- Dynamics is the study of the respiratory system during conditions of flow of gas into and out of the lungs.
- Driving pressure is equal to flow multiplied by resistance.
- Under conditions of laminar flow, changes in driving pressure are proportional to changes in flow.
- Under conditions of turbulent flow, changes in driving pressure are proportional to the square of the flow.
- The Reynolds number predicts whether flow in tubes is laminar or turbulent. Gases of lower density are more likely to be associated with laminar flow, as are gases traveling at low velocity.
- Poiseuille's law states that airway resistance is inversely proportional to the fourth power of the radius. Small increases in the radius of an airway greatly diminish the resistance in that airway.
- The arrangement of tubes that are connected is important in determining the total resistance of the system. For tubes arranged in series, the total resistance is the sum of each of the individual resistances. For tubes arranged in parallel, the total resistance is less than the resistance of any of the individual tubes. Because of the parallel arrangement of the millions of small airways in the periphery of the lung, most of the lung's resistance is located in the more central airways.
- As air moves from the millions of small airways in the periphery of the lung to the single trachea, the velocity of the gas increases. This phenomenon contributes to the presence of turbulent flow in the central airways.
- Bernoulli's principle states that as the velocity of a gas increases, the pressure exerted by the gas decreases. This phenomenon contributes to the decrease in the pressure in the airway during exhalation as air moves from the alveolus to the mouth.
- Small airways in the periphery of the lung are tethered open by the elastic recoil of the lung tissue through which they travel.
- The smooth muscle in the walls of medium-sized airways is under the control of the autonomic nervous system. Bronchial smooth muscle tone is a major determinant of the cross-sectional area and, consequently, the resistance of the medium-sized airways.
- The transmural pressure across the airway is always positive in a normal person during quiet breathing. During a forced exhalation, transmural pressure may become negative, which predisposes to airway narrowing and collapse in small, noncartilaginous airways.
- During a forced exhalation, pressure in the airway decreases as gas moves from the alveolus to the mouth because of several factors, including airway resistance, Bernoulli's principle, and the development of turbulent flow. When the pressure in the airway equals the pleural pressure, the transmural pressure is 0, the EPP is reached, and airway compression may follow.
- If the EPP is reached in peripheral airways, the condition of flow limitation exists; further increases in pleural pressure will not result in an increased flow.
- The elastic recoil of the lung is a major determinant of the location of the equal pressure point. Low elastic recoil pressure, caused by low lung volumes or diseases such as emphysema, shifts the equal pressure point toward the periphery of the lung.
- Most of the expiratory portion of the flow–volume loop represents conditions of flow limitation; at a given volume, as soon as the flow associated with a maximal expiratory effort at that volume is reached, further increases in pleural pressure will not result in increased flow.
- Time constants tell us about the rate at which gas is exchanged between an alveolus and the atmosphere.