Ambient air pressure =760 mm Hg ●= Density of air molecules xpiration Inspiration 63 mm H 758 mm Hg. 1.Contraction of ribs(internal intercostals) 1. Elevation of ribs(external intercostals) 2. Intrathoracic volume decreased 2. In trathoradic volume increased 3. Intrathoracic pressure rises 3. Intrathoracic pressure falls 4. Therefore, air flows out of airway 4. Therefore, air flows in to airway down its pressure gradient down its pressure gradient