Airway Resistance and Dynamics

Airway resistance is the opposition the conducting airways offer to airflow, defined as the pressure difference driving the flow divided by the flow it produces. The dynamics of the airways — how their calibre changes with lung volume, flow rate, and transmural pressure — determine where most resistance lies and why flow becomes limited during a forced expiration.

Definition

Airway resistance is the ratio of the driving pressure difference between the alveoli and the airway opening to the airflow it produces; it reflects the frictional and geometric opposition to gas movement through the conducting airways and depends strongly on airway radius.

Scope

This topic covers the definition and determinants of airway resistance, the distribution of resistance along the bronchial tree, the dependence of airway calibre on lung volume, and the dynamic compression that limits expiratory flow. It is a reference account of airway mechanics and provides no clinical management advice.

Core questions

• How is airway resistance defined in terms of driving pressure and flow?
• Why does airway radius have such a large effect on resistance?
• Where along the bronchial tree does most airway resistance reside?
• How does dynamic airway compression produce expiratory flow limitation?

Key concepts

• Airway resistance
• Laminar and turbulent flow
• Radius dependence
• Distribution of resistance
• Lung-volume dependence of calibre
• Dynamic airway compression
• Equal pressure point

Key theories

Radius dependence of resistance

For laminar flow, resistance varies inversely with a high power of airway radius, so small changes in calibre — from smooth-muscle tone, secretions, or wall thickening — produce large changes in resistance; resistance also falls as lung volume rises and airways are pulled open.

Dynamic compression and the equal pressure point

During forced expiration, pleural pressure can exceed the pressure inside the airways at a point downstream of the alveoli; beyond this equal pressure point the airway is compressed, so maximal flow is set by lung recoil and the resistance of the segment upstream rather than by expiratory effort.

Mechanisms

Airflow through the airways is opposed by resistance that, for laminar flow, depends very strongly on airway radius, so the calibre of the airways is the dominant determinant of resistance. Although individual small airways are narrow, they are so numerous and their combined cross-section so large that most measurable resistance in the normal lung lies in the medium-sized bronchi rather than the smallest airways. Airway calibre increases as the lung inflates, because the surrounding parenchyma exerts radial traction that holds the airways open, so resistance falls at higher lung volumes. During a forced expiration the rise in pleural pressure that drives air out also compresses the airways; downstream of the point where airway and pleural pressures become equal, the airway narrows dynamically, and from there maximal flow is determined by the lung's elastic recoil and the resistance upstream — the basis of expiratory flow limitation.

Clinical relevance

Increased airway resistance, from bronchoconstriction, mucosal swelling, secretions, or loss of the parenchymal traction that holds airways open, is the mechanical hallmark of obstructive ventilatory patterns, and it raises the resistive work of breathing. Dynamic compression explains why forced expiratory measurements reflect airway function. This entry describes physiology and measurement and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

Methods for measuring airway resistance and related flows were established in classic plethysmographic and forced-oscillation studies and are applied within standardized lung-function frameworks; the interpretation of resistance and flow measurements is set out in international lung-function statements.

History

Direct measurement of airway resistance became possible in the 1950s with body plethysmography and forced-oscillation techniques introduced by DuBois and colleagues. In the 1960s Mead, Macklem and co-workers explained expiratory flow limitation through dynamic airway compression, linking airway resistance, lung recoil, and maximal flow into a coherent account of airway dynamics.

Key figures

• Arthur B. DuBois
• Jere Mead
• Peter Macklem

Seminal works

• dubois-1956
• mead-1967

Frequently asked questions

Why does a small change in airway diameter cause a large change in resistance?

For laminar flow, resistance varies inversely with a high power of the airway radius, so even a modest narrowing — from smooth-muscle contraction, swelling, or secretions — sharply increases the resistance to airflow.

Where in the lung is most airway resistance located?

In the normal lung most measurable resistance is in the medium-sized bronchi. The smallest airways are individually narrow but so numerous, with such a large combined cross-sectional area, that together they contribute relatively little resistance.