Acid Base Balance Stephen P. DiBartola, DVMStephen P. DiBartola, DVM Department of Veterinary...

Acid Base BalanceAcid Base BalanceAcid Base BalanceAcid Base Balance• Stephen P. DiBartola, DVMStephen P. DiBartola, DVM

• Department of Veterinary Clinical Department of Veterinary Clinical SciencesSciences

• College of Veterinary MedicineCollege of Veterinary Medicine

• Ohio State UniversityOhio State University

• Columbus, OH 43210Columbus, OH 43210

The Nephronauts

Acid Base BalanceAcid Base BalanceAcid Base BalanceAcid Base Balance

• Who cares about Who cares about acid base acid base balance?balance?

Now that I have this data, what does it mean?

Acid Base Balance: Why pH?Acid Base Balance: Why pH?Acid Base Balance: Why pH?Acid Base Balance: Why pH?

• Nanoequivalent concentration of HNanoequivalent concentration of H++ versus milliequivalent concentrations versus milliequivalent concentrations of other electrolytes (Naof other electrolytes (Na++, K, K++, Cl, Cl--, , HCOHCO33

--) in body fluids) in body fluids

• Normal ECF [HNormal ECF [H++] is 40 nEq/L or ] is 40 nEq/L or 0.00004 mEq/L versus [K0.00004 mEq/L versus [K++] of 4.0 ] of 4.0 mEq/LmEq/L


• pH = -logpH = -log1010[H[H++] = log] = log1010(1/[H(1/[H++])])

• [H[H++] = 40 nEq/L = 4 X 10] = 40 nEq/L = 4 X 10-8-8 Eq/L Eq/L

• pH = -logpH = -log1010(4 X 10(4 X 10-8-8) = 7.4) = 7.4

• Normal pH = 7.37-7.42 and extreme Normal pH = 7.37-7.42 and extreme range compatible with life is 6.8 to 8.0range compatible with life is 6.8 to 8.0


• HA = HHA = H++ + A + A--

• Acid = Proton donor (HA)Acid = Proton donor (HA)

• Base = Proton acceptor (ABase = Proton acceptor (A--))

Acid Base Balance: Daily Acid Base Balance: Daily metabolismmetabolism

Acid Base Balance: Daily Acid Base Balance: Daily metabolismmetabolism

• Metabolism of sulfur-containing Metabolism of sulfur-containing amino acids in proteins yields amino acids in proteins yields sulfuric acid and metabolism of sulfuric acid and metabolism of phospholipids yields phosphoric phospholipids yields phosphoric acid: 40-80 mEq/day of FIXED or acid: 40-80 mEq/day of FIXED or NON-VOLATILE acidNON-VOLATILE acid

• Metabolism of carbohydrates and Metabolism of carbohydrates and fats yields 15,000-20,000 mMol COfats yields 15,000-20,000 mMol CO22 or or VOLATILE acidVOLATILE acid


• Why is COWhy is CO22 an acid? (How can it be a an acid? (How can it be a proton donor?)proton donor?)

• Carbonic anhydrase in red cells and Carbonic anhydrase in red cells and renal tubular cells facilitates the renal tubular cells facilitates the reaction: COreaction: CO22 + H + H22O = HO = H22COCO33

• Carbonic acid is an obvious proton Carbonic acid is an obvious proton donor: Hdonor: H22COCO33 = H = H++ + HCO + HCO33

--

Acid Base Balance: BuffersAcid Base Balance: BuffersAcid Base Balance: BuffersAcid Base Balance: Buffers

• A substance that can accept A substance that can accept protons (Hprotons (H++ ions) and minimize a ions) and minimize a change in pHchange in pH

• A solution of a weak acid and its A solution of a weak acid and its saltsalt

Titration curve of a buffer Titration curve of a buffer solutionsolution

Titration curve of a buffer Titration curve of a buffer solutionsolution

pHpH

pKapKa

mE

q s

tro

ng

mE

q s

tro

ng

Aci

dA

cid

Bas

eB

ase

Buffer zoneBuffer zone

pKa +/- 1.0 pH unitpKa +/- 1.0 pH unit

Acid Base Balance: BuffersAcid Base Balance: BuffersAcid Base Balance: BuffersAcid Base Balance: Buffers

