Heart rate increases during inhalation and decreases during exhalation. During exhalation the diaphragm and intercostal muscles relax. These receptors signal the respiratory center to initiate inhalation or exhalation. This cycle of exhalation and inhalation is repeated to provide continuous locomotion. They assist as muscles of exhalation in the breathing process during forceful exhalation. Proper full inhalation and complete exhalation were key to this. Inhaled benzene is primarily expelled unchanged through exhalation. These odorants enter the nasal cavity during exhalation. FRC is the amount of air left in the lungs after normal exhalation. RV is amount of air left in the lungs after a forced exhalation. Exhalation in mechanical ventilation is almost always completely passive. Air is therefore expelled from the respiratory system in the act of exhalation. The contraction of smooth muscles in the walls of the lung results in exhalation. When the circuit is interrupted by an inhibitory signal the muscles relax causing exhalation. There is collapse of a main stem bronchus on exhalation.