The application of flow principles to complex airway systems, such as the respiratory tract, is challenging despite their effectiveness in smooth cylindrical tubes. The airways exhibit characteristics of both laminar and turbulent flow due to their intricate nature. Even during quiet breathing, turbulent flow occurs in the trachea as indicated by a Reynolds number greater than 2000. Turbulence is further promoted by irregularities and obstructions caused by the glottis and vocal cords. As gas travels distally, the total cross-sectional area increases significantly, resulting in decreased gas velocities and a shift towards laminar flow in the smaller airways, even during maximal ventilation. Larger airways (nose, mouth, glottis, and bronchi) generally experience turbulent flow, while smaller airways predominantly exhibit laminar flow. The audible breath sounds heard with a stethoscope reflect turbulent airflow, while laminar flow in the smaller airways makes it challenging to detect small airway diseases using a stethoscope.

Airflow resistance plays a crucial role in determining airflow rates within the airways. The resistance to airflow (Raw) varies across airways of different sizes. As airways progressively decrease in size and the number of airway branches increases significantly from the trachea to the alveolus, Raw is the sum of the resistance in each individual airway (Raw = Rlarge + Rmedium + Rsmall). According to Poiseuille's equation, one might assume that the smallest airways are the primary site of airway resistance. However, the major site of resistance in the bronchial tree is located in the first eight generations of airways.

In a normal lung, the smallest airways contribute minimally to the overall resistance of the bronchial tree (Fig. 22.1). This is primarily due to two factors: (1) a substantial decrease in airflow velocity as the effective cross-sectional area increases, resulting in laminar flow, and (2) the presence of airway branches in parallel rather than in series within each generation. The resistance of parallel airways is inversely proportional to the sum of the individual resistances. Therefore, the contribution of small airways to resistance is negligible. For instance, if three tubes each have a resistance of 3 cm H2O and are in series, the total resistance (Rtot) is the sum of the individual resistances (Equation 22.4). However, if the tubes are in parallel (as in small airways), the total resistance is the sum of the inverse of the individual resistances (Equation 22.5). This is in contrast to the pulmonary blood vessels, where most of the resistance is located in the small vessels. Consequently, as airway diameter decreases, individual airways offer increased resistance, but the presence of numerous parallel pathways and larger cross-sectional areas reduces resistance at each branching generation.
Although these relationships apply well to smooth cylindrical tubes, application of these principles to a complicated system of tubes such as the airways is difficult. As a result, much of the flow in the airways demonstrates characteristics of both laminar and turbulent flow. In the trachea, for example, even during quiet breathing the Reynolds number is greater than 2000. Hence turbulent flow occurs in the trachea even during quiet breathing. Turbulence is also promoted by the glottis and vocal cords, which produce some irregularity and obstruction in the airways. As gas flows distally the total cross-sectional area increases dramatically, and gas velocities decrease significantly. As a result, gas flow becomes more laminar in the smaller airways even during maximal ventilation. Overall the gas flow in the larger airways (nose, mouth, glottis, and bronchi) is turbulent, whereas the gas flow in the smaller airways is laminar. Breath sounds heard with a stethoscope reflect turbulent airflow. Laminar flow is silent, which is why it is difficult to “hear” small airway disease with a stethoscope.
Airflow resistance is the second major factor that determines rates of airflow in the airways. Airflow resistance in the airways (Raw) differs in airways of different size. In moving from the trachea toward the alveolus, individual airways become smaller while the number of airway branches increases dramatically. Raw is equal to the sum of the resistance of each of these airways (i.e., Raw = Rlarge + Rmedium + Rsmall). From Poiseuille’s equation, one might conclude that the major site of airway resistance is in the smallest airways. In fact, however, the major site of resistance along the bronchial tree is in the first eight generations of airways.
.08 .06 Respiratory zoneConducting zone Airway generation .02 .04 0 0 5 10 15 20 Resistance (cm H2 O/L/sec) •Fig. 22.1 Airwayresistanceasafunctionoftheairwaygeneration.Inanormallung,mostoftheresistancetoairflowoccursinthefirsteightairwaygenerations.
The smallest airways contribute very little to the overall total resistance of the bronchial tree (Fig. 22.1 ). The reason for this is twofold: (1) airflow velocity decreases substantially as the effective cross-sectional area increases (i.e., flow becomes laminar), and (2) most importantly, the airway branches in each generation exist in parallel rather than in series. The resistance of airways in parallel is the inverse of the sum of the individual resistances; therefore the overall contribution to resistance of the small airways is very small. As an example, assume that each of three tubes has a resistance of 3 cm H2O. If the tubes are in series, the total resistance (Rtot) is the sum of the individual resistances:
Equation 22.4
If the tubes are in parallel (as they are in small airways), the total resistance is the sum of the inverse of the individual resistances:
Equation 22.5
This relationship is in marked contrast to the pulmonary blood vessels, in which most of the resistance is located in the small vessels (see ). Thus as airway diameter decreases, the resistance offered by each individual airway increases, but the large increase in the number of parallel pathways and cross-sectional area reduces the resistance at each generation of branching.