Osmo Larity

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    275:S169-S184, 1998.Advan in Physiol EduSusan P. Bagby and William M. BennettOSMOLARITY/WATER REGULATIONDISORDERS OF TOTAL BODY FLUIDVOLUME/NA CONTENT REGULATION VERSUSDIFFERENTIATING DISORDERS OF ECF

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    can be found at:Advances in Physiology EducationaboutAdditional material and information

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    This infomation is current as of September 19, 2011.

    Physiological Society. ISSN: 1043-4046, ESSN: 1522-1229. Visit our website at http://www.the-aps.org/.the American Physiological Society, 9650 Rockville Pike, Bethesda MD 20814-3991. Copyright 1998 by the Americanand in the broader context of general biology education. It is published four times a year in March, June, September and December by

    is dedicated to the improvement of teaching and learning physiology, both in specialized coursesAdvances in Physiology Education

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    DIFFERENTIATING DISORDERS

    OF ECF VOLUME/ NA CONTENT REGULATION

    VERSUS DISORDERS OF TOTAL BODY FLUIDOSMOLARITY/ WATER REGULATION

    Susan P. Bagby and William M. Bennett

    Portland Veteran s Affairs Medical Center, Portlan d 97207; and Divi sion of

    Nephrology/ Hypertension , Or egon Health Sciences Un iversity, Port lan d, Or egon 97201

    When one is teaching students who are new to concepts of body fluid and

    electrolyte regulation, a major challenge is to convey the separate but

    interactive natureof the two systems that respectively regulate extracellular

    fluid(ECF) volume/Nacontentandtotal bodyfluidosmolarity/water. Wehavedeveloped

    a series of strategies/tools designed to both optimize conceptual understanding and

    translate conceptsto clinical practice. These include 1) clear delineation of the distinct

    differencesbetween thetwo homeostatic systems,reinforced in instructional objectives,

    lectures, and small group sessions; 2) anticipation and direct confrontation of the

    common Na content Na concentration error, soliciting student participation in

    oustingthismisconception; 3) modificationof terminologyto clarifybodyfluid reality;4)

    facilitation of active problem-based learning in small group sessions focused on clinical

    casesof increasingcomplexity (50%of coursehours); 5) useof whole body/single-eyeful

    graphicsto conveyessentialdetails within aclinically meaningful context; 6) standardiza-

    tion of diagnostic algorithms and pathophysiological graphs across lecturersand coursecomponents; and 7) provision of hands-on instruction/practice in physical examination

    of ECFvolume in parallel with conceptual learning, thus emphasizingtheimportanceof

    the bedside exam in detecting disorders of ECF volume/Na content. These approaches

    require an enthusiastic and well-prepared faculty committed to a high level of

    consistency and are designed for second-year students with a solid basic science

    background.

    AM. J. PHYSIOL. 27 5 ( ADV. PHYSIOL. EDUC. 20): S169 S184 , 199 8.

    In our collective years of teaching medical students,

    no concept hasbeen morechallengingto convey than

    theseparate-but-interactivenatureof thetwo distincthomeostatic systems that regulate extracellular fluid

    (ECF) volume/ECF Na content on one hand versus

    total body fluid (TBF) osmolarity/water on the other

    hand. Few conceptual distinctions are so crucial to

    safe clinical management of body fluid disorders.

    Teaching this topic in the second year, and seeing

    little evidence of useful retention by students in thethird year, convinced us that fresh approaches were

    needed.

    GOAL

    Our goal is to train clinicians whose understanding offluid/electrolyte concepts permits differentiation ofclinical abnormalitiesof ECFvolume/Nacontent regu-lation from abnormalities of TBF osmolarity/water

    A P S R E F R E S H E R C O U R S E R E P O R T

    1043 - 4046 / 98 $5.00 COPYRIGHT 1998THE AMERICAN PHYSIOLOGICAL SOCIETY

    VOLUME20: NUMBER 1 ADVANCESIN PHYSIOLOGY EDUCATION DECEMBER 1998

    S169

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    regulation and leadsto appropriate treatment of thesedisorders, whether present alone or in combination.

    By choice, we have focused on homeostasis at thelevel of thewhole organism and have, over theyears,deliberately omitted myriad important details at cellu-lar and molecular levels. On the other hand, ourapproach provides a framework for ready incorpora-tion of such details and can facilitate their conceptualintegration in courses designed for different levels ofcomplexity.

