The relationship of gas flow, pressure, and resistance in the airways is described for airwaay resistance: Raw = PTA/flow, where Raw is airway resistance and PTA is pressure difference between the mouth and the alveolus, or the transairway pressure *[Key Point 2-2]. Flow is the gas flow during inspiration. Resistance is usually expressed in centimeters of water per liter per second [cm H2O/L/sec]. In normal, conscious individuals with a gas flow of 0.5 L/sec, resistance is about 0.6 to 2.4 cm H2O/[L/sec][Box 2-4]. the actual amount varies over the entire respiratory cycle. The variation occurs because flow during spontaneous ventilation usually is slower at the beginning and end of the cycle and faster in the middle.