Intrapulmonary pressure
when air moves out of the lungs, the air pressure decreases
Inspiration happens when the pressure inside the lungs is lower than the atmospheric pressure (outside) and air rushes into the lungs. Expiration is when the air inside the lungs is higher than the atmospheric pressure and the air rushes out of the lungs. If the intrapleural pressure (pressure within the pleura of the lungs) isn't maintained then the pressure in the lungs can't differentiate between inspiration and expiration and so the lung collapses.
atmospheric
The intrapulmonary pressure is the pressure in the alveoli. Intrapulmonary pressure rises and falls with the phases of breathing, but it ALWAYS eventually equalizes with the atmospheric pressure.
when pressure inside the lungs is lower then outer atmosphere
reduces intra-thoracic pressure which creates suction that draws air into lungs
by the air pressure we have inside our lungs
The pressure inside the lungs decreases as the ribcage moves out and up. Air from outside basically gets pushed in by other air molecules due to the pressure gradient (air moves from a high pressure to a low pressure)
The act of inhaling is to create low pressure in the lungs, causing the air in the atmosphere to rush in as it is moving from a higher pressure (outside in the atmosphere) to the lower pressure (created in the lungs). However the fact that air does move into the lungs means that there is no net change in pressure.
AnswerAir is actually forced into the lungs by atmospheric pressure. But here's the rest of the story. The diaphragm, a muscle below the lungs, contracts and actually makes the space in which the lungs are located, the thorax, get bigger. When this space gets bigger, the pressure inside it decreases. There is now a difference in pressure between the lung cavity and the outside air with the outside air having more pressure. Air goes from where the pressure is higher to where it is lower, and so it goes into the lungs. That's called inspiration or inhaling. When the diaphragm relaxes, the thorax gets smaller and the pressure increases in the lungs and air is forced out. Click the link for more information.
You use the respirerotry system. To be precise when air is to be sucked in, the brain askes the lungs to expand, which naturally creates a low pressure inside the lungs. Similarly when the lungs are compressed the air inside is flushed out.
The condition in which there is air inside the thoracic cavity and outside of the lungs is called a pneumothorax.