It creates a pressure gradient when it contracts by the volume of the lungs, which the pressure in the lungs. when the pressure in the lungs, or the falls below air enters the lungs via during forced inspiration, other muscles called assist in the process. the process of expiration is largely due to thethis causes the volume of the lungs to which intrapulmonary pressure. when intrapulmonary pressure is atmospheric pressure, .

1. It creates a pressure gradient when it contracts by increasing the volume of the lungs, which decreases the pressure in the lungs.

Looking at the ideal gas equation, you can conclude that volume is inversely related to the pressure. Increasing volume of lungs will result in a decrease in the pressure of the lungs.

2. When the pressure in the lungs, or the intrapulmonary pressure, falls below atmospheric pressure, air enters the lungs via inspiration.

If the lungs pressure keeps decreasing, eventually it will fall below the atmospheric pressure. Air moves from higher pressure into the lower pressure area. This will cause the air outside come inside the lungs. The event is called inspiration.

3. During forced inspiration, other muscles called external intercostals assist in the process.
Other than the diaphragm, the external intercostals could also assist in the inspiration. External intercostals will expand the chest outward, while the diaphragm pulls the lung. Both of them will cause decreased pressure in the lungs.

4. The process of expiration is largely passive due to the lungs’ elastic recoil.

Unlike the inspiration, expiration is mostly passive. The chest wall is made of ribs that connected with elastic connective tissue. The tissue is stretched at inspiration and when then muscle that causes inspiration relax, the connective tissue elastic force will press the chest wall.

5. This causes the volume of the lungs to decrease, which increases intrapulmonary pressure.

The pressure from the elastic tissue causing the lungs shrink. As the volume is inversely related to the pressure, the decreased volume will result in increased pressure.

6. When intrapulmonary pressure is higher than atmospheric pressure, expiration occurs. 

The pressure of the lungs keep increases and eventually, it reaches a point where the pressure is higher than the atmospheric pressure. 
Air moves from higher pressure into the lower pressure area so the air inside the lungs will be forced to flow outside. The event is called expiration.

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