Breathing Cycle: Normal, Quiet Breathing Rest period between breathing cycles diaphragm is at equilibrium position alveolar pressure = atmospheric pressure → alveolar pressure = 0 no pressure gradient between mouth/nose (atmospheric pressure) and alveoli login to view 1 more bullet intrapleural pressure < 0 caused by opposing forces of lungs login to view 1 more bullet keeps lungs inflated volume = functional residual capacity (FRC) volume of gas that remains in lungs following normal tidal volume (TV) expiration equilibrium volume of lungs Inspiration diaphragm contracts → ↑ intrathoracic volume → ↓ intrathoracic pressure ↓ intrathoracic pressure → ↓ alveolar pressure alveolar pressure < atmospheric pressure → alveolar pressure < 0 login to view 1 more bullet inspired volume = TV end-inspiration volume = FRC + TV Expiration passive process diaphragm relaxes → ↓ intrathoracic volume → ↑ intrathoracic pressure ↑ intrathoracic pressure → ↑ alveolar pressure alveolar pressure > atmospheric pressure → alveolar pressure > 0 login to view 1 more bullet expired volume = TV end-expiration volume = FRC