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Acid Base BalanceDr M A Maleque Molla, FRCP(Ed), FRCPCH

April 20 , 2015

Terminology & Definitions:Acid - Any substance that can donate proton (H+) e.g. H2CO3, NH3, HCL, Base- Any substance that can accept proton e.g. HCO3,PO4, proteinAcidemia- pH< normal range(7.35-7.45). i.e. H+ in the blood above normal rangeAlkalemia-pH>normal (7.35-7.45). i.e. H+ in the blood less than normal rangeAn acidosis & alkalossis is a pathologic process that causes an increase or decrease in the hydrogen ion concentration, Acidosis -Blood pH than 7.45

Terminology & Definitions (Cont..)pH-Negative log of the hydrogen ion concentrationpH= pK + log([base]/[acid])Represents the hydrogen concentrationNeutral pH is 7 at temp 250 C, 6.8 at 370C (Water)Normal pH of body fluids:Arterial blood is 7.4 (7.35-7.45)Venous blood and interstitial fluid is 7.35Intracellular fluid pH 7.0pH compatible with life 6.8-7.8

Buffers are substances that attenuate the change in pH that occurs when acids or bases are added to the body. Acid-Base BalanceDef: Maintenance of a normal balance between production and excretion of acid or alkali by the body, resulting in a stable concentration of H+ in body fluids. Acid load in the bodyAn adult normally produces 1-2 mEq/kg/24 hr of hydrogen ions. Children produce 2-3 mEq/kg/24 hr of hydrogen ions. There are 2 types of acids that can potentially contribute to the daily acid load; Carbonic acid (H2CO3) or volatile acidNon carbonic or nonvolatile acids e.g. HCL and H2SO4 The 3 principal sources of hydrogen ions:Dietary protein metabolism, Incomplete metabolism of carbohydrates and fat, Losses of bicarbonate in the stool e.g. GE

In order to maintain acid-base homeostasis, acid production must balance the neutralization or excretion.Daily H+ BalanceINPUTMmol/dayOUTPUTMmol/dayVolatile: CO213000Lungs13000 Lactate1500Liver, Kidney1500Nonvolatile: Protein(SO4)45Titratable acid30 Phosphate(PO4)30Ammonia40Others12Why acid-base balance needed?Regulation of normal pH is necessary because;Cellular enzymes and other metabolic processes, function optimally at normal pH.Chronic derangements in acid-base status may interfere with normal growth and developmentAcute, severe changes in pH can be fatal. How acid-base balance regulates?Body strictly maintains pH in a range from 7.35-7.45Changes in H+ concentration are prevented is by body's buffering system. Control of normal acid-base balance depends on; Cellular buffersLungsKidneys,

1. Intracellular and extracellular buffersImportant cellular buffers Bicarbonate/carbonic acid bufferNon bicarbonate buffers

Blood Buffer systemBicarbonate/carbonic acid bufferMost abundant buffer in ECF Function almost instantaneously within seconds.

10Blood Buffer systemBicarbonate/carbonic acid buffer.

Cells utilizing O2 & produces CO2

CO2 enter RBC & combines with water to form carbonic acid(H2CO3), which dissociates to form H+ and HCO3- :

HCO3- is pumped out to plasmaAt the alveoli, HCO3- re-enter the RBC and the above equation is driven to the left, re-producing CO2 & eliminated by the lung.H2O+CO2 H2CO3 H+ + HCO3-11Bicarbonate Buffer system

Buffer system (Cont..)2. Non bicarbonate buffersProtein buffers: Extracellular proteins, mostly albuminIntracellular proteins, including hemoglobin. Proteins are effective buffers, due to the presence of the amino acid histidine, which can bind or release hydrogen ions. Protein buffers both hydrogen ions(H+) and carbon dioxide(CO2).

Buffer system (Cont..)Phosphate Buffers Phosphate is an intracellular buffer & important buffer in the UrineHas a major role in the elimination of H+ via the kidneyCan bind up to 3 hydrogen molecules.At a physiologic pH, most phosphate exists as either HPO42 or H2PO41Buffer system (Cont..)BoneBone composed of compounds such as sodium bicarbonate and calcium carbonate dissolution of bone releases base & can buffer an acid load.In contrast, bone formation, by consuming base, helps buffer excess base.2. Respiratory Control mechanisms on pHWorks within minutes to control pH- maximal in 12-24 hoursMajor source of acid in the body is CO2CO2 react with water produce 12,500 mEq of H+ daily

Excess CO2 & H+ in the blood act directly on respiratory centers causing increase rate & depth of respiration & eliminate CO2In case of alkalosis(pH>7.45), respiratory center is inhibited and there is retention of CO2 H2O+CO2 H2CO3 H+ + HCO3-163. Renal Control Mechanisms on pHKidney takes several hours to days to act & restore pH to, or close to normal.Regulates plasma bicarbonate & pH by;Reabsorption of filtered HCO3- ;Kidneys handles around 4000 mEq of HCO3 daily.Almost all HCO3 are absorbed in renal tubules.Excreting excess H + by formation of titratable acid e.g. H2PO4- & NH4+ in the distal tubule

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From Thibeodeau GA PattonKI, Anatomy & Physiology, 5th ed,2004Respiratory & Renal response to acidosisIn Summery18Disorder in acid base balanceDisorder in acid base balanceACIDOSIS (pH7.45)Metabolic alkalosisRespiratory alkalosis

Metabolic acidosis Def: pH 26mEq/L- Respiratory AlkalosisStep 3: If respiratory acidosis? Acute or Chronic

Look for bicarbonate

Normal slightly raised HCO3 Acute Rp. acidosis

High HCO3 Chronic Rp. acidosis

Stepwise interpretation of blood gas Step 4: For metabolic acidosis

Look for anion gap

Nomal anion gap 12 4 mmol/l

Anion gap 12 4mmol/l =Normal or non-anion gap acidosisAnion gap > 16 mmol/l- Anion gap metabolic acidosis

Stepwise interpretation of blood gasAnion gapDef: Calculated difference between anion & cation electrolytes Anion gap = (Na++K+ ) - (Cl- + HCO3-). Nomal anion gap 12 4 mmol/l Anion gap metabolic acidosis (anion gap > 16):DKA(diabetic hyperglycemia) Lactic acidosis (sepsis, left ventricular failure) Uremia Alcohol, methanol, ethylene glycol, paraldehyde, salicylates poisoning Normal or non-anion gap acidosis (anion gap 12 4)GI loss of HCO3 e.g. diarrhea Renal loss of HCO3; Renal tubular acidosis Compensation for respiratory alkalosis Ureteral diversion Decrease anion Gap: (anion gap