两句话看得不太懂

This results from frictional resistance in the airways. In the healthy subject, the small airways make only a small contribution to total airway resistance, because their aggregate cross-sectional area increases to very large values after about the eighth generation (see Fig. 1.5). Overall airway resistance is therefore dominated by the resistance of the larger airways.

气道阻力源于气道的摩擦阻力。健康人小气道的摩擦阻力占气道阻力的很小部分，因为小气道总横截面积在第8级分支后增加非常多（见图1.5）。因此总气道阻力主要取决于较大的气道。

Gas flow along pulmonary airways is complex compared with the theoretical tubes described earlier and consists of a variable mixture of both laminar and turbulent flow. Both the velocity of gas flow and airway diameter (and therefore Reynolds number) decrease in successive airway generations from a maximum in the trachea to almost zero at the start of the pulmonary acinus (generation 15). In addition, there are frequent divisions with variable lengths of approximately straight airway between. Finally, in large-diameter airways the entrance length is normally greater than the length of the individual airway. As a result of these purely physical factors, laminar flow cannot become established until approximately the 11th airway generation. Predominantly turbulent flow in the conducting airways has two practical implications. First, the physical characteristics of the airway lining will influence frictional resistance more with turbulent than with laminar flow, so changes in airway lining fluid consistency (page 165) will have a significant effect.Second, gas mixtures containing helium (low Reynolds number) are more beneficial in overcoming increased resistance in large airways and of less benefit in small airway disease such as asthma.

与前述理论管道相比，肺气道内气流更加复杂，并且是层流和湍流不同程度的混合。随气道分级增加，气流速度和气道直径（以及由此的雷诺数）下降，流速从气管中的最大值一直降到肺泡起始部（第15级）接近于零。另外，还经常出现不同长度的近似直线气道。最后，大直径气道的长度（形成层流所需的距离）通常大于分出的单个气道的长度。由于这些纯物理因素，大概到第11级气道才开始出现层流。传导性气道以湍流为主，有2个现实意义。首先，与层流相比，湍流时气道黏膜（物理特性）对摩擦阻力的影响更大，因此气道黏膜表面液体黏稠度的改变对摩擦阻力有重大影响（见第165页）。其次，含氦气的混合气体（低雷诺数）更利于克服增加的大气道阻力，而对小气道疾病如哮喘益处较小。

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