Question

As with the capillaries in the circulatory system, there is low combined (overall) resistance in the conducting zone despite the tubes' decreasing radius. Why is the resistance low?

a) the tubes branch extensively
b) the tubes have smoother surfaces for air flow
c) the tubes have cilia to better push the air through
d) a decreasing radius does not affect air flow

Answer 1

Low resistance in the conducting zone is due to the extensive branching of the tubes, which increases the overall cross-sectional area; this counters the effect of the decreasing tube radius.

Step-by-step explanation:

The resistance is low in the conducting zone of the respiratory system, not because the tubes have smoother surfaces or are lined with cilia, but because the tubes branch extensively. When the conducting tubes branch, they significantly increase the total cross-sectional area. As a result, even though the individual tubes get smaller, the overall area that the air can flow through gets much larger, similar to blood flow in capillaries.

The physics behind this phenomenon lies in the fact that resistance to flow is inversely proportional to the fourth power of the radius of the vessel. Therefore, when numerous small tubes branch off from a larger one, their combined cross-sectional area is much greater than that of the single larger tube. This increased area allows air to flow with less resistance, despite the decreased radius of each individual tube.