• Effectiveness of a buffer depends Effectiveness of a buffer depends upon:upon:• Prevailing pH of body fluid to be Prevailing pH of body fluid to be

defendeddefended• Concentration of buffer in that body Concentration of buffer in that body

fluidfluid• pKa of bufferpKa of buffer• ““Special circumstances” (e.g., Special circumstances” (e.g.,

bicarbonate is part of an open system)bicarbonate is part of an open system)

Important body buffersImportant body buffersImportant body buffersImportant body buffers

• Extracellular fluid (ECF)Extracellular fluid (ECF)• BicarbonateBicarbonate

• Intracellular fluid (ICF)Intracellular fluid (ICF)• PhosphatesPhosphates• Proteins (e.g., Hb in RBC)Proteins (e.g., Hb in RBC)

• Bone carbonateBone carbonate

Important body buffers: Important body buffers: BicarbonateBicarbonate

Important body buffers: Important body buffers: BicarbonateBicarbonate

• pKa is 1.3 units below pKa is 1.3 units below physiologic pHphysiologic pH

• ECF concentration is high (24 ECF concentration is high (24 mEq/L)mEq/L)

• In equilibrium with COIn equilibrium with CO22 (i.e. part (i.e. part of an open system)of an open system)

COCO22 + H + H22O = HO = H22COCO33 = H = H++ + HCO + HCO33--

Important body buffers: Important body buffers: PhosphatePhosphate

Important body buffers: Important body buffers: PhosphatePhosphate

• Important in ICF where concentration Important in ICF where concentration is high (40 mEq/L)is high (40 mEq/L)

• Relatively unimportant in ECF where Relatively unimportant in ECF where concentration is low (2 mEq/L)concentration is low (2 mEq/L)

• Important in urine (titratable acidity) Important in urine (titratable acidity) due to pKa of 6.8 (distal tubular fluid due to pKa of 6.8 (distal tubular fluid pH = 6.0 to 7.0)pH = 6.0 to 7.0)

Important body buffers: Important body buffers: ProteinsProteins

Important body buffers: Important body buffers: ProteinsProteins

• Plasma proteins play limited role Plasma proteins play limited role in ECF bufferingin ECF buffering

• Intracellular proteins (especially Intracellular proteins (especially Hb in RBC) play major role in Hb in RBC) play major role in bufferingbuffering

Important body buffers: Bone Important body buffers: Bone carbonatecarbonate

Important body buffers: Bone Important body buffers: Bone carbonatecarbonate

• Very large store of potential Very large store of potential bufferbuffer

• Important role in long term Important role in long term response to chronic acidosisresponse to chronic acidosis

Henderson-Hasselbach Henderson-Hasselbach equationequation


• HA HA H H++ + A + A-- ( (vv11 = k = k11[HA])[HA])

• HH++ + A + A-- HA ( HA (vv22 = k = k22[H[H++][A][A--])])

• At equilibrium: At equilibrium: vv11 = = vv22

• kk11[HA] = k[HA] = k22[H[H++][A][A--]]

• kk11/k/k22 = K = Kaa = [H = [H++][A][A--]/[HA]]/[HA]



• KKaa = [H = [H++][A][A--]/[HA]]/[HA]

• pH = pKpH = pKaa + log([A + log([A--]/[HA])]/[HA])



• pH = pKpH = pKaa + log([HCO + log([HCO33--]/[H]/[H22COCO33])])

• COCO22 dissolved in plasma potentially dissolved in plasma potentially can form Hcan form H22COCO33 so: so:

pH = pKpH = pKaa + log([HCO + log([HCO33--]/[dissolved CO]/[dissolved CO22 + H + H22COCO33])])



• At the temperature and ionic strength At the temperature and ionic strength of ECF there are 6,800 HCOof ECF there are 6,800 HCO33

-- ions and ions and 340 molecules of dissolved CO340 molecules of dissolved CO22 for for each molecule of Heach molecule of H22COCO33. Therefore, . Therefore, HH22COCO33 can be neglected. can be neglected.

