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It is approximately 500 nanometers (nm) thick.Most of the alveolar surface is surrounded in this manner bypulmonary capillaries, allowing for “sheet flow” of bloodaround alveolar sacs, with a broad surface area for gas exchange(Figs. 13.7 and 13.8).Alveoli are lined by type I and type II epithelial cells:■Type I epithelial cells compose more than 90% of thesurface area. Their squamous structure is adapted for gasdiffusion across the alveolar-capillary membrane.■Type II epithelial cells are cuboidal cells that secretesurfactant, a complex lipoprotein that lines the surfaceof alveoli and reduces their surface tension, increasingcompliance of the lung (see “Surfactant and SurfaceTension” in Chapter 14).PULMONARY VOLUMES AND CAPACITIESTo describe pulmonary function in health and disease, it isnecessary to understand the volumes and capacities associatedwith the lungs and breathing.
There are four basic pulmonaryvolumes:Pulmonary Ventilation and Perfusion and Diffusion of GasesSegmentalbronchusSubdivision andStructure ofIntrapulmonaryAirways151Terminal bronchioleSmooth muscleElastic fibersAlveolusBronchiCartilageLargesubsegmentalbronchi(about 5generations)RespiratorybronchiolesAlveolar ductsSmall bronchi(about 15generations)Alveolar sacsand alveoliBronchiolesAcinusLobuleTerminalbronchiolesRespiratorybronchiolesAlveolarducts andalveolar sacsAcinusPoresof KohnFigure 13.5 Intrapulmonary Airways After branching up to 23 times, the conducting system leadsto the terminal bronchioles in the pulmonary lobules.
Respiratory bronchioles and alveolar ducts give riseto alveolar sacs, the major site of gas exchange. The respiratory zone (gas exchange region) consists ofstructures distal to the terminal bronchioles (alveolar sacs, ducts, and respiratory bronchioles).■■■■Tidal volume (VT): The volume of air inhaled and exhaledduring breathing.
Resting VT is approximately 500 mL.Residual volume (RV): The volume remaining in thelungs after a maximal exhalation.Expiratory reserve volume (ERV): The additionalvolume a subject is capable of exhaling after a normal,quiet expiration.Inspiratory reserve volume (IRV): The additionalvolume a subject is capable of inhaling after a normal,quiet inspiration.Four capacities associated with pulmonary function are:■■Total lung capacity (TLC): The volume of gas in thelungs after maximal inspiration. TLC is approximately7 L in healthy adults.Vital capacity (VC): The maximum volume of air thata subject can exhale after maximal inspiration.
Thenormal value is approximately 5 L. Forced vital capacity(FVC) is vital capacity measured during expiration atmaximum force.152Respiratory PhysiologyMucusNerveCiliated cellsSerous cellGoblet (mucous) cellKulchitsky cellBasement membraneBasal cell Brush cellBasal cell Goblet cell Nerve(discharging)Trachea and large bronchi. Ciliated and goblet cells predominant, with someserous cells and occasional brush cells and Clara cells. Numerous basal cellsand occasional Kulchitsky cells are present.NervesCiliated cellsClara cellCrosssectionBasement membraneBasal cellClara cellBronchioles. Ciliated cells dominant and Claracells progressively increase distally along airways.Goblet cells and serous cells decrease distallyand are absent in terminal bronchioles.Magnified detail of ciliumFigure 13.6 Ultrastructure of Tracheal, Bronchial, and Bronchiolar Epithelium The tracheaand bronchi are lined mainly by pseudostratified columnar epithelium and goblet cells, with several other,less common cell types.
The goblet cells have the important function of mucus secretion. Kulchitsky cellsare neuroendocrine-like cells that secrete paracrine factors and are part of the “diffuse neuroendocrinesystem” (DNES). The functions of brush and serous cells are not well defined; basal cells are pulmonaryepithelial stem cells. The epithelial lining of bronchioles contains columnar epithelial cells; goblet cells arelost in terminal bronchioles.
