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Possible associatedhemothorax must also be considered.Open Pneumothorax168Respiratory PhysiologyTospirometerACBDuring a slow expiration from TLC, flow is periodically interrupted and measurements are made of lung volumeand of transpulmonary pressure. Transpulmonary pressure is the difference between alveolar and pleural pressures.Pleural pressure is determined from pressure in the esophagus.
Because there is no airflow, alveolar pressure is thesame as pressure at the airway opening.A1009070⌬VCompliance ⫽ ⌬PC6050⌬V40⌬PTime3020FRCRV0Lung vol. (% TLC)80B152025510Transpulmonary pressure (cm H2O)1030Figure 14.4 Measurement of Elastic Properties of the Lung Lung compliance (ΔV/ΔP) is ameasure of lung distensibility and can be measured by determining transpulmonary pressure at various lungvolumes (A, B, and C). Transpulmonary pressure is the difference between alveolar and pleural pressure.Alveolar pressure is the same as pressure measured at the airway opening when flow is stopped; pleuralpressure is measured by esophageal balloon catheter. Note that lung compliance is highest at residualvolume, and is reduced at greater lung volumes.
TLC, total lung capacity.deflation curves is mainly associated with the effects of surfacetension during inflation, when surface tension at the liquidair interface of the lung must be overcome as volume rises.When a fluid-filled lung (in which surface tension is not anissue) is exposed to the same manipulations, compliance isgreater and hysteresis is less pronounced.Like the lungs, the chest wall also is characterized by compliance and elastance. These properties become apparent whenthe chest wall is punctured, creating a pneumothorax (see“Pneumothorax” Clinical Correlate).SURFACTANT AND SURFACE TENSIONSurface tension is the elastic-like force at the surface of aliquid (at the gas-liquid interface) caused by intermolecularattraction of the liquid molecules at that surface.
In the lung,surface tension reduces lung compliance and has the potentialfor causing collapse of small airways. The potential problemsof surface tension and low pulmonary compliance are overcome by the production of surfactant by type II alveolarepithelial cells. Surfactant is a complex lipoprotein containing the phospholipid dipalmitoyl phosphatidyl choline. It isThe Mechanics of Breathingand reduced slope of the pressure–volume relationship.
Obstructive lung disease does not directly affect lung compliance but mayultimately change compliances. For example, in emphysema,often associated with tobacco smoking and COPD, elastic fibersin the lungs are destroyed, alveolar architecture is compromised,and lung compliance is increased (elasticity is reduced). FRC andtotal lung capacity are increased, and the slope of the pressure–volume relationship is greater. Despite increased compliance andgreater tidal volume, emphysema sufferers have reduced ability toexchange gas, based on the destruction of alveoli.CLINICAL CORRELATECompliance in Pulmonary DiseasesLung diseases are often classified as restrictive and obstructivediseases. Restrictive diseases are characterized by reduced functional volume of the lung, whereas obstructive diseases are characterized by reduced flow rate.
Restrictive diseases includeinterstitial lung diseases (for example, idiopathic pulmonaryfibrosis, sarcoidosis, and asbestosis); examples of obstructive lungdiseases include chronic obstructive pulmonary disease (COPD)and asthma. In restrictive diseases, compliance of the respiratorysystem is reduced, resulting in lower FRC and total lung capacity,Work of Breathing⫺⫺FRC⫺⫺⫺⫺B. Obstructive disease⫺⫺nti oraCionratpiB’Bn⫺⫺rapisnIFRC E02468 10 12⫺Intrapleural pressure (cm H2O)In disorders characterized by airway obstruction, work toovercome flow-resistance is increased; elastic work ofbreathing remains unchanged.A⫺⫺C. Restrictive disease⫺⫺⫺⫺⫺⫺⫺⫺Lung vol. (L)1⫺Expira1 D⫺⫺sInE A02468 10 12Intrapleural pressure (cm H2O)ti o⫺BB’Work performed on lung during breathing can be determinedfrom dynamic pressure–volume loop.
