water, electrolyte, and acid-base balance

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  • 24-*



  • 24-*Water, Electrolyte and Acid-Base Balance

  • 24-*Water BalanceTotal body water for 150 lb. male = 40LFluid compartments65% ICF35% ECF25% tissue fluid8% blood plasma, lymph2% transcellular fluid (CSF, synovial fluid)

  • 24-*Water Movement in Fluid CompartmentsElectrolytes play principle role in water distribution and total water content

  • 24-*Water GainPreformed wateringested in food and drink

    Metabolic waterby-product of aerobic metabolism and dehydration synthesis

  • 24-*Water LossRoutes of lossurine, feces, expired breath, sweat, cutaneous transpirationLoss varies greatly with environment and activityrespiratory loss : with cold, dry air or heavy workperspiration loss : with hot, humid air or heavy workInsensible water lossbreath and cutaneous transpirationObligatory water lossbreath, cutaneous transpiration, sweat, feces, minimum urine output (400 ml/day)

  • 24-*Fluid Balance

  • 24-*Regulation of Fluid IntakeDehydration blood volume and pressure blood osmolarityThirst mechanismsstimulation of thirst center (in hypothalamus)angiotensin II: produced in response to BPADH: produced in response to blood osmolarityhypothalamic osmoreceptors: signal in response to ECF osmolarityinhibition of salivationthirst center sends sympathetic signals to salivary glands

  • 24-*Satiation MechanismsShort term (30 to 45 min), fast actingcooling and moistening of mouthdistension of stomach and intestine

    Long term inhibition of thirstrehydration of blood ( blood osmolarity)stops osmoreceptor response, capillary filtration, saliva

  • 24-*Dehydration, Thirst, and Rehydration

  • 24-*Regulation of OutputControlling Na+ reabsorption (changes volume)as Na+ is reabsorbed or excreted, water follows

    Action of ADH (changes concentration of urine)ADH secretion (as well as thirst center) stimulated by hypothalamic osmoreceptors in response to dehydrationaquaporins synthesized in response to ADHmembrane proteins in renal collecting ducts to channel water back into renal medulla, Na+ is still excretedeffects: slows in water volume and osmolarity

  • 24-*Secretion and Effects of ADH

  • 24-*Disorders of Water BalanceFluid deficiency volume depletion (hypovolemia)total body water , osmolarity normalhemorrhage, severe burns, chronic vomiting or diarrheadehydration total body water , osmolarity riseslack of drinking water, diabetes, profuse sweating, diureticsinfants more vulnerablehigh metabolic rate demands high urine excretion, kidneys cannot concentrate urine effectively, greater ratio of body surface to massaffects all fluid compartmentsmost serious effectscirculatory shock, neurological dysfunction, infant mortality

  • 24-*Water Loss & Fluid Balance1) profuse sweating2) produced by capillary filtration3) blood volume and pressure drop, osmolarity rises4) blood absorbs tissue fluid to replace loss5) fluid pulled from ICF

  • 24-*Fluid ExcessVolume excessboth Na+ and water retained, ECF isotonicaldosterone hypersecretion

    Hypotonic hydrationmore water than Na+ retained or ingested, ECF hypotonic - can cause cellular swelling

    Most serious effects pulmonary and cerebral edema

  • 24-*Blood Volume & Fluid IntakeKidneys compensate very well for excessive fluid intake, but not for inadequate intake

  • 24-*Fluid SequestrationExcess fluid in a particular location

    Most common form: edemaaccumulation in the interstitial spaces

    Hematomashemorrhage into tissues; blood is lost to circulation

    Pleural effusionsseveral liters of fluid may accumulate in some lung infections

  • 24-*ElectrolytesFunctionchemically reactive in metabolismdetermine cell membrane potentialsaffect osmolarity of body fluidsaffect bodys water content and distribution

    Major cationsNa+, K+, Ca2+, H+

    Major anionsCl-, HCO3-, PO43-

  • 24-*Sodium - FunctionsMembrane potentials

    Accounts for 90 - 95% of osmolarity of ECF

    Na+- K+ pump exchanges intracellular Na+ for extracellular K+creates gradient for cotransport of other solutes (glucose)generates heat

    NaHCO3 has major role in buffering pH

  • 24-*Sodium - HomeostasisPrimary concern - excretion of dietary excess0.5 g/day needed, typical diet has 3 to 7 g/dayAldosterone - salt retaining hormone # of renal Na+/K+ pumps, Na+ and K+ reabsorbedhypernatremia/hypokalemia inhibits releaseADH - blood Na+ levels stimulate ADH releasekidneys reabsorb more water (without retaining more Na+)ANP (atrial natriuretic peptide) from stretched atriakidneys excrete more Na+ and H2O, thus BP/volumeOthers - estrogen retains water during pregnancyprogesterone has diuretic effect

  • 24-*Sodium - ImbalancesHypernatremiaplasma sodium > 145 mEq/Lfrom IV salinewater retension, hypertension and edema

