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Chapter26 Fluids_ Electrolytes_ Acid-Base

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					                                           Chapter 26 Fluids, Electrolytes , Acid-Base
                                                               fluid homeostasis
•   maintain osmotic equilibrium of all body fluids
•   maintain fluid volume
•   maintain cell volume
•   maintain electrolyte levels
•   maintain optimum pH

                                                             integration of systems
•   renal
•   cardiovascular
•   respiratory
•   neural
•   endocrine
•   behaviors
                                                                       Water
•   50 – 75 % of body weight                               ~ 42 L
•   universal solvent
•   ICF = intracellular fluid                              ~ 28L
•   ECF = extracellular fluid                              ~ 14 L
            • blood                    plasma ~ 5L
            • tissue fluid             interstitial
            • lymph vessels            lymph
            • meninges                 CSF
            • joints                   synovial fluid

                                                                     water flow
•   continuous exchange between ECF’s and ICF
•   same osmolarity            285 – 300 mOsm / L

•   water movement :
•   osmosis
     –   towards high particle concentration
     –   any change in solute concentration leads to water flow
•   filtration
     –   any pressure gradient leads to water flow

                                                                    mass balance
•   water intake = water output                  ~ 2.5 L
•   1o intake        = beverages
     –   also        food
                     metabolic water
•   1o output        = urine
     –   also        sweat
                     respiratory
                                                             obligatory water loss
•   insensible water loss      ~ 900 ml / day
     –   respiratory
     –   skin
     –   feces
•   sensible water loss        ~ 500 ml / day
     –   amt urine to get solutes out of the body

                                                             additional water loss
•   varied urine volume
     –   where?
•   to balance intake
•   to correct blood volume (BP)

                                                          regulation of water volumes
•   hypothalamus :
     –   osmoreceptors         water conservation
          • affects rate of ADH release
     –   thirst                         water intake
•   renal :
     –   RAAS       renin-angiotensin-aldosterone system
          • stim by:            blood volume , low BP
     –   GFR                   Δ ~ blood volume

                                                              need more water ?
•   need more water                     too much water
•   hypothalamus                        hypothalamus
     –   thirst                ?                 thirst            ?
     –   ADH                   ?                 ADH               ?

•   kidney                              kidney
     –   renin                 ?                  renin            ?
     –   GFR                   ?                  GFR              ?

                                                                   disorders
•   dehydration
     –   hemorrhage
     –   vomiting, diarrhea
     –   skin       -          sweat , burns
•   hypotonic hydration
     –   extreme intake
     –   renal insufficiency
•   edema
                                                                    solutes
•   electrolytes               dissociate into ions

•   non-electrolytes
     –   glucose
     –   proteins
     –   lipids
                                                           Electrolytes
•   molecules that dissociate in water
•   + ions           =        cations
•   - ions           =        anions

•   functions:       osmolarity
                     acid – base balance
                     tissue / organ functions

•   mEq/L            =         milliequivalent per liter
                     electric charges / L
                     = moles x electric charge
•   1 mEq/L          =        1 mOsm

                                                             cations
•   Na+ sodium                ECF
                              nerve, muscle
                              osmolarity
•   K+    potassium           ICF
                              nerve, muscle
•   Ca++ calcium              bones, teeth
                              nerve (NT release)
                              muscle contraction
                              cardiac conduction
                              blood clotting
•   Mg++ magnesium            bone
                              ATP
                                                             anions
•   Cl-
          chloride            ECF
                              HCl
•   HCO3- bicarbonate         buffer system
                              CO2 transport
•   HPO4-- phosphate          ICF
                              bones, teeth
                              DNA, RNA, ATP
                              phosphate buffer
•   proteins -                buffers
                              plasma osmolarity

                                                             sodium
•   most abundant cation in ECF
•   mostly NaCl and NaHCO3
•   142 mEq/L

•   accounts for most osmolarity of plasma and ECF
•   primary molecule in movement of water
•   primary in movement of many ions

•   no Na receptors have been found
•   regulation tied to        osmolarity
                              blood pressure (volume)
                                                       what’s wrong with a little sodium ?
•   daily Na intake would raise Osm to 305-310 mOSm
•   cells shrink
•   increased blood volume / BP
•   nerve depolarization
                                                                  sodium regulation
•                              +
    kidney reabsorbs 90% Na w/o hormonal control
•   aldosterone     increases Na reabsorption
     –   stim:      low BP             via renin-angiotensin
                    sympathetic                 renin
                    hi K
•   ANP             decreases Na reabsorption
                    inhibits aldosterone , renin
•   water intake             lowers osm

                                                           sodium regulation – part 2
•   estrogen        like aldosterone                Na rebsorb
                    “retain water”
•   progesterone blocks aldosterone                Na excreted
•   hi cortisol      Na and water reabsorption                       edema

                                                                      potassium
•   main cation ICF
•   affects resting membrane potential
•   esp. neurons, muscles, heart

•   any change in K+ affects nerves, muscles, heart:
•   high K+ in ECF                     cells depolarize
•          +
    low K in ECF                       hyperpolarize
•   low pH (hi H ) +
                                       hyperpolarize

                                                                 potassium regulation
•   10-15 % lost in urine regardless of body’s need

•   aldosterone      K+ secreted
     –   stim:                K+ in ECF / plasma
                             renin-angiotensin

•   Addison’s                hyperkalemia
•   diuretics                possible hyperkalemia
•   water (intake) hypokalemia

                                                                       calcium
•   functions       ??