Clara cells are secretory cells of the bronchioles.■■Functional residual capacity (FRC): The volume remaining in the lungs after expiration during normal, quietbreathing.Inspiratory capacity (IC): The maximum volume thatcan be inspired after expiration during normal, quietbreathing.SpirometryThese capacities and volumes are measured by spirometryand related techniques. In spirometry, the subject breathes inand out of a device known as a spirometer. Essentially, a spirometer is composed of two vessels: one contains water andFull appreciation of pulmonary volumes and capacitiesrequires understanding the relationships between theseparameters. For example, total lung capacity is the sum of fourof the described volumes:TLC = RV + ERV + VT + IRVSimilarly, vital capacity, functional residual capacity, and inspiratory capacity can be equated to the sums of particular lungvolumes:VC = IRV + VT + ERVFRC = ERV + RVIC = VT + IRVPulmonary Ventilation and Perfusion and Diffusion of GasesTerminal bronchiole153Pulmonary vein(to left side of heart)Pulmonaryartery (fromright sideof heart)Bronchial artery(from left side ofheart, via aorta)RespiratorybronchiolesCapillary plexuson alveolusPulmonaryvein (toleft sideof heart)Capillary plexuseson alveolar sacs(cut away in places)SeptumPleuraSeptumPleuraFigure 13.7 Intrapulmonary Blood Circulation The pulmonary circulation is a low-pressure, lowresistance circulation.
Blood from the right ventricle and pulmonary artery is distributed to the pulmonarycapillaries, where gas exchange takes place. The interface between the alveolar lumen and pulmonarycapillary blood consists of a single layer of alveolar epithelium, basement membrane, and the one-cell-layerthick capillary endothelium. Capillaries cover alveoli in this manner, providing for efficient gas exchange.the other floats upside down in the first (Fig.
13.9). As thesubject breathes through the attached tube, air flows in andout of the inner vessel, which consequently moves up anddown. This up-and-down motion is recorded as a spirogram(Fig. 13.10), which is calibrated to reflect changes in volumeof the inner vessel.Tidal volume is measured by spirometry during normal, quietbreathing and is the difference between end inspiratory andend expiratory levels (see Fig. 13.10). Note that the actualend inspiratory and end expiratory volumes are unknown,because the spirometer records changes in volume rather thanactual volume inside the lung.
By having the subject expiremaximally, expiratory reserve volume can be determined,because ERV is the difference between the resting end expiratory level and maximal expiratory level. Likewise, inspiratoryreserve volume can be measured by having the subject inspiremaximally and comparing the maximal inspiratory levelto the end inspiratory level during normal, quiet breathing.Vital capacity (VC) and inspiratory capacity (IC) can be154Respiratory PhysiologyType IIalveolar cellLamellar bodiesSurface-active layer (surfactant)CapillarylumenCapillarylumenType I alveolar celland nucleusAlveolus (airspace)Alveolar macrophageEndothelial cell and nucleusTight cell junctionsCapillarylumenEndothelial (loose) cell junctionsInterstitiumCapillarylumenAlveolus(airspace)Interstitial cellFused basement membranesType II alveolar cellFigure 13.8 Ultrastructure of Pulmonary Alveoli and Capillaries The alveolar epithelium consists of type I and type II alveolar epithelial cells.
Type I cells constitute the largest surface area; type II epithelial cells secrete surfactant. Gas diffusion takes place across the thin alveolar-capillary membranecomposed of alveolar epithelium, basement membrane, and capillary endothelium. Interstitial tissue isminimal in most of the interface between the alveolar epithelium and capillary endothelium in healthylungs.determined by similar comparisons: VC is the differencebetween the maximal inspiratory and expiratory levels; IC isthe difference between maximal inspiratory level and restingend-expiratory level.Measurement of Capacities (FRC, RV, and TLC)In order to measure total lung capacity, residual volume, andfunctional residual capacity, one of these parameters must bemeasured indirectly, for example by nitrogen washout, heliumdilution, or body plethysmography.
In the helium dilutiontechnique, a small volume of helium is added to the spirometer before the subject begins the test, resulting in a knowninitial helium concentration within the volume of the system.When the subject begins breathing (beginning at FRC), thisinitial helium concentration becomes diluted, as the gas equilibrates between the lungs of the subject and the spirometer.Because helium does not diffuse through the alveolarcapillary membrane and is inert, a stable equilibrium isreached quickly. This final concentration is dependent onlyon the initial concentration of helium within the spirometerand the volume of the spirometry system plus FRC.
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