Chapter10-RespiratorySystemSTRUCTUREANDFUNCTIONMortonLippmannTherespiratorysystemextendsfromthebreathingzonejustoutsideofthenoseandmouththroughtheconductiveairwaysintheheadandthoraxtothealveoli,whererespiratorygasexchangetakesplacebetweenthealveoliandthecapillarybloodflowingaroundthem.Itsprimefunctionistodeliveroxygen(O2)tothegas-exchangeregionofthelung,whereitcandiffusetoandthroughthewallsofthealveolitooxygenatethebloodpassingthroughthealveolarcapillariesasneededoverawiderangeofworkoractivitylevels.Inaddition,thesystemmustalso:(1)removeanequalvolumeofcarbondioxideenteringthelungsfromthealveolarcapillaries;(2)maintainbodytemperatureandwatervapoursaturationwithinthelungairways(inordertomaintaintheviabilityandfunctionalcapacitiesofthesurfacefluidsandcells);(3)maintainsterility(topreventinfectionsandtheiradverseconsequences);and(4)eliminateexcesssurfacefluidsanddebris,suchasinhaledparticlesandsenescentphagocyticandepithelialcells.Itmustaccomplishallofthesedemandingtaskscontinuouslyoveralifetime,anddosowithhighefficiencyintermsofperformanceandenergyutilization.Thesystemcanbeabusedandoverwhelmedbysevereinsultssuchashighconcentrationsofcigarettesmokeandindustrialdust,orbylowconcentrationsofspecificpathogenswhichattackordestroyitsdefencemechanisms,orcausethemtomalfunction.Itsabilitytoovercomeorcompensateforsuchinsultsascompetentlyasitusuallydoesisatestamenttoitselegantcombinationofstructureandfunction.MassTransferThecomplexstructureandnumerousfunctionsofthehumanrespiratorytracthavebeensummarizedconciselybyaTaskGroupoftheInternationalCommissiononRadiologicalProtection(ICRP1994),asshownin聽figure10.1聽.Theconductiveairways,alsoknownastherespiratorydeadspace,occupyabout0.2litres.Theyconditiontheinhaledairanddistributeit,byconvective(bulk)flow,totheapproximately65,000respiratoryacinileadingofftheterminalbronchioles.Astidalvolumesincrease,convectiveflowdominatesgasexchangedeeperintotherespiratorybronchioles.Inanycase,withintherespiratoryacinus,thedistancefromtheconvectivetidalfronttoalveolarsurfacesisshortenoughsothatefficientCO2-O2聽exchangetakesplacebymoleculardiffusion.Bycontrast,airborneparticles,withdiffusioncoefficientssmallerbyordersofmagnitudethanthoseforgases,tendtoremainsuspendedinthetidalair,andcanbeexhaledwithoutdeposition.Figure10.1Morphometry,cytology,histology,functionandstructureoftherespiratorytract聽andregionsusedinthe1994ICRPdosimetrymodelAsignificantfractionoftheinhaledparticlesdodepositwithintherespiratorytract.Themechanismsaccountingforparticledepositioninthelungairwaysduringtheinspiratoryphaseofatidalbreatharesummarizedin聽figure10.2聽.Particleslargerthanabout2碌minaerodynamicdiameter(diameterofaunitdensityspherehavingthesameterminalsettling(Stokes)velocity)canhavesignificantmomentumanddepositbyimpactionattherelativelyhighvelocitiespresentinthelargerairways.Particleslargerthanabout1碌mcandepositbysedimentationinthesmallerconductiveairways,whereflowvelocitiesareverylow.Finally,particleswithdiametersbetween0.1and1碌m,whichhaveaverylowprobabilityofdepositingduringasingletidalbreath,canberetainedwithintheapproximately15%oftheinspiredtidalairthatisexchangedwithresiduallungairduringeachtidalcycle.Thisvolumetricexchangeoccursbecauseofthevariabletime-constantsforairflowinthedifferentsegmentsofthelungs.Duetothemuchlongerresidencetimesoftheresidualairinthelungs,thelowi...
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