inspiration - sequence of events

Step 1
Inspiratory muscles contract: diaphragm descends, rib cage rises
Step 2
Thoracic cavity volume increases
Step 3
Plural membranes move as thoracic cavity moves, so intrapulmonary volume increases
Step 4
Intrapulmonary pressure drops
Step 5
Air flows into lungs down its pressure gradient until Intrapulmonary pressure is equal to atmospheric pressure