Download expiration is called the functional residual capacity

Tidalvolumeisnormalbreathing.MAXinspirtionfollowedbyMAXexpiration;the
exhaledvolumeisthevitalcapacityandthegas(ofthemaximallyinspiredbreath)
thatremainsinthelungsiscalledtheresidualvolume.Thevolumeofgas
remaininginthelungafteranormal
expirationiscalledthefunctional
residualcapacity.FRCandRV
havetobemeasuredbyheliumdilution
techniquesorbodyplethysmography,
notaspirometer.
AverageTidalVolume=500ml,
Totalvent=7500ml(RRof15)
Anatomicaldead-space=150ml
Alveolargas=~3000ml,
Alveolarventilation=5250ml/min
Pulmonarycapilaryblood=70ml
Pulmonarybloodflow=5000ml/min
Alveolarventilationiswhatreallymatters-isthevolumeoffresh(non-deadspace)gasthat
enterstherespiratoryzone(areaofgasexchange)perminute.Itcanbedeterminedbythe
CO2outputofthebodydividedbythefractionalconcentrationofCO2intheexpiredgas
(alveolargasequation).TheconcentrationofCO2(andit'spartialpressure)inalveolargasis
inverselyrelatedtothealveolarventilation-thisisimportantbecauseifalveolarventilation
HALVES,thenpaCO2doubles.Innormalbodies,thepCO2ofarterialbloodandinthe
alveoliareprettymuchidentical.
Va-Ventilationofalveoli
Vco2=rateofCO2production
Pco2=partialpressureofCO2
K=constantof863mmHg
Thisisthevolumeoftheconductingairways.Normalvalueisaround150ml,butincreaseswith
largerbreathsbecauseofthetractionexertedonthebronchibythesurroundinglung
parenchyma.DSalsodependsonthesizeandpostureofthepatient-usually2mg/kgin
uprightposition.FOWLER'SMETHODestimatesdeadspacevolumeusingavalveboxanda
continuousnitrogenanalyseratthelips.-followingasinglebreathof100%O2,nitrogenwill
risesasthedeadspacegasisincreasinglywashedoutbyalveolargas,finallyauniformconc
isseenontheanalyseras'pure'alveolargascomesout.
DSgivesinformationonhowmuchoftotalventilationthatreachesbothventilatedandperfused
alveoliandthusallowsgasexchangebetweenalveoliandpulmonaryblood.
RealisingthatCO2retentioncanbeaneffectnotonlyoflowtotalventilationbutalsoof
increaseddeadspaceisIMPORTANT.
Moreover,deadspacewillgiveinsightintothematchingofventilationandperfusion.
ExpiredCO2comesfromalveolargas,notanatomicaldeadspace.Therefore,Bohr's
methodcancalculatephysiologicaldeadspace.ITISTHEVOLUMEOFGASTHATDOES
NOTELIMINATECO2.YouhavetocomparearterialandexpiredCO2tocalculatethe
mismatch.ThisisimportantwhenconsideringPE'sandmanyotherlungdiseases.
VD=Physiologicaldeadspace
VT=Tidalvolume
PaCO2=ArterialCO2
PeCO2=ExpiredCO2
Thiswasidentifiedwhentheycalculatedwhereradiationofinhaledxenonwas
detectedbycounterslinedupagainsttheposteriorchestwall.Lowerportions
ofthelungshadmorexenondetectedinthem.
Thetopofthelungswereleastwellventilated.Thisisallduetogravitywhenstandingup.
Whensupine-apexandbaseoflungarethesame,butthePOSTERIORlobesarebetterventilated.
Whenlateral-thedependentareaisbetterventilated.
Intrapleuralpressureishigheratthebasesofthelungs(lessnegative)
-andexpandsbetteroninspiration,thereforeventilationisbetter....(largeoversimplification!)
ThisconceptisextremelyimportantinsicklungsandparticularlyinICU-whichiswhysome
practitionerselecttoPRONEpeopletochangethe
dependentareaofthelung,improveventilationin
thehealthierportionsoflungandimprove
oxygenation.
West'sPhysiology+http://www.ncbi.nlm.nih.gov/pubmed/16682925
ProfessorWesthasawonderfulrepositoryofonlinevideotutorialsonrespiratoryphysiology
-http://meded.ucsd.edu/ifp/jwest/resp_phys/