• Dissolved CODissolved CO22 is related to pCO is related to pCO22 by by its solubility coefficient:its solubility coefficient:

Dissolved CODissolved CO22 = 0.03 X pCO = 0.03 X pCO22

Henderson-Hasselbach equation Henderson-Hasselbach equation (clinically relevant form)(clinically relevant form)

Henderson-Hasselbach equation Henderson-Hasselbach equation (clinically relevant form)(clinically relevant form)

• pH = pKpH = pKaa + log([HCO + log([HCO33--]/.03xpCO]/.03xpCO22))

• pH = 6.1 + log([HCOpH = 6.1 + log([HCO33--]/.03xpCO]/.03xpCO22))

• Shows that pH is a function of the Shows that pH is a function of the RATIORATIO between bicarbonate and between bicarbonate and pCOpCO22

Acid Base Balance: Physiologic Acid Base Balance: Physiologic lines of defenselines of defense

Acid Base Balance: Physiologic Acid Base Balance: Physiologic lines of defenselines of defense

• Physicochemical buffering by ECF Physicochemical buffering by ECF and ICF buffers (seconds)and ICF buffers (seconds)

• Alterations in ventilation to produce a Alterations in ventilation to produce a change in pCOchange in pCO22 (seconds to minutes) (seconds to minutes)

• Renal readjustment of body HCORenal readjustment of body HCO33--

stores (hours to days)stores (hours to days)

• pH = pKpH = pKaa + log(KIDNEYS/LUNGS) + log(KIDNEYS/LUNGS)

Primary Acid Base DisordersPrimary Acid Base DisordersPrimary Acid Base DisordersPrimary Acid Base Disorders

• Metabolic disturbancesMetabolic disturbances• Metabolic acidosisMetabolic acidosis• Metabolic alkalosisMetabolic alkalosis

• Respiratory disturbancesRespiratory disturbances• Respiratory acidosisRespiratory acidosis• Respiratory alkalosisRespiratory alkalosis

Acid Base DisordersAcid Base DisordersAcid Base DisordersAcid Base Disorders

Acidosis and alkalosisAcidosis and alkalosisversusversus

Acidemia and alkalemiaAcidemia and alkalemia

• Each Each primaryprimary (metabolic or respiratory) (metabolic or respiratory) disturbance is accompanied by a disturbance is accompanied by a secondarysecondary (opposing) response in the (opposing) response in the other system (respiratory or metabolic)other system (respiratory or metabolic)

• pH is returned nearly but not completely pH is returned nearly but not completely to normalto normal

• Overcompensation does not occurOvercompensation does not occur

Acid Base Disorders: Acid Base Disorders: Principles of interpretationPrinciples of interpretation


DisorderDisorder pHpH [H[H++]] Primary Primary disturbancedisturbance

SecondarySecondaryresponseresponse

Metabolic Metabolic acidosisacidosis

[HCO[HCO33--]] pCOpCO22

Metabolic Metabolic alkalosisalkalosis

[HCO[HCO33--]] pCOpCO22

Respiratory Respiratory acidosisacidosis

pCOpCO22 [HCO[HCO33--]]

Respiratory Respiratory alkalosisalkalosis

pCOpCO22 [HCO[HCO33--]]


• SimpleSimple Primary disturbance and expected Primary disturbance and expected adaptive (secondary) responseadaptive (secondary) response

• MixedMixed Two separate primary disturbances Two separate primary disturbances present simultaneously in the same present simultaneously in the same individualindividual

Must know expected adaptive Must know expected adaptive (compensatory) response to recognize (compensatory) response to recognize mixed disturbancesmixed disturbances


Primary disorderPrimary disorder Compensatory responseCompensatory responseMetabolic acidosisMetabolic acidosis 0.7-1.2 mm 0.7-1.2 mm pCO pCO22 per 1.0 mEq/L per 1.0 mEq/L HCO HCO33

--

Metabolic alkalosisMetabolic alkalosis 0.7 mm 0.7 mm pCO pCO22 per 1.0 mEq/L per 1.0 mEq/L HCO HCO33--

Acute respiratory acidosisAcute respiratory acidosis 0.15 0.15 mEq/L HCO mEq/L HCO33-- per 1.0 mm per 1.0 mm pCO pCO22

Chronic respiratory acidosisChronic respiratory acidosis 0.35 0.35 mEq/L HCO mEq/L HCO33-- per 1.0 mm per 1.0 mm pCO pCO22

Acute respiratory alkalosisAcute respiratory alkalosis 0.25 mEq/L 0.25 mEq/L HCO HCO33-- per 1.0 mm per 1.0 mm pCO pCO22

Chronic respiratory alkalosisChronic respiratory alkalosis 0.55 mEq/L 0.55 mEq/L HCO HCO33-- per 1.0 mm per 1.0 mm pCO pCO22

Compensation for metabolic acidosisCompensation for metabolic acidosisCompensation for metabolic acidosisCompensation for metabolic acidosis