    Hyponatremiaplasma sodium < 130 mEq/Lresult of excess body water, quickly corrected by excretion of excess water

  • 24-*Potassium - FunctionsMost abundant cation of ICF

    Determines intracellular osmolarity

    Membrane potentials (with sodium)

    Na+-K+ pump

  • 24-*Potassium - Homeostasis90% of K+ in glomerular filtrate is reabsorbed by the PCT

    DCT and cortical portion of collecting duct secrete K+ in response to blood levels

    Aldosterone stimulates renal secretion of K+

  • 24-*Secretion and Effects of Aldosterone

  • 24-*Potassium - ImbalancesMost dangerous imbalances of electrolytes

    Hyperkalemia-effects depend on rate of imbalanceif concentration rises quickly, (crush injury) the sudden increase in extracellular K+ makes nerve and muscle cells abnormally excitable slow onset, inactivates voltage-gated Na+ channels, nerve and muscle cells become less excitable

    Hypokalemiafrom sweating, chronic vomiting or diarrheanerve and muscle cells less excitablemuscle weakness, loss of muscle tone, reflexes, arrthymias

  • 24-*Potassium & Membrane Potentials

  • 24-*Chloride - FunctionsECF osmolarity most abundant anions in ECFStomach acid required in formation of HClChloride shift CO2 loading and unloading in RBCspHmajor role in regulating pH

  • 24-*Chloride - HomeostasisStrong attraction to Na+, K+ and Ca2+, which it passively follows

    Primary homeostasis achieved as an effect of Na+ homeostasis

  • 24-*Chloride - ImbalancesHyperchloremia result of dietary excess or IV saline

    Hypochloremiaresult of hyponatremia

    Primary effects pH imbalance

  • 24-*Calcium - FunctionsSkeletal mineralization

    Muscle contraction

    Second messenger


    Blood clotting

  • 24-*Calcium - HomeostasisPTHCalcitriol (vitamin D)Calcitonin (in children)these hormones affect bone deposition and resorption, intestinal absorption and urinary excretionCells maintain very low intracellular Ca2+ levels to prevent calcium phosphate crystal precipitationphosphate levels are high in the ICF

  • 24-*Calcium - ImbalancesHypercalcemiaalkalosis, hyperparathyroidism, hypothyroidism membrane Na+ permeability, inhibits depolarizationconcentrations > 12 mEq/L causes muscular weakness, depressed reflexes, cardiac arrhythmias

    Hypocalcemia vitamin D , diarrhea, pregnancy, acidosis, lactation, hypoparathyroidism, hyperthyroidism membrane Na+ permeability, causing nervous and muscular systems to be abnormally excitable very low levels result in tetanus, laryngospasm, death

  • 24-*Phosphates - FunctionsConcentrated in ICF asphosphate (PO43-), monohydrogen phosphate (HPO42-), and dihydrogen phosphate (H2PO4-)

    Components of nucleic acids, phospholipids, ATP, GTP, cAMP, creatine phosphate

    Activates metabolic pathways by phosphorylating enzymes

    Buffers pH

  • 24-*Phosphates - HomeostasisRenal controlif plasma concentration drops, renal tubules reabsorb all filtered phosphate

    Parathyroid hormone excretion of phosphate

    SImbalances not as criticalbody can tolerate broad variations in concentration of phosphate

  • 24-*Acid-Base BalanceImportant part of homeostasismetabolism depends on enzymes, and enzymes are sensitive to Ph

    Normal pH range of ECF is 7.35 to 7.45

    Challenges to acid-base balancemetabolism produces lactic acids, phosphoric acids, fatty acids, ketones and carbonic acids

  • 24-*Acids and BasesAcidsstrong acids ionize freely, markedly lower pHweak acids ionize only slightly

    Basesstrong bases ionize freely, markedly raise pHweak bases ionize only slightly

  • 24-*BuffersResist changes in pH convert strong acids or bases to weak ones

    Physiological buffersystem that controls output of acids, bases or CO2urinary system buffers greatest quantity, takes several hoursrespiratory system buffers within minutes, limited quantity

    Chemical buffer systemsrestore normal pH in fractions of a secondbicarbonate, phosphate and protein systems bind H+ and transport H+ to an exit (kidney/lung)

  • 24-*Bicarbonate Buffer SystemSolution of carbonic acid and bicarbonate ionsCO2 + H2O H2CO3 HCO3- + H+

    Reversible reaction important in ECFCO2 + H2O H2CO3 HCO3- + H+ lowers pH by releasing H+ CO2 + H2O H2CO3 HCO3- + H+raises pH by binding H+

    Functions with respiratory and urinary systemsto lower pH, kidneys excrete HCO3-to raise pH, kidneys excrete H+ and lungs excrete CO2

  • 24-*Phosphate Buffer SystemH2PO4- HPO42- + H+as in the bicarbonate system, reactions that proceed to the right release H+ and pH, and those to the left pH

    Important in the ICF and renal tubuleswhere phosphates are more concentrated and function closer to their optimum pH of 6.8constant