Breathing involves two stages, which are well-explained in the NCERT Solutions for Class 11 Biology Chapter 14 Breathing and Exchange of Gases:

• Inspiration: The diaphragm contracts and flattens, and external intercostal muscles lift the ribs, increasing thoracic volume and decreasing pressure inside the lungs, causing air to flow in.

• Expiration: The diaphragm and intercostal muscles relax, decreasing thoracic volume and increasing lung pressure, pushing air out