Figure 48.11 Into the Lungs and Out Again (A) Inhalation is an active process spurred by contraction of the diaphragm. (B) Exhalation generally is a passive process as the diaphragm relaxes. (C) Strong inhalations and exhalations also involve the external and internal intercostal muscles. (D) Between breaths, the intra-alveolar pressure is the same as the external barometric pressure, but the interpleural pressure is still negative and keeps the lungs from collapsing. During inhalation the interpleural pressure becomes more negative, expanding the lungs and causing the intra-alveolar pressure to become negative, which pulls air into the lungs.