• HH++ buffered by ECF HCO buffered by ECF HCO33- - &&

Hb in RBC; Plasma Pr and Pi: Hb in RBC; Plasma Pr and Pi: negligible role (sec-min)negligible role (sec-min)

• Ventilation lowers pCOVentilation lowers pCO22 (min) (min)

• HH++ buffered by interstitial buffered by interstitial HCOHCO33

- - (30 min)(30 min)

• HH++ buffered by ICF Pr and Pi buffered by ICF Pr and Pi (hrs)(hrs)

• Renal regeneration of HCORenal regeneration of HCO33- -

(2-6 days)(2-6 days)

Compensation for metabolic acidosisCompensation for metabolic acidosisCompensation for metabolic acidosisCompensation for metabolic acidosis

• HH++ buffered by ECF HCO buffered by ECF HCO33- -

&& Hb in RBC; Plasma Pr Hb in RBC; Plasma Pr and Pi: negligible role and Pi: negligible role (sec-min)(sec-min)

• Ventilation lowers pCOVentilation lowers pCO22 (min)(min)

• HH++ buffered by interstitial buffered by interstitial HCOHCO33

- - (30 min)(30 min)• HH++ buffered by ICF Pr and buffered by ICF Pr and

Pi (hrs)Pi (hrs)• Renal regeneration of Renal regeneration of

HCOHCO33- - (2-6 days)(2-6 days)

Compensation for respiratory acidosisCompensation for respiratory acidosisCompensation for respiratory acidosisCompensation for respiratory acidosis

Bicarbonate cannot participate in Bicarbonate cannot participate in buffering of Hbuffering of H++ arising from arising from respiratory acidosis:respiratory acidosis:

COCO22 + H + H22O = HO = H22COCO33 = H = H++ + HCO + HCO33--

From respiratory acidosisFrom respiratory acidosis

Compensation for respiratory acidosisCompensation for respiratory acidosisCompensation for respiratory acidosisCompensation for respiratory acidosis

• Bicarbonate is formed as the COBicarbonate is formed as the CO22 + H + H22O = O = HH22COCO33 = H = H++ + HCO + HCO33

-- equilibrium is pushed equilibrium is pushed to the rightto the right

• HH++ formed is buffered by Hb in RBC formed is buffered by Hb in RBC (seconds to minutes)(seconds to minutes)

• HH++ enters other cells and is buffered by enters other cells and is buffered by proteins and phosphates (hours)proteins and phosphates (hours)

• Renal excretion of HRenal excretion of H++ and generation of and generation of new bicarbonate follows (2 to 6 days)new bicarbonate follows (2 to 6 days)

Compensation for acid base Compensation for acid base disturbancesdisturbances

Compensation for acid base Compensation for acid base disturbancesdisturbances

• Respiratory compensation for Respiratory compensation for metabolic disorders should be metabolic disorders should be complete in 24 hourscomplete in 24 hours• ““Acute” is < 24-48 hrsAcute” is < 24-48 hrs• ““Chronic” is > 24-48 hrsChronic” is > 24-48 hrs

• Metabolic (renal) compensation for Metabolic (renal) compensation for respiratory disorders is slower and respiratory disorders is slower and requires 2 to 6 daysrequires 2 to 6 days

Concept ofConcept ofAnion GapAnion Gap

Acid Base Disorders: Acid Base Disorders: InterpretationInterpretation

Acid Base Disorders: Acid Base Disorders: InterpretationInterpretation

• Is an acid base disturbance present?Is an acid base disturbance present?• What is the What is the primaryprimary disturbance? disturbance?• Is the Is the secondarysecondary (adaptive) response (adaptive) response

as expected?as expected?• What underlying disease process is What underlying disease process is

responsible for the acid base responsible for the acid base disturbance?disturbance?

