Lungs want to be small. The aveoli surface tension causes lungs to draw the avoli in together. Pressure is the key to the lungs, when the pressure is high the volume of the lungs is small. (smaller higher pressure) When the pressure is low the volume of the lungs is high. The diaphram is contracting when it expands and becomes flat. this increases the volume potential the pressue goes down the lungs fill up with air. The air goes down the trachea, pharx, bronchials, to the avoli then the O2 goes from the avoli to the cap heads. CO2 is from the cap heads back to the Avoili. When The pressure is down it is lower than the atmospheric pressure. Air flows down into the lungs.
this is active. atp requires energy