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.