Acid Base Disorders: InterpretationAcid Base Disorders: InterpretationAcid Base Disorders: InterpretationAcid Base Disorders: Interpretation

• Arterial blood gas from a dog: pH 7.27, HCOArterial blood gas from a dog: pH 7.27, HCO33--

12 mEq/L, pCO12 mEq/L, pCO22 27 mmHg (normal: pH 7.39, 27 mmHg (normal: pH 7.39, HCOHCO33

-- 22 mEq/L, pCO 22 mEq/L, pCO22 37 mmHg) 37 mmHg)• Is an acid base disturbance present?Is an acid base disturbance present?• YES (look at the pH)YES (look at the pH)

• Of what general type?Of what general type?• ACIDOSIS (pH 7.27 < 7.39)ACIDOSIS (pH 7.27 < 7.39)

• Metabolic or respiratory?Metabolic or respiratory?• pCOpCO22 is LOW (can’t be respiratory acidosis) is LOW (can’t be respiratory acidosis)• HCOHCO33

-- is LOW (must be METABOLIC ACIDOSIS) is LOW (must be METABOLIC ACIDOSIS)


• Is secondary (adaptive) response as expected?Is secondary (adaptive) response as expected?• Observed HCOObserved HCO33

-- is 10 mEq/L is 10 mEq/L lowerlower than than “normal” (22-12)“normal” (22-12)

• ““Normal” dog can lower pCONormal” dog can lower pCO22 1 mmHg for every 1 mmHg for every 0.7-1.2 mEq/L decrement in HCO0.7-1.2 mEq/L decrement in HCO33

-- (use 1.0 (use 1.0 mEq/L as “average”)mEq/L as “average”)

• Expected pCOExpected pCO22 = 37-10 = 27 mmHg = 37-10 = 27 mmHg• Observed pCOObserved pCO22 = 27 mmHg = 27 mmHg• Conclusion: YES, adaptive response is as Conclusion: YES, adaptive response is as

expected. This is a simple metabolic acidosis expected. This is a simple metabolic acidosis with respiratory compensationwith respiratory compensation


• Arterial blood gas from a dog sick for 1 week: Arterial blood gas from a dog sick for 1 week: pH 7.33, HCOpH 7.33, HCO33

-- 29 mEq/L, pCO 29 mEq/L, pCO22 57 mmHg 57 mmHg (normal: pH 7.39, HCO(normal: pH 7.39, HCO33

-- 22 mEq/L, pCO 22 mEq/L, pCO22 37 37 mmHg)mmHg)

• Is an acid base disturbance present?Is an acid base disturbance present?• YES (look at the pH)YES (look at the pH)

• Of what general type?Of what general type?• ACIDOSIS (pH 7.33 < 7.39)ACIDOSIS (pH 7.33 < 7.39)

• Metabolic or respiratory?Metabolic or respiratory?• HCOHCO33

-- is HIGH (can’t be metabolic acidosis) is HIGH (can’t be metabolic acidosis)• pCOpCO22 is HIGH (must be RESPIRATORY ACIDOSIS) is HIGH (must be RESPIRATORY ACIDOSIS)


• Is secondary (adaptive) response as expected?Is secondary (adaptive) response as expected?• Observed pCOObserved pCO22 is 20 mmHg is 20 mmHg higherhigher than than

“normal” (57-37)“normal” (57-37)• ““Normal” dog can increase HCONormal” dog can increase HCO33

-- 3.5 mEq/L for 3.5 mEq/L for every 10 mmHg increment in pCOevery 10 mmHg increment in pCO22 (in “chronic” (in “chronic” disturbance)disturbance)

• Expected HCOExpected HCO33-- = 22+7 = 29 mEq/L = 22+7 = 29 mEq/L

• Observed HCOObserved HCO33-- = 29 mEq/L = 29 mEq/L

• Conclusion: YES, adaptive response is as Conclusion: YES, adaptive response is as expected. This is a simple respiratory acidosis expected. This is a simple respiratory acidosis with metabolic compensationwith metabolic compensation


• Even in Even in simplesimple disturbances, disturbances, calculated compensatory calculated compensatory pCOpCO22 and and HCOHCO33

-- values usually won’t match values usually won’t match observed values because calculations observed values because calculations are based on “average” valuesare based on “average” values

• Do not diagnose a Do not diagnose a mixedmixed disturbance disturbance unless calculated value is > 2 to 3 unless calculated value is > 2 to 3 mmHg (pCOmmHg (pCO22) or mEq/L (HCO) or mEq/L (HCO33

--) different ) different from observed valuefrom observed value


• Arterial blood gas from a dog: pH 7.05, HCOArterial blood gas from a dog: pH 7.05, HCO33--

12 mEq/L, pCO12 mEq/L, pCO22 44 mmHg (normal: pH 7.39, 44 mmHg (normal: pH 7.39, HCOHCO33

