Which explains the relationship between the gas pressure gradient of the atmosphere and the alveolar air that allows inhalation?

Answer 1

Sub atmospheric pressure in the lungs is generated through sudden expansion caused by the diaphragm and accessory muscles; this sudden and brief pressure gradient is recognized by airflow from the atmosphere into the lungs until they are filled. Once the lungs are filled and achieve a greater than atmospheric pressure another pressure gradient is established and with the assistance of elastic recoil allow exhalation to occur. This end of exhalation provides a temporary period of equilibrium between the lung and outside atmosphere where no air flow occurs.

The lungs never totally deflate. The remaining volume left in the lungs after passive exhalation is referred to as the Functional Residual Capacity or FRC. A person can forcefully exhale a portion of their FRC but cannot eliminate all of it; a remaining portion referred to as the residual volume remains. A combination of factors involving surface tension of the alveoli and 0 pressure gradient prevent the elimination of RV.