1625915643-5d53d156c9525bd62bd0d3434ecdc231 (843955), страница 45
Текст из файла (страница 45)
At FRC, the mechanical system is at rest, and thepressure within the airways and alveolar space is equal toatmospheric pressure (see Fig. 14.2A). In this state, musclesof the chest wall are relaxed, and the outward elastic recoilpressure of the chest wall is equal and opposite to the inwardelastic recoil pressure of the lungs.
In other words, at FRC,although the tendency of the chest wall is to expand and thetendency of the lungs is to collapse, these two forces are inbalance, keeping the volume of the lungs at FRC. The algebraic sum of the negative recoil pressure created by the chestwall and the positive recoil pressure of the lungs is zero;pleural pressure (pressure in the pleural cavity) is negative(subatmospheric). Note the balance between chest wall recoiland lung recoil pressures at FRC in Figure 14.3.Forces during Inspiration and ExpirationInspiration occurs when the rib cage expands and the diaphragm moves downward, creating negative pressure withinthe lungs (see Fig. 14.2B).
Pleural pressure is more negativethan in the resting state, as the chest wall exerts greater outwardpressure. Air flows into the lung until alveolar pressure reachesatmospheric pressure. At the end of inspiration, duringnormal, quiet breathing (tidal volume of 500 mL), chest wall164Respiratory PhysiologyMuscles of InspirationPrincipalMuscles of ExpirationAccessoryQuietbreathingSternocleidomastoid(elevatessternum)ActivebreathingExpirationresults frompassive recoilof lungsScalenusanteriormiddleposterior(elevate andfix upper ribs)Internalintercostals,exceptinterchondralpartExternalintercostals(elevate ribs)Abdominalmuscles(depress lowerribs, compressabdominalcontents)Interchondralpart of internalintercostals (alsoelevates ribs)Diaphragm(domes descend,increasinglongitudinaldimension ofchest andelevatinglower ribs)RectusabdominisExternalobliqueInternalobliqueTransversusabdominisFigure 14.1 Respiratory Muscles Contraction of the diaphragm is the main factor producing inspiration during normal, quiet breathing; expiration is a passive process in this type of breathing, caused bypassive recoil of the lungs.
Active breathing requires the activity of additional muscles and involves energyexpenditure for both inspiration and expiration.recoil pressure is still negative, but recoil pressure of the totalrespiratory system is positive due to increased recoil pressureof the lungs (see Fig. 14.3). At total lung capacity, both chestwall and lung recoil pressures are positive.During expiration, with the relaxation of inspiratory muscles,the elastic recoil pressure of the respiratory system (increaseddue to its expansion) results in elevation of alveolar pressureabove atmospheric pressure, causing outward airflow untilatmospheric pressure is reached in the lungs (see Fig.
14.2C).The system returns to FRC, unless air is actively expiredbeyond that level; expiration is eventually limited by the largenegative elastic recoil pressure of the chest wall as residualvolume (RV) is approached.During normal, quiet breathing, inspiration is mainlydriven by the contraction of the muscular diaphragm,whereas expiration is a passive process, driven by the positiveelastic recoil pressure of the respiratory system achieved duringinspiration. In this type of breathing, pleural pressure is alwaysnegative.
During active expiration, contraction of abdominalmuscles compresses the viscera, forcing the diaphragm upward,producing positive pleural pressure. Thus, in contrast to quietbreathing, in which work is performed only for inspiration,energy is expended for both inspiration and expiration in activebreathing.as elastic recoil pressure. The elastic recoil of the lungs andchest wall can be related to pleural pressure:COMPLIANCE, ELASTANCE, ANDPRESSURE–VOLUME RELATIONSHIPSElastance is defined as the tendency of a hollow organ toreturn to its original size when distended; it can be quantified■■Elastic recoil of the lungs is equal to alveolar pressureminus pleural pressure.Elastic recoil of the chest wall is equal to pleural pressureminus atmospheric pressure.The Mechanics of BreathingA.
At restElastic recoil of chest wall (pleuralpressure minus pressure atsurface of chest)⫺1. Respiratory muscles are at rest.2. Recoil of lung and chest wall are equalbut opposite.3. Pressure along tracheobronchial tree isatmospheric.4. There is no airflow.⫺⫺Elastic recoil of lung (alveolarpressure minus pleural pressure)⫺⫺Pleural pressure (subatmospheric;determined from esophageal pressure)⫺⫺⫺Alveolar pressure(atmospheric)Pleural pressure (increasinglysubatmospheric)B. During inspirationⴚInspiratory muscles contract and chestexpands; alveolar pressure becomessubatmospheric with respect topressure at airway opening. Airflows into lungs.ⴚⴚⴚⴚⴚⴚAlveolar pressure(subatmospheric)ⴚC.