-- 22 mEq/L, pCO 22 mEq/L, pCO22 37 mmHg) 37 mmHg)• Is an acid base disturbance present?Is an acid base disturbance present?• ABSOLUTELY! (look at the pH)ABSOLUTELY! (look at the pH)

• Of what general type?Of what general type?• ACIDOSIS (pH 7.05 << 7.39)ACIDOSIS (pH 7.05 << 7.39)

• Metabolic or respiratory?Metabolic or respiratory?• pCOpCO22 is HIGH (could be respiratory acidosis) is HIGH (could be respiratory acidosis)• HCOHCO33

-- is LOW (could be metabolic acidosis) is LOW (could be metabolic acidosis)


• Is secondary (adaptive) response as expected?Is secondary (adaptive) response as expected?• NONO• If simple metabolic acidosis, pCOIf simple metabolic acidosis, pCO22 should should

be low in responsebe low in response• If simple respiratory acidosis, HCOIf simple respiratory acidosis, HCO33

-- should should be high in responsebe high in response

• Conclusion: This is a Conclusion: This is a mixedmixed metabolic and metabolic and respiratory acidosis. The extremely low pH respiratory acidosis. The extremely low pH alerts you to the possibility of a mixed alerts you to the possibility of a mixed disturbancedisturbance

Acid Base Disorders: InterpretationAcid Base Disorders: InterpretationAcid Base Disorders: InterpretationAcid Base Disorders: Interpretation• Arterial blood gas from a dog with sudden onset of Arterial blood gas from a dog with sudden onset of

gastric dilatation/volvulus: pH 7.38, HCOgastric dilatation/volvulus: pH 7.38, HCO33-- 12 mEq/L, 12 mEq/L,

pCOpCO22 21 mmHg (normal: pH 7.39, HCO 21 mmHg (normal: pH 7.39, HCO33-- 22 mEq/L, 22 mEq/L,

pCOpCO22 37 mmHg) 37 mmHg)• Is an acid base disturbance present?Is an acid base disturbance present?• If so, it’s not obvious from pHIf so, it’s not obvious from pH

• Of what general type?Of what general type?• From pCOFrom pCO22 could be respiratory alkalosis or from could be respiratory alkalosis or from

HCOHCO33-- could be metabolic acidosis could be metabolic acidosis

• Metabolic or respiratory?Metabolic or respiratory?• pCOpCO22 is LOW (could be respiratory alkalosis) is LOW (could be respiratory alkalosis)• HCOHCO33

-- is LOW (could be metabolic acidosis) is LOW (could be metabolic acidosis)


• Is secondary (adaptive) response as expected?Is secondary (adaptive) response as expected?• If primary metabolic acidosisIf primary metabolic acidosis• 10 mEq/L decrement in HCO10 mEq/L decrement in HCO33

-- (22-12) (22-12)• Expected pCOExpected pCO22 = 27 mmHg (37-10) = 27 mmHg (37-10)• Observed pCOObserved pCO22 = 21 mmHg = 21 mmHg

• If primary If primary acuteacute respiratory alkalosis respiratory alkalosis• 16 mmHg decrement in pCO16 mmHg decrement in pCO22 (37-21) (37-21)• Expected HCOExpected HCO33

-- = 18 mEq/L (22-4) = 18 mEq/L (22-4)• Observed HCOObserved HCO33

-- = 12 mEq/L = 12 mEq/L • Conclusion: Conclusion: MixedMixed metabolic acidosis and metabolic acidosis and

respiratory alkalosisrespiratory alkalosis


• IsIs mixed mixed metabolic acidosis and metabolic acidosis and respiratory alkalosis compatible with respiratory alkalosis compatible with acute gastric dilatation/volvulus?acute gastric dilatation/volvulus?• YESYES• Metabolic acidosis due to shock and Metabolic acidosis due to shock and

decreased tissue perfusiondecreased tissue perfusion• Respiratory alkalosis due to Respiratory alkalosis due to

hyperventilation induced by pain or hyperventilation induced by pain or septicemiasepticemia


• What if dog had been sick with some What if dog had been sick with some other disorder for 1 week?other disorder for 1 week?• If primary If primary chronicchronic respiratory alkalosis respiratory alkalosis• 16 mmHg decrement in pCO16 mmHg decrement in pCO22 (37-21) (37-21)• Expected HCOExpected HCO33

