress condition characterized by dyspnea
, labored breathing or gasping. Air hunger can be very distressing and triggers strong reactions to restore breathing.
In mammals (with the notable exception of seals and some burrowing mammals), the breathing reflex is triggered by excess of carbon dioxide rather than lack of oxygen. In particular, this means that air hunger is not always experienced during asphyxiation. In oxygen-deprived environments, respiration continues to cycle out carbon dioxide but does not bring in sufficient oxygen. Without the required carbon dioxide accumulation, victims may not realize they are being asphyxiated until other symptoms appear, or at all.