In order for you to inhale, your intercostal muscles between ribs lift your ribs slightly out and up. At the same time your diaphragm (the elastic floor of your ribcage) flattens down. This results in a slightly larger ribcage that creates an imbalance of pressure and new air is sucked in. This is the busy, working, active stage of each breath cycle.
The resting stage means that all breathing muscles simply let go and relax. The ribs drop down once more and the diaphragm domes upward into its resting position. And there we have an exhalation as the breath is squeezed out.
The distance the diaphragm must move from its domed up relaxed position when the lungs are relatively empty to the flattened position of inhalation is only about half an inch. However, this slight change of total volume is enough to move all the air you need.
Your breathing muscles also have somewhat easier duty during sleep when you tend to breathe more efficiently.
Be well. Breathe beautifully.
