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Acid and Base Balance andImbalance

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pH Review

pH = - log [ H+]H+ is really a proton

Range is from 0 - 14If [H+] is high, the solution is acidic; pH < 7If [H+] is low, the solution is basic or

alkaline ; pH > 7

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Acids are H+ donors.Bases are H+ acce p tors, or give u p OH- insolution.Acids and bases can be: ± Strong ± dissociate com p letely in

solutionHC l, NaO H

± Weak ± dissociate only partially insolution

Lactic acid, carbonic acid

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T he Body and pH

Homeostasis of pH is tightly controlledExtracellular fluid = 7.4

Blood = 7. 35 ± 7.4 5< 6 .8 or > 8.0 death occursAcidosis (acidemia) below 7. 35

Alkalosis (alkalemia) above 7.4 5

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Small changes in pH can produce

major disturbancesMost enzymes function only with narrowpH ranges

Acid-base balance can also affectelectrolytes (Na +, K+, C l-)Can also affect hormones

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T he body produces more acids

than basesAcids take in with foodsAcids p roduced by metabolism of li p ids

and p roteinsCellular metabolism produces CO 2.CO 2 + H20 H2CO 3 H+ + HC O 3

-

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Control of Acids

1. Buffer systemsT ake u p H+ or release H+ as conditions

changeBuffer pairs ± weak acid and a baseExchange a strong acid or base for a

weak oneResults in a much smaller pH change

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Bicarbonate buffer

Sodium Bicarbonate (Na HC O 3 ) andcarbonic acid ( H2CO 3 )

Maintain a 20:1 ratio : HC O 3-

: H2CO 3

HC l + Na HC O 3 H2CO 3 + Na C l

NaO H + H2CO 3 Na HC O 3 + H2O

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P hos phate buffer

Major intracellular buffer H+ + HP O 4

2 - H2P O4 -

OH- + H2P O 4- H2O + H2P O 4

2 -

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P rotein Buffers

Includes hemoglobin, work in blood and ISFCarboxyl grou p gives u p H+

Amino Grou p acce p ts H+

Side chains that can buffer H+ are p resent on27 amino acids.

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2 . Res p iratory mechanisms

Exhalation of carbon dioxideP owerful, but only works with volatile

acidsDoesn¶t affect fixed acids like lactic acidCO 2 + H20 H2CO 3 H+ + HC O 3

-

Body pH can be adjusted by changing rateand de p th of breathing

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3 . Kidney excretion

Can eliminate large amounts of acidCan also excrete base

Can conserve and p roduce bicarb ionsMost effective regulator of pHIf kidneys fail, pH balance fails

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Rates of correction

Buffers function almost instantaneouslyRes p iratory mechanisms take several

minutes to hoursRenal mechanisms may take severalhours to days

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Acid-Base Imbalances

pH < 7. 35 acidosispH > 7.4 5 alkalosis

T he body res ponse to acid-baseimbalance is called compensationMay be complete if brought back within

normal limitsPartial compensation if range is stilloutside norms.

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Com pensation

If underlying problem is metabolic,hyperventilation or hy poventilation canhel p : respiratory compensation .If problem is res p iratory, renalmechanisms can bring about metaboliccompensation.

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Alkalosis

Alkalosis causes over excitability of the centraland peripheral nervous systems.NumbnessLightheadedness

It can cause : ± Nervousness ± muscle s pasms or tetany ± Convulsions ± Loss of consciousness ± Death

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Res p iratory Acidosis

C arbonic acid excess caused by bloodlevels of CO 2 above 4 5 mm Hg.Hypercapnia ± high levels of CO 2 in bloodChronic conditions: ± De pression of res p iratory center in brain that

controls breathing rate ± drugs or head

trauma ± P aralysis of res p iratory or chest muscles ± Em physema

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Res p iratory Acidosis

Acute conditons: ± Adult Res p iratory Distress Syndrome

± P ulmonary edema ± P neumothorax

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Com pensation for Res p iratory Acidosis

Kidneys eliminate hydrogen ion and retainbicarbonate ion

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Signs and Sym p toms of Res p iratory Acidosis

BreathlessnessRestlessness

Lethargy and disorientationT remors, convulsions, comaRes p iratory rate ra p id, then graduallyde pressedSkin warm and flushed due to vasodilationcaused by excess CO 2

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T reatment of Res p iratory Acidosis

Restore ventilationIV lactate solution

T reat underlying dysfunction or disease

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Res p iratory Alkalosis

Carbonic acid deficitpC O 2 less than 35 mm Hg (hy poca pnea)

Most common acid-base imbalanceP rimary cause is hy perventilation

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Com pensation of Res p iratory Alkalosis

Kidneys conserve hydrogen ionExcrete bicarbonate ion

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T reatment of Res p iratory Alkalosis

T reat underlying causeBreathe into a pa per bag

IVChloride containing solution ± C l-ions

re p lace lost bicarbonate ions

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Metabolic Acidosis

B icarbonate deficit - blood concentrations of bicarb dro p below 22 mEq/L

Causes: ± Loss of bicarbonate through diarrhea or renaldysfunction

± Accumulation of acids (lactic acid or ketones) ± Failure of kidneys to excrete H+

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Sym p toms of Metabolic Acidosis

Headache, lethargyNausea, vomiting, diarrhea

ComaDeath

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Com pensation for Metabolic Acidosis

Increased ventilationRenal excretion of hydrogen ions if

possibleK+ exchanges with excess H+ in E CF(H+ into cells, K + out of cells)

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T reatment of Metabolic Acidosis

IV lactate solution

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Metabolic Alkalosis

B icarbonate excess - concentration inblood is greater than 26 mEq/LCauses: ± Excess vomiting = loss of stomach acid ± Excessive use of alkaline drugs ± Certain diuretics

± Endocrine disorders ± Heavy ingestion of antacids ± Severe dehydration

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Com pensation for Metabolic Alkalosis

Alkalosis most commonly occurs withrenal dysfunction, so can¶t count onkidneysRes p iratory com pensation difficult ±hypoventilation limited by hy poxia

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Sym p toms of Metabolic Alkalosis

Res p iration slow and shallowHyperactive reflexes ; tetany

Often related to de p letion of electrolytesAtrial tachycardiaDysrhythmias

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T reatment of Metabolic Alkalosis

Electrolytes to re p lace those lostIV chloride containing solution

T reat underlying disorder

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Diagnosis of Acid-Base Imbalances

1. Note whether the pH is low (acidosis) or high (alkalosis)

2 . Decide which value, pC O 2 or HC O 3

-

, isoutside the normal range and could bethe cause of the p roblem. If the cause isa change in pC O 2 , the p roblem isres p iratory. If the cause is HC O 3 - thep roblem is metabolic.

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3 . Look at the value that doesn¶t corres pondto the observed pH change. If it is insidethe normal range, there is nocom pensation occurring. If it is outside thenormal range, the body is partially

com pensating for the p roblem.

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Exam p le

Apatient is in intensive care because hesuffered a severe myocardial infarction 3 days ago. T he lab re ports the followingvalues from an arterial blood sam p le: ± pH 7. 3

± HC O 3 - = 20 mEq / L ( 22 - 26 )

± pC O2 = 3 2 mm Hg ( 35 - 45 )

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Diagnosis

Metabolic acidosisWith com pensation

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