Inspiration occurs when the is less than the

The intrapulmonary pressure; atmospheric pressure

Intrapulmonary pressure refers to the pressure that exist in your pleural cavity. Atmospheric pressure, refers to the pressure that exist in earth's atmosphere. Air will move from high pressure to low pressure area. So, in order for the air to sucked in, we will expand our lung to reduce our intrapulmonary pressure and allow air to move from our environment into our body.

Inspiration occurs when the intrapulmonary pressure is less than the atmospheric pressure.

Explanation:

On inspiration (incorporation of air into the body) the diaphragm muscle and intercostal muscles contract. The diaphragm descends from its position and the intercostal muscles, as a consequence of the posture they adopt, raise the ribs. When this happens, the volume of the rib cage increases together with the volume of the lungs, and more space is available. As the volume of the lungs rises, the air pressure in them decreases. Due to the difference in pressures (now greater outside) the air quickly penetrates the airways into the lungs. This air intake is the inspiration. The opposite effect is known as exhalation.