-- = 13.2 mEq/L (22-8.8) = 13.2 mEq/L (22-8.8)• Observed HCOObserved HCO33

-- = 12 mEq/L = 12 mEq/L• Difference is < 2 mEq/LDifference is < 2 mEq/L

• Conclusion of Conclusion of simplesimple chronic respiratory chronic respiratory alkalosis would be justifiedalkalosis would be justified

Renal regulation of acid base balanceRenal regulation of acid base balanceRenal regulation of acid base balanceRenal regulation of acid base balance

• Role of kidneys is preservation of body’s Role of kidneys is preservation of body’s bicarbonate stores. Accomplished by:bicarbonate stores. Accomplished by:• Reabsorption of 99.9% of filtered bicarbonateReabsorption of 99.9% of filtered bicarbonate

• Regeneration of titrated bicarbonate by Regeneration of titrated bicarbonate by excretion of:excretion of:• Titratable acidity (mainly phosphate)Titratable acidity (mainly phosphate)• Ammonium saltsAmmonium salts

All of these things are accomplishedAll of these things are accomplishedby secretion of hydrogen ionsby secretion of hydrogen ions

All of these things are accomplishedAll of these things are accomplishedby secretion of hydrogen ions …by secretion of hydrogen ions …All of these things are accomplishedAll of these things are accomplishedby secretion of hydrogen ions …by secretion of hydrogen ions …

• If secreted HIf secreted H++ ions combine with filtered ions combine with filtered bicarbonate, bicarbonate is reabsorbedbicarbonate, bicarbonate is reabsorbed

• If secreted HIf secreted H++ ions combine with ions combine with phosphate or ammonia, net acid phosphate or ammonia, net acid excretion and generation of excretion and generation of newnew bicarbonate occurbicarbonate occur

It all depends on what buffer the secreted HIt all depends on what buffer the secreted H++

encounters in the tubular fluid, which in turn encounters in the tubular fluid, which in turn is a function of where we are in the nephron!is a function of where we are in the nephron!

Renal reabsorption of bicarbonateRenal reabsorption of bicarbonateRenal reabsorption of bicarbonateRenal reabsorption of bicarbonate

• Proximal tubule: Proximal tubule: 70-85%70-85%

• Loop of Henle: Loop of Henle: 10-20%10-20%

• Distal tubule and Distal tubule and collecting ducts: collecting ducts: 4-7%4-7%

Factors affecting renal bicarbonate Factors affecting renal bicarbonate reabsorptionreabsorption

Factors affecting renal bicarbonate Factors affecting renal bicarbonate reabsorptionreabsorption

• Filtered load of bicarbonateFiltered load of bicarbonate• Extracellular fluid volumeExtracellular fluid volume• Prolonged changes in pCO2Prolonged changes in pCO2• Plasma chloride Plasma chloride

concentrationconcentration• Plasma potassium Plasma potassium

concentrationconcentration• Hormones (e.g., Hormones (e.g.,

mineralocorticoids, mineralocorticoids, glucocorticoids)glucocorticoids)

Titratable acidityTitratable acidityTitratable acidityTitratable acidity

• Occurs when secreted HOccurs when secreted H++ encounter & titrate encounter & titrate phosphate in tubular fluidphosphate in tubular fluid

• Refers to amount of strong Refers to amount of strong base needed to titrate urine base needed to titrate urine back to pH 7.4back to pH 7.4

• 40% (15-30 mEq) of daily 40% (15-30 mEq) of daily fixed acid loadfixed acid load

• Relatively constant (not Relatively constant (not highly adaptable)highly adaptable)

Ammonium excretionAmmonium excretionAmmonium excretionAmmonium excretion

• Occurs when Occurs when secreted Hsecreted H++ combine combine with NHwith NH33 and are and are trapped as NHtrapped as NH44

++ salts salts in tubular fluidin tubular fluid

• 60% (25-50 mEq) of 60% (25-50 mEq) of daily fixed acid loaddaily fixed acid load

• Very adaptable (via Very adaptable (via glutaminase glutaminase induction)induction)

Ammonium excretionAmmonium excretionAmmonium excretionAmmonium excretion

• Large amounts of Large amounts of HH++ can be excreted can be excreted without extremely without extremely low urine pH low urine pH because pKbecause pKaa of of NHNH33/NH/NH44

++ system is system is

very high (9.2)very high (9.2)

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Acid Base Balance Stephen P. DiBartola, DVMStephen P. DiBartola, DVM Department of Veterinary...

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