Arterial blood gas tensions are maintained at appropriate levels through a feedback system which adjusts ventilation and has multiple inputs and outputs. Inputs to the system arise mainly from peripheral and central chemoreceptors, but there are other inputs from mechanoreceptors and higher brain centers. Output from the system travels to chest wall and upper airway muscles. Need to conserve energy consumption by the respiratory system may also affect controller activity. The response times of the system may be important clinically but are not measured by conventionally used tests of respiratory control. Instability in respiratory control may contribute to the recurrent periods of apnea seen during sleep.