5.2: Airway Resistance

So with radius having such a powerful effect on airway resistance we would expect that the early and larger generations of airways would offer the least resistance to flow, and resistance would increase as we descended deeper into the lung to the smaller and later airway generations. Figure 5.4 shows the opposite is true—that airway resistance decreases as the airway generations are descended. This is because the total cross-sectional area increases with each generation—while the early and large airways are wide, they are few. The lower and smaller airways are much more numerous, and so collectively they have a greater cross-sectional area and therefore offer less resistance.

Figure 5.4: Airway resistance down the bronchial tree.

The highest point of resistance is actually the midsize bronchioles. There are a couple of clinically important points to make here:

1. If radius is reduced by disease, such as during inflammation of the airway wall or contraction of airway smooth muscle causing bronchoconstriction, then resistance is markedly increased, and to maintain flow, the pressure differential must be increased. Increasing this pressure differential means increasing the work of breathing.
2. The vast total cross-sectional area of the lower airways means that a significant amount of damage can be done before symptoms arise. This "silent zone" means that a disease may be significantly established and be in a relatively late stage before the patient becomes symptomatic and goes to the physician.

Airway Resistance and Lung Volume

The airways without cartilaginous support significantly change their radius when the lung expands due to the radial traction. In brief, parenchymal fibers tethered to the alveoli and exterior of the airways allow the airways to be pulled open by the expanding alveoli when lung volume increases (illustrated in figure 5.5).

Figure 5.5: Radial traction decreases airway resistance as lung volume increases.

This increase in airway diameter means that airway resistance falls as lung volume increases. This is demonstrated by figure 5.6; as lung volume increases, then airway resistance falls exponentially.

Figure 5.6: Airway resistance and lung volume.

The inverse is also true, that as lung volume decreases, airway radius declines. This may happen to a sufficient extent to allow small airways to collapse. It is worth noting here that respiratory patients frequently breathe at higher lung volumes. While there are mechanical reasons for this that we will discover in the next chapter, the higher lung volume may at least improve airway conductance (although it carries many other disadvantages).

Airway Resistance and Neural Control

As well as the lung volume effect, the tone of airway smooth muscle is also a powerful determinant of airway radius and therefore resistance. The muscle is arranged in a ring pattern around the airway circumference. Contraction of the smooth muscle causes bronchoconstriction, decreasing the airway radius. Relaxation of the smooth muscle allows bronchodilation.

Airway smooth muscle is under the control of the autonomic nervous system. Parasympathetic release of acetylcholine causes activation of muscarinic receptors. This causes a rise in intracellular calcium that activates the smooth muscle. Muscle is relaxed by sympathetic stimulation of β₂ adrenergic receptors. These β₂ receptors are the target of bronchodilator drugs, such as albuterol, that resolve the inappropriate contraction of smooth muscle seen in the hypersensitive airways of asthmatics.

The bronchoconstrictive pathway is utilized by the irritant reflex that is initiated by airway wall receptors detecting the arrival of inspired particulates. This defensive reflex results in bronchoconstriction, presumably to limit the entry of more particulates.

A number of inflammatory mediators also cause bronchoconstriction and probably play a significant role in the bronchoconstriction of asthma (which frequently also involves airway inflammation).

Low airway PCO₂ also has a direct stimulatory effect on airway smooth muscle and a bronchoconstrictive effect. This is presumably to shunt air to other regions of the lung and away from regions where overventilation caused the low PCO₂.

Summary

So now you should be able to understand how the type of flow, airway radius, lung volume, and autonomic nervous system all influence airway resistance and so can either oppose or promote the flow of air in the lung.

References, Resources, and Further Reading

Text

Levitsky, Michael G. "Chapter 2: Mechanics of Breathing." In Pulmonary Physiology, 9th ed. New York: McGraw Hill Education, 2018.

West, John B. "Chapter 7: Mechanics of Breathing—How the Lung Is Supported and Moved." In Respiratory Physiology: The Essentials, 9th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams and Wilkins, 2012.

Widdicombe, John G., and Andrew S. Davis. "Chapter 3." In Respiratory Physiology. Baltimore: University Park Press, 1983.

Figures

Figure 5.1: Laminar flow. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/5.1_20220125/mode/1up

Figure 5.2: Turbulent flow. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/5.2_20220125/mode/1up

Figure 5.3: Transitional flow. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/5.3_20220125/mode/1up

Figure 5.4: Airway resistance down the bronchial tree. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/5.6_20220125/mode/1up

Figure 5.5: Radial traction decreases airway resistance as lung volume increases. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/5.7_20220125/mode/1up

Figure 5.6: Airway resistance and lung volume. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/5.8_20220125/mode/1up