Gas flow in the airways can be categorized into two major patterns: turbulent and laminar. Airway resistance, which is the change in pressure per unit of flow, varies depending on the type of flow. Turbulent flow has higher resistance compared to laminar flow. The first eight airway generations are the main site of airway resistance. Lung volume and gas density play a role in regulating airway resistance, with resistance decreasing as lung volume increases and gas density decreases. Neural and humoral agents also have an impact on airway resistance.

Pulmonary function tests, such as spirometry, flow-volume loop, and body plethysmography, can detect abnormalities in lung function even before symptoms arise. These test results are compared with those obtained from normal individuals and may differ based on factors such as sex, ethnicity, age, and height. Chronic obstructive pulmonary disease (COPD) is characterized by increased lung volumes and airway resistance, as well as decreased expiratory flow rates. Emphysema, a specific type of COPD, is further characterized by increased lung compliance. Restrictive lung diseases, on the other hand, are characterized by decreased lung volume, normal expiratory flow rates and resistance, and a significant decrease in lung compliance.

The equal pressure point refers to the point where the pressure inside the airway matches the pressure surrounding it. In normal individuals, the location of the equal pressure point is dynamic and moves closer to the alveolus as lung volume and elastic recoil decrease. In individuals with COPD, the equal pressure point is closer to the alveolus regardless of lung volume. Expiratory flow limitation occurs at the equal pressure point.

Breathing requires energy to overcome the mechanical properties of the lung. Respiratory muscle fatigue is a common cause of respiratory failure. Individuals adjust their respiratory rate to minimize work. Those with increased airway resistance breathe at lower frequencies to minimize work, while individuals with restrictive lung diseases minimize work by shallow breathing at high frequencies.

The dynamic compliance of the lung is always lower than the static compliance and increases during activities like exercise, sighing, and yawning.
There are two major patterns of gas flow in the airways: turbulent and laminar.
2.
Resistance to airflow is the change in pressure per unit of flow. Airway resistance varies with the inverse of the fourth power of the radius and is higher in turbulent than in laminar flow. The major site of airway resistance is the first eight airway generations. Airway resistance decreases with increases in lung volume and with decreases in gas density. Airways resistance is also regulated by neural and humoral agents.
3.
Pulmonary function tests (spirometry, flow-volume loop, body plethysmography) can detect abnormalities in lung function before individuals become symptomatic. Test results are compared with results obtained in normal individuals and vary with sex, ethnicity, age, and height. COPD is characterized by increases in lung volumes and airway resistance and by decreases in expiratory flow rates. Emphysema, a specific type of COPD, is further characterized by increased lung compliance. Restrictive lung diseases are characterized by decreases in lung volume, normal expiratory flow rates and resistance, and a marked decrease in lung compliance.
4.
The equal pressure point is the point at which the pressure inside and surrounding the airway is the same. The location of the equal pressure point is dynamic. Specifically, as lung volume and elastic recoil decrease, the equal pressure point moves toward the alveolus in normal individuals. In individuals with chronic obstructive pulmonary disease (COPD), the equal pressure point at any lung volume is closer to the alveolus. Expiratory flow limitation occurs at the equal pressure point.
5.
Energy is expended during breathing to overcome the inherent mechanical properties of the lung. Respiratory muscle fatigue is the most common cause of respiratory failure. Individuals breathe at a respiratory rate to minimize work. For individuals with increased airway resistance, work is minimized by breathing at lower frequencies. For individuals with restrictive lung diseases, work is minimized by shallow breathing at high frequencies.
6.
The dynamic compliance of the lung is always less than the static compliance and increases during exercise, sighing, and yawning.
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Crapo RO, et al. Reference spirometric values using techniques and equipment that meet ATS recommendations. Am Rev Respir Dis. 1981;123:659-664.
Otis AB. A perspective of respiratory mechanics. J Appl Physiol. 1983;54:1183-1187.
Otis AB, et al. Mechanics of breathing in man. J Appl Physiol. 1950;2:592-607.
Leff AR, Schumacker PT. Respiratory Physiology: Basics and Applications. Philadelphia: Saunders; 1993. Lumb AB. Nunn’s Applied Respiratory Physiology. 8th ed. St. Louis: Elsevier; 2016.
Mead J, MacKlem PT. Mechanics of breathing. In: Handbook of Physiology. Section 3, The Respiratory System. Bethesda, MD: American Physiological Society; 1986. Wiley Online Library doi:10.1002/cphys.cp0303fmo1.
Ventilation, Perfusion, and Ventilation/ Perfusion Relationships
Upon completion of this chapter, the student should be able to answer the following questions: 1.
Define two types of dead space ventilation, and describe how dead space ventilation changes with tidal volume.
2.
Describe the composition of gas in ambient air, the trachea, and the alveolus, and understand how this composition changes with changes in oxygen fraction and barometric pressure.
3.
Use the alveolar air equation to calculate the alveolar-arterial difference for oxygen (AaDO2).
4.
Understand the alveolar carbon dioxide equation and identify how it changes with alterations in alveolar ventilation.
5.