Final answer:
Inspiration or inhalation occurs when the pressure in the lungs is less than atmospheric pressure, allowing air to flow into the lungs. Expiration or exhalation occurs when the pressure is greater than atmospheric pressure, forcing air out of the lungs.
Step-by-step explanation:
When the pressure in the lungs is lower than the atmospheric pressure, the process occurring is inspiration (or inhalation). During inhalation, the muscles around the chest expand and the diaphragm moves downward. This action reduces pressure inside the lungs to below atmospheric pressure (negative gauge pressure), which causes air to flow into the lungs due to the creation of a pressure gradient. The principle guiding this process is that air travels from areas of higher pressure to areas of lower pressure. Conversely, during expiration (or exhalation), the muscles relax and the elasticity of lung tissue causes the lungs to recoil. This increases the intrapulmonary pressure above the atmospheric pressure, resulting in air being pushed out of the lungs. The ability to breathe - to have air enter and leave the lungs - is dependent on the differences between atmospheric pressure and the pressure within the lungs.