•   hypocalcemia             tetany                ( Na permeability)
•   hypercalcemia            inhibits neurons
                                                             calcium regulation
•   blood levels important, not bone levels
•   renal constant PCT reabsorption

•   PTH parathyroid hormone            blood Ca
     –    bone                         Ca to blood
     –    small intestine              Ca absorption
     –    kidney                       reabsorption (DCT)

•   calcitonin                         blood Ca
     –     Ca deposition to bone

                                                                  anions
•   Cl-
                     follows Na+

                     exchanged for HCO3-
•   HCO3-            amount varies to control pH


                                                             Acid – Base , pH
•   pH = parts hydrogen
•   acid increases H+        eg. HCl ; -COOH
•   base decreases H     +
                             eg. NaOH ; NaHCO3

•   in blood :
     –    increased pH                alkalosis > 7.45
     –    decreased pH                acidosis < 7.35
•   ICF                      7.0

                                                              the pH problem
•   pH ~ free H ions+


•   most bodily functions are affected by pH changes !
     –    protein functions (3D shape depends on H bonds)
     –    enzymes
     –    Na+ and K+ concentrations

•   pH extremes
     –    < 7.0      CNS depressed             coma, death
     –    > 7.8     CNS overexcited            convulsions
           respiratory arrest

                                                         the body produces acids
•   amino acids
•   fatty acids
•   keto acids
•   lactic acid
•   H+ from gastric HCl
•   CO2 is the largest source of acid (H+)
                                                            acid – base homeostasis
•   buffer systems                        fastest                     weak, short term

•   respiratory mechanisms                slower    (few minutes)     stronger    75% effective

•   renal mechanism                       slowest (several hours)     strongest          long term

                                                                    buffer systems
•   buffer                       =        weak acid or weak base
•   buffer system =              weak acid + weak base

•   strong acid + buffer              weak acid
•   strong acid + weak base  weak acid + salt

•   strong base + buffer              weak base
•   strong base + weak acid  weak base + water

                                                                3 buffer systems
•   bicarbonate system
           • blood and ECF
•   phosphate system
           • kidney , ICF
•   protein system
           • ICF
                                                           bicarbonate buffer system
•   bicarbonate ion                       HCO3-

•   weak acid                    carbonic acid                         H2CO3
•   weak base                    sodium bicarbonate                    NaHCO3

•   strong acid + weak base            weak acid + salt
•    HCl        +      NaHCO3  H2CO3 + NaCl

•   strong base + weak acid           weak base + water
•    NaOH          +       H2CO3  NaHCO3 + H2O

                                                            constant source of HCO3-
•   all cells produce HCO3   -


•   alkaline reserve

•   CO2 + H2O              H2CO3           H+ + HCO+-

                                                            phosphate buffer system
•   weak acid                    sodium dihydrogen phosphate          Na H2PO4
•   weak base                    sodium monohydrogen phosphate Na2HPO4


•   strong acid + weak base  weak acid + salt
•    HCl           + Na2HPO4          Na H2PO4 + NaCl

•   strong base + weak acid  weak base + water
•    NaOH          + Na H2PO4  Na2HPO4 + H2O
                                                            protein buffer system
•   amino acid               = weak acid and a weak base
          • amino group      = weak base      NH2
          •
          • carboxyl                  = weak acid         COOH

•   eg. Hemoglobin

                                                            respiratory mechanism
•   respiratory rate affects pH
•   by changing concentration of CO2 in blood
•   stimulus:
     –   H+ concentration affects carotid/aortic chemoreceptors
     –   CO2 concentration affects medulla

•   compensates for metabolic causes of pH imbalance

                                                         ventilation corrects blood pH
•   CO2 + H2O           H2CO3         +
                                       H + HCO3      -




•    rate   CO2   pH
     –  CO2 + H2O         H2CO3  H+ + HCO3-

•    rate   CO2   pH
     –    CO2 + H2O       H2CO3  H+ + HCO3-


•    respiratory rate       causes       blood pH
     –   to  blood pH           ? respiratory rate
     –   to  blood pH           ? respiratory rate


                                                               renal mechanism
•   “ultimate acid-base regulatory organ”
•   can buffer any pH imbalance               lactic acid
                                              uric acid
                                              ketones
                                              CO2
                                              bases

•   H+ and HCO3- secretion / reabsorption

•   wide range of urine pH

                                                    HCO3- reabsorption affects blood pH
•          -
    HCO3 filtered in glomerulus
•   constantly reabsorbed to blood

•   to raise blood pH         HCO3- reabsorption
•   to lower blood pH         HCO3- reabsorption

•
    new HCO3- from CO2 or glutamine
                                                              acid-base imbalances
•   respiratory                 caused by respiratory problem
     –   respiratory acidosis           poor CO2 exchange
                                        respiratory disease
     –   respiratory alkalosis          hyperventilation

•   metabolic                           caused by non-respiratory problem
     –   metabolic acidosis             diarrhea
                                        lactic acid (exercise)
                                        ketosis (diabetes)
     –   metabolic alkalosis            antacids
                                        constipation
     –   kidney diseases                usually acidosis

                                                                 pH compensations
•   respiratory mechanism
     –   will compensate for metabolic and renal causes
     –   can’t compensate for respiratory causes
     –   to  blood pH              ? respiratory rate
     –   to  blood pH              ? respiratory rate

•   renal mechanism
     –   will compensate for metabolic and respiratory causes
     –   can’t compensate for renal causes
     –   to  blood pH                 ? HCO3- reabsorption
     –   to  blood pH                 ? HCO3- reabsorption

				
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posted:11/30/2011
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