Illustration of the flow-volume curve with an extra and intrathoracic obstruction. An intrathoracic obstruction doesn?t produce an obstruction on inhalation as the negative pleural pressure allows an expansion of tissue. On the other hand, the obstruction happens during the exhalation phase (sibilances) due to the absence of that negative pleural pressure. An extrathoracic obstruction produces an obstruction during inhalation (stridor) due to the absence of negative pleural pressure. During exhalation, the positive pressure produces an expansion of tissue.