The pressure within the tiny gas-exchanging structures of the lungs. When this pressure is positive, it exceeds atmospheric pressure. When it is negative, it is less than the pressure of ambient gases. Gases flow from higher to lower pressures; so when alveolar pressures are higher than atmospheric pressures, respiratory gases tend to be exhaled. When alveolar pressures are less than atmospheric pressures, gas flows into the lungs.
Air pressure in the alveoli and bronchial tree. It fluctuates below and above atmospheric pressure during breathing; this causes air to enter or leave the lungs.