    THE WORKING REALITY

    Body-fluid osmolarity is regulated by hypothalamicsensors, which detect osmolarity, and by effectors,which modify water handling (intake and excretion).

    Because water moves freely throughout the bodyfluids (crossing vascular and cellular boundariesreadily), this system regulates intracellular fluid (ICF)and ECF compartments as a single unit, the TBFcompartment. Thus, at steady state, theosmolarity ofICF and ECF are always equal. When disorders of TBFosmolarity/water regulation occur, ICF and ECFosmo-laritywill both bealtered.

    Despite this dependable concordance of TBF osmolarregulation, the specific solute molecules that consti-tute the ICF versus ECF osmoles are distinct, and it isthe energy-dependent separation of these solutes at

    thecell membranethat createstheICF/ECFboundary.Thissolute-based subcompartmentalizationof theTBFspace creates the possibility that the ICF and the ECFcompartments may change their respective volumeseitherconcordantly(whenwater or anoncompartmen-talized solute is in excess or deficit) or discordantly(when a compartmentalized solute is in excess ordeficit). For example, if water is ingested in excess (oran evenly distributed solutesuch as ureais ingested inexcess, triggering water retention), the increase inTBF water will be distributed to the ICF and ECFcompartments in proportion to their initial sizes; thustheir respective volumes will change proportionately(i.e., by a similar percent increase) with two-thirds ofthewater excess goinginto theICF and one-thirdintotheECF (Fig. 1, upper pathway).

    In contrast, in response to theadditionof asolute thatis restricted to oneof thetwo compartments, homeo-statically retained waterafter equilibrationwill re-

    side in the compartment containing the extra solute.To understand the impact of this aspect of osmoregu-lation on the ECF compartment, it is helpful toconsider what would happen to theECF Volume afterNaCl ingestion if no ECF volume/Na content regula-tory system were operative. When NaCl, which isconfined to the ECF, is ingested without water (Fig. 1,lower pathway), ECF hypertonicity transiently drawswater from the ICF and also activates thirst/waterretention. If water ingestion is prevented and osmoticequilibration viawater shifts proceedsto completion,the resulting TBF osmolarity is above normal butlower than the ECF osmolarity immediately after saltingestion. Furthermore, ECFvolumeis now increased,whereas ICF volume is decreased by the volume ofwater shifted to the ECF. This state exists until water

    ingestion occurs; water will then be homeostaticallyretained in a volume sufficient to restoretheosmolar-ityof both ICF andECF to normal. To achieve this, thewater must be retained in sufficient quantity to re-place the volume of ICF water initially shifted to theECF (therebynormalizing ICF volume and osmolarity)andmust further increasetheECFwater byan amountsufficient to normalize ECF osmolarity. If there is stillno ability to regulate ECF volume or Na content, theingested NaCl would persist as excess ECF NaClcontent. The osmoregulatory system would thus re-tain sufficient water to dilute an elevated ECF Naconcentration (and osmolarity) to normal, mandating

    that the total ECF water content be increased. Thus,compared with the status before NaCl ingestion, theentirevolumeof new water gained in ourhypotheti-cal system would exactly equal the volume of waterretained within the ECF by the osmotic action of thecompartmentalizedNaCl;therehasbeenno n etchangein theICF volume or osmolarity.

    Thus if the TBF osmolarity/water regulating systemwere operative in isolation, with no mechanism formonitoring/modifyingECF volume, then thetransientECF hyperosmolarity after NaCl ingestion would beroutinely normalized at the expense of an increasedECF volume and ECF Na content (without change inICF volume or solute content). The wide range oftolerable Na intakes that we take for granted wouldlikely lead to hypertension, pulmonary edema, anddeath with stroke or heart failure as cumulativepositive NaCl balance ensued and obligated isotonicECFwater retention in response to osmoregulation.

    A P S R E F R E S H E R C O U R S E R E P O R T

    VOLUME 20 : NUMBER 1 ADVANCES IN PHYSIOLOGY EDUCATION DECEMBER 1998

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    Fortunately, operating in parallel with TBF osmoregu-lation, thebodyis equipped with aseparateregulatorysystem that limits both ECF volume expansion andcontraction, therebyprotecting the hemodynamicallycritical intravascular compartment from unopposed

    osmoregulation. ECF volume is regulated by intravas-cular sensors, which detect mechanical stretch (thusvascular volume) and by effectors which modify ECFNa content. In fact, altering Na content of the ECFcompartment can effect a change in ECF vo

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