Biomedical Engineering - Bioengineering of Physiological Control Systems

Cardiac Output Regulation

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SYSTEM /(( / RCULAT / ①N ma=PERIPHERALcircolatori2 85%totalbloodV15%total bloodV ' ,/"MICROCIRCULATION""\ : |I HLU I/'aARTERIES : VEINS : theMAJORRESISTNEtheMAJORRESERVOIRCOMPONENTforbloodvolumePaE- t.EE?IvIIFsi:niricantsP)oo-FPra(orPf ) RaRuFILUNGPRESSORECaCvBIGGERCompliance: / BigAVvsscuoleAPF-÷ELECTRICALMODELforSYSTEMICCIRCULATION"¥thisisWHYthe%Co=VR [ a=2%( vMechanismisPOSSIBLE Ra =170%RvHOWEVERisgodtotakeintoconsiderationtheFactthat( vempiricalyresultedtobeNOTLINEARVESSELS ' RESISTANCEREE-µ% etheRESISTANCEsinceAP=RQ,thepressoredrop.inVESSELS dramatically -addition alby →invesselsdraunaticallyincreaseasPoiseuiue'slawINCREASESasthevessel'stheresistance(R)increases,wrt µ RADIUSDECREASEStuesauneflow(Q)ButactuallyitisSupposedtohadanyforlargeCOMPLIANCEplaysanImportant | tlbes.RoleinpressoreREGULATIONpormodernesofELASTICITA COMPLIANCE((p ) =DPYOUNG ' SMODULUS(e) ishowmucuavessechamgesitsvolumeresuetingin agmaaaapaggunearopwreamywpy.pagfanimd.ca foroftheSTIFFNESS ofanelasticSolidMaterialtheHIGHERE,thetesstheelongatiom(all)UnderthesemePRESSORE (Fla)→ It is an example of NON LINEAR model that works around an EQUILIBRIUM POINT (or Working Point) in this case determined by the balance of 2 interacting steady-state curves: 1. CO, which is determined by the heart properties 2. VR, which depends on the peripheral circulation It is not just CO = VR, which is the trivial consequence of the constant total blood volume (VT, alias, volemia) that circulates. It is by far more than this! “The cardiac output (CO) in a compensated heart is equal to and determined by the amount of blood flowing into the heart (VR), and may be increased or diminished within very wide limits according to the inflow.” (Starling, 1918) This mechanism is important since it protects the lungs and the heart from the excess of volume load wrt to a wide range of VR variations. CARDIACOutputREGULATION see-0--0InthisapptoachtheSMALLCIRCULATIONisOVERLOOKEDandLUMPEDintheHLUfaggetafaggetaWhitetheI/0dynamicsoftheHLUthatregulatesco,accordingtoVR,describedinStarlingLawistakenintoaccountandplays ] aCentralroleexpeciauyonVenauscircnlatiomproperties STARLING 'sLAWco 9Pmsby9 ¥(=diCv)SPINALANESTHESIA= sympatheticinhibition The heart works as a dynamic pump. Its contractility depends primarily upon their contraction and relaxation properties of the myocardium ThePUMPINGHEART \ CARDIACMUSCLE:whosecontractoristriggeredbyACTIONPOTENTIALsTheTENSIONtheMuscle'scelldeve/opdependsontheINITIALFIBERLENGTH^NB . ThisholdsWitkinaLIMITEDRANGE ✓ 'ÈÉÈaddobbato"" [ PHYSIOLOGICALLIMIT" RANGEofINCREASINGCONTRACT /LITYrangeoflemghtstowhichmycocardialcellsareabletorespondwithPROPORTIONALSTRENGTHtoimcreasimgSTRETCHNON-NORMALCONDITIONtheStretchofthemyocardiumexceedsthePHYSIOLOGICALLIMIT,itistaDILATEDanditcannotdevelopaproportioneForceofcontracton.→HEARTFAILURERESTvsCONIRACTWhenaMuscleisACTIVEitabilitytocontract,i.e.togeneratehigherAPwithSmollerchamgesinvolumeis higherthan in RESTCONDITIONS :monocolo•ACTIVE:SmollerDV → bargerDP VENTRICULARFUNCTIONSMALLERCURVE• RELAXED : bargerDU moiSmollerDPCOMPLIANCE@DIASTOLE@SYSTOLELARGERCOMPLIANCE))_,#ÈÈ :ÈÉ I:* !DEADVOLUME(vd)isthetheVolumeofbloodinthet,È"FW-s.isVENIRNEatNULLTRANSMURALPRESSORE(Vds#Voi) 1. End of the filling phase of the previous cycle ( 5 → 1 ) the heart is relaxed and the external filling pressure provokes the filling accordingly to the ventricle diastolic compliance 2. The ventricle activates, provoking a steep isovolumetric rise in pressure (1 →2) 3. Ejection phase ( 2→4 ), divided into two tracts: (2→3) arterial pressure (AP) increases due to ejection faster than the windkessel runoff to the peripheral resistance (3→4) arterial pressure decreases since ejection is still positive but less than the WK runoff Then when the volume of the heart reaches the end systolic volume ejection ends (point 4), at this point the ventricle compliance is balanced by the WK compliance. 4. Ventricle relaxation (4 → 5) immediately follows with a fall of pressure to the diastolic P-V characteristics (point 5). A non-isolated left ventricle will not interact without reservoirs having a virtually infinite compliance. Conversely, ejection and filling will follow the pressure profiles determined by the interacting compartments (aorta and pulmonary veins), therefore inevitably a more complex P-V loop results. VENTRKLEDYNAMICS - PVLOOPISOLATEDVEIVIRICLE(Ideal)Hp:coCOMPLIANCE(Cv→co ) a>1FKLINGPHASEB-,Vs-"¥"¥EnergytheENDDIASTOLEVOLUME(Vis)isdeterminiedbytheFKLINGPRESSURE(Pt)e>aISOVOLUMETRICCONTRACT/ONVD,B->Pathe theventridecontractsraisingpressoretoprovatetheejectiouDicoassEJECTIONPHASEPa,VD> VsTEnergy} seav>oBloodFlowsout,thevolumegoesbacktoitsminimum3>aISOVOLUMETRICRELAXATION(Pa-Ps,Vs) thenutricegoesbacktoRentConditiontoallowtueRE-fittingENERGY:a-1Vf>Vi→Di>0:thesystemRECENESENERGYsthatisStordasELASTICENERGY,thankstoCOMPLIANCE,andteamUsenetforEJECTION2>3Vg