Work to overcome elasticforces is represented by area of trapezoid EABCD. Additionalwork required to overcome flow-resistance during inspirationis represented by area of right half of loop AB’CBA.Lung vol. (L)⫺⫺ntiopi⫺CDEx⫺Lung vol. (L)1A. Normal169CDntioirapExBiI n spra tB’ionAFRC E02468 10 12⫺Intrapleural pressure (cm H2O)Restrictive lung diseases result in increase of elastic workof breathing; work to overcome flow-resistance is normal.Work of Breathing in Obstructive and Restrictive Lung Disease Compared to work performedin normal breathing (A), in obstructive disease, while compliance may be unchanged, the work of breathingis increased by the elevated airway resistance (B).
In restrictive disease, lung compliance is low, and theelastic work of breathing is increased (C).170Respiratory PhysiologyAirSalineExcised lungdistendedby airExcised lungdistendedby saline200Pressure–volume relationships ofair-filled and saline-filled lungs. Lungsfilled with liquid require a lowerpressure to maintain a given volumethan do lungs filled with air, becauseof elimination of liquid-air interface.Vol (mL)Saline-filled100Air-filledRV51030152025Pressure (cm H2O)3540Figure 14.5 Compliance and Surface Forces of the Lung Measurement of the pressure–volumerelationship in an isolated lung results in the illustrated pressure–volume loops.
Greater pressure is requiredto inflate an air-filled lung compared with a saline-filled lung, due to the surface tension at the air-alveolarinterface in the former. Surfactant is produced by type II alveolar epithelial cells and reduces surface tensionof the air-filled lung; without it, even greater force would be required to inflate the lungs with air. RV, residualvolume.amphipathic and lines the surface of the alveolar epitheliumand small airways (with hydrophilic regions of the phospholipid oriented toward the epithelial surface and hydrophobicregions facing the lumen). Surfactant reduces surface tensionof airways and alveoli and increases lung compliance, reducing the work of breathing.where ΔP is the pressure gradient from one end of a tubeto the other, r4 is the radius of the tube to the fourth power,η is the viscosity of air, and L is the length of the tube.Thus, based on this relationship, airflow (Q) through a tubeshould be:■■■■AIRWAY RESISTANCEFlow of air in and out of the lungs is dependent on the pressure gradient from the opening of the mouth to the alveoli.At the end of inspiration or expiration, the gradient is zero.Poiseuille’s law, presented in the context of blood flow inSection 3, also applies to flow of air (Q) through tubes (seeFig.
14.5):Q=ΔPπr 4η8LDirectly proportional to the longitudinal pressure gradient (inflow pressure minus outflow pressure).Inversely proportional to the length of the tube.Inversely proportional to the viscosity of air.Directly proportional to the fourth power of the radiusof the tube.In the respiratory system as a whole, the greatest resistance toflow actually occurs in medium-sized airways (fourth to eighthgeneration). Recall from Section 3 that in parallel tubes, totalresistance is less than the resistance of the individual tubes.Considering both the diameter and number of tubes at thislevel, resistance is higher in the medium-sized bronchi (inaggregate) than in the larger or smaller airways.The Mechanics of BreathingThe effects of surface tension in the lung have often beenincorrectly presented in textbooks in the context ofLaplace’s law, which states that in a sphere, surface tensioncreates pressure (P):P=2Trwhere T is surface tension and r is radius of the sphere.
According to this construct, in an isolated alveolus, this pressure is thepressure tending to cause its collapse (or the equivalent pressurerequired to keep the alveolus open). Laplace’s law would lead tothe prediction that when multiple spherical units are subjectedto the same inflation pressure through a branching tube, largerunits would expand and smaller units would collapse, as thehigher surface tension in smaller units causes air to flow towardlarger units in which surface tension is lower.
Textbooks havetypically illustrated this situation as a Y-tube with two alveoliattached, or as alveoli in the configuration of a “bunch of grapes.”According to this line of argument, a major role of surfactant isto overcome the effects of surface tension in alveoli of differentsizes and promote more uniform inflation of the lungs.This argument is incorrect for several reasons: alveoli are actually prismatic in shape (appearing polygonal in histological sections) and exist in a honeycomb-like geometry with commonwalls, sometimes even with pores between them. Thus, alveolido not act as independent units, and inflation of an alveolusaffects inflation of adjacent alveoli. Rather than viewing surfactant’s role in terms of alveolar surface tension, its effects on theoverall compliance of the lung and on small airway patencyshould be emphasized.(From Prange H: Laplace’s Law and the alveolus: A misconception of anatomyand a misapplication of physics.
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