Airway radius and resistance to airflow
Airway resistance is the opposition to the flow of air through the airways, and it rises steeply as the radius of an airway falls.
Remember
The narrower the airway, the greater the resistance to airflow, so bronchoconstriction (tightening of the smooth muscle in the airway wall) raises resistance steeply, while widening of the airways in inspiration lowers it.
Memory aid
Drinking through a thinner straw: halve the width and the effort rises 16-fold, so every widening of the airway makes breathing easier.
Picture drinking through a thinner straw: halving the width makes the effort rise 16-fold, which is the same steep effect that a narrower airway has on resistance.
Airway resistance is the opposition to the flow of air through the airways. The most important factor is the radius of the airway. Resistance is inversely related to the radius (as the radius decreases, the resistance increases), so a narrower airway has a much higher resistance.
The effect of a change in radius is large, because resistance rises steeply as the radius falls. For smooth (laminar) flow, resistance is inversely proportional to the radius to the power of 4, so halving the radius increases resistance 16-fold. This is why even a small amount of narrowing causes a large fall in airflow.
In an asthma attack, bronchoconstriction (tightening of the smooth muscle in the airway wall) narrows the airway, and mucus can also narrow the lumen (the inside of the tube). Either change markedly increases resistance, so more pressure is needed to move the same amount of air and the work of breathing rises. Turbulent flow (chaotic, swirling flow) also raises resistance, because it needs a larger pressure difference to move the same amount of air.
In inspiration the chest enlarges and the small airways widen, so their radius increases and resistance falls. In expiration the opposite happens: the small airways narrow and resistance rises.
The bronchioles, the narrowest airways, lie in vast numbers side by side, arranged in parallel, so their combined resistance is low. The highest total resistance lies in the trachea and larger bronchi.
Key facts
- Airway resistance
- the opposition to the flow of air through the airways.
- Main determinant
- airway radius, because a narrower airway has a much higher resistance.
- Size of the effect
- for smooth flow, halving the radius raises resistance 16-fold.
- Breathing in
- the small airways widen, so resistance falls.
- Breathing out
- the small airways narrow, so resistance rises.
- Flow pattern
- turbulent flow raises resistance, because it needs a larger pressure difference to move the same amount of air.
- Effort
- higher resistance needs more pressure to move the same amount of air, so the work of breathing rises.
Exam traps
- The bronchioles, the narrowest airways, do not hold the largest share of the resistance, because vast numbers of them lie side by side and their combined resistance is low.
- Halving the radius does not simply double the resistance: for smooth laminar flow it multiplies resistance by sixteen.
- Resistance is not higher in inspiration than in expiration: the airways widen when breathing in, which lowers resistance, and narrow when breathing out, which raises it.
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