At the end of the expiratory cycle, before the ventilator cycle into inspiration, normally no flow of gas occurs; the alveolar and mouth pressures are equal. Because flow is absent, resistance to flow is also absent. When the ventilator cycle on and creates a positive pressure at the upper airway, the gas attempts to move into the lower pressure zones in the alveoli. However, this movement is impeded or even blocked by having to pass through the endotrachial tube and the upper conductive airways. Some molecules are slowed as they collide with the tube and the bronchial walls; in doing this they exert energy [pressure] against the passages. This causes the airways to expand [Figure 2-4], and as a result, some of the gas molecules[pressure] remain in the airway and do not reach the alveoli. In addition, as the gas molecules flow through the airway and the layers of gas flow over each other, resistance to flow, called viscous resistance, occurs.