During expirationElastic recoil of lung(increased)ⴚInspiratory muscles relax; recoil of lungcauses alveolar pressure to exceedpressure at airway opening. Air flows outof lung.ⴚ⫺⫺⫺ⴙ⫺Force of muscularcontractionPleural pressure(subatmospheric)ⴚAlveolar pressure(greater than atmospheric)Figure 14.2 Forces during Quiet Breathing Normal, quiet breathing is produced mainly by contraction of the diaphragm, resulting in inspiration as alveolar pressure falls below atmospheric pressure; expiration occurs when the diaphragm relaxes and recoil pressure of the lungs elevates alveolar pressure aboveatmospheric pressure. The dynamic interactions of elastic recoil pressure of the lungs and chest wall andcontraction and relaxation of the diaphragm producing airflow are illustrated (A–C). Pleural pressure is alwaysnegative during quiet breathing.165Respiratory PhysiologyA.
At residual volumeB. At functionalC. At largerElastic recoil of chestresidual capacitylung volumewall directed outwardElastic recoils of lungElastic recoil ofis large. Recoil of lungand chest wall arechest wall becomesdirected inward isequal but oppositesmaller, and recoilvery smallof lung increasesD. At approximately 70% oftotal lung capacityEquilibrium position ofchest wall (its recoilequals zero)E. At total lung capacityElastic recoil of both lungand chest wall directedinward, favoring decreasein lung volumePressure at airway opening(mouth pressure)Airway closedand airflowstopped atvarious lungvolumesPressure–volume relationships of respiratory systemE100100Respiratorymusclesrelaxed8080DC heLust wallngandchest wLungall16660% TLC4060C% VC40B20FRC20F. Pressure recorded at airway opening issame as alveolar pressure when airflow isstopped; provides a measure of elasticrecoil of respiratory system whenrespiratory muscles are relaxed0ARV⫺30⫺20⫺1001020Pressure (cm H2O)3040G.
Elastic recoil pressure of respiratory system is algebraic sumof recoil pressures of lung and chest wallFigure 14.3 Elastic Properties of Respiratory System: Lung and Chest Wall Elastic recoilpressure of the respiratory system is the algebraic sum of recoil pressures of the lungs and chest wall (G).These pressures can be measured at various lung volumes from residual volume to total lung capacity (A–E),and represent the forces resulting in inspiration and expiration. Above functional residual volume (FRC), thenet recoil pressure of the respiratory system is positive, and relaxation of inspiratory muscles will resultin expiration; above FRC, net recoil pressure is negative and relaxation of expiratory muscles will causeinspiration.
At FRC, the system is in equilibrium. Elastic recoil when respiratory muscles are relaxed can bedetermined by stopping airflow (F). RV, residual volume.The Mechanics of BreathingLung compliance (CL) is a measure of the distensibility of thelung and is the inverse of lung elastance. Thus, CL is measuredas the change in lung volume resulting from a change in transpulmonary pressure, where transpulmonary pressure is thepressure across the lung, or the difference between alveolarand pleural pressure (Fig.
14.4). Experimentally, compliancecan be measured in an isolated lung (Fig. 14.5). Referring toCLINICAL CORRELATEPneumothoraxPneumothorax is a condition in which air accumulates in thepleural cavity, due to injury to the chest wall or lungs or as a resultof pulmonary disease. Ordinarily, the pleural cavity contains onlya few milliliters of fluid.
Introduction of air into the pleural spacethrough an open (sucking) pneumothorax will uncouple themechanical forces associated with the chest walls and lung, causingthe experimental preparation in Figure 14.5 in which the lungis filled with air, as pressure applied to the lung is changed, thevolume of the lung changes. The slope of this curve is thecompliance of the lung.
Note that the slope differs betweeninflation and deflation, with the volume at a given pressurebeing lower during inflation than during deflation, a phenomenon known as hysteresis. The difference in the inflation andcollapse of the lung on the damaged side, thereby affecting ventilation in that lung. In a tension pneumothorax, air is able to enterthe pleural space but cannot leave (a piece of tissue acts as a oneway valve). Air accumulates with each breath, raising intrathoracicpressure and thereby causing severe dyspnea (shortness of breath)and circulatory collapse.
Tension pneumothorax is an unstablecondition requiring immediate medical intervention.PathophysiologyAirAirExpirationInspirationAir enters pleural cavity through open, suckingchest wound. Negative pleural pressure is lost,permitting collapse of ipsilateral lung andreducing venous return to heart. Mediastinumshifts, compressing opposite lung.167As chest wall contracts and diaphragm rises, air isexpelled from pleural cavity via wound. Mediastinumshifts to affected side and mediastinal flutter furtherimpairs venous return by distortion of venae cavae.Patient often cyanotic and in severe respiratorydistress or in shock. Immediate closure ofsucking wound imperative, preferably bypetrolatum gauze pad, but if not available,by palm or anything at hand.Chest strapped over packingon top of petrolatum gauze.Thoracostomy tube attachedto underwater-seal suction drainageor Heimlich valve may be indicatedto promote reexpansion of lung.Wound debridement may benecessary.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
