18 Lung Ventilation and Gas Exchange I

Summary of respiratory cycle mechanics of breathing

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Summary of Respiratory Cycle
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Mechanics of Breathing Breathing requires effort by respiratory muscles for: 1. Lung Compliance 2. Resistive work (moving air through airways)
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The lung is an elastic structure; when it is inflated away from its rest position, it tends to recoil back. Lung compliance is the physical property of the lung that tells how much pressure change is required to produce a given change is volume. Lung Compliance = Volume/Pressure 1. Elastic Properties
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Graphical Representation of Lung Compliance
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Lung Compliance is Determined by 2 Physical Factors : 1) Elasticity of connective tissue 2) Surface tension at air-water interface
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Smoking-Induced Emphysema Causes Abnormally High Lung Compliance
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Surface Tension is Caused by Adhesion of Molecules (From: nsf.gov)
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Laplace’s Equation for Surface Tension P = 2T/r Where P = Transmural pressure, T = Wall tension, r = Alveolar radius Characteristics of Pulmonary Surfactant 1) Reduces T, thereby reducing pressure required to inflate lungs 2) Surfactant lowers T more at small r and this helps stabilize interconnected alveolae of different sizes
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Stabilizing Effect of Surfactant
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2. Flow Resistive Properties Flow = ΔP/R = (Pao – Palv)/Raw Where: Pao = pressure at airway opening Palv = pressure in alveoli Raw = resistance of airway Radius of airways is the major factor that determines airway resistance R 1/(radius)4 (Poiseuille’s Law for Laminar Flow)
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Airway Narrowing is a Major Problem in Asthma (from: nih.gov)
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Dynamic Lung Hyperinflation Caused by Abnormal Expiration is a Major Problem in Asthma (From: Critical Care 9: 581-587, 2005)
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Flow Restriction During Forced Expiration (Airway Resistance Rises in Parallel to Pressure Increase)
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