Under normal conditions air moves from higher pressure system to a lower pressure system
Air moves to lower pressure. This concept can be observed in the lungs and heart. When you trying to inhale, the muscle in ribs or diaphragm will pull the rib cage and decrease its pressure up to -1mm Hg. This will cause the air from atmosphere enter the lungs.
In expiration, the muscle will relax and the air pressure inside lungs will become +1 mm Hg, makes the air pushed out.
