2021A14 Discuss how the body handles a metabolic acidosis.

 

List:

·       Intro

·       Dilution

·       Buffering

·       Compensation

·       Correction

 

Intro:

Definition

·     Excess production or addition of metabolic acid or excess loss of base

Diagnosis

·     HH approach: pH <7.4, PaCO2 <40mmHg, HCO3- <24mmol/L

·     Stewart approach: SID <42 (where SID = [Na + K + Ca + Mg] – [Cl + Lac] )

Classification

·     NAGMA: loss of HCO3- or base (e.g. diarrhoea, renal tubular acidosis)

·     HAGMA: addition of fixed acid (e.g. lactic acidosis, ketoacidosis)

Summary of response

1.    Dilution

2.    Buffering: ICF and ECF

3.    Compensation: respiratory and renal

4.    Correction of the underlying cause

 

Dilution (immediate)

Purpose

·     Minimise fall in local or plasma pH

Mechanism

·     Distribution of acid across body fluid compartments

 

Buffering – minutes:

Purpose

·     Resists the fall in pH when acid is added or base removed

Mechanism

·     System comprises a weak acid and its conjugate base

·     H+ readily exchanged

·     AH <-> A- + H+

·     Hence ↓[H+]

Ideal properties

·     Abundant

·     Rapid

·     pKa = prevailing pH +/- 1

·     Open-ended

 

Intracellular buffering (60%)

Proteins

·     Imidazole groups of histidine residues

·     Abundant

·     pKa = pH ICF = 6.8

Phosphate

·     H2PO4- <-> H+ + HPO42-

·     High concentration (30-60mmol/L)

·     pKa = pH of ICF = 6.8

 

Extracellular buffering (40%)

Bicarbonate

·     Most important ECF buffer

·     CO2 + H2O <-> H+ + HCO3-

·     ↑[H+] -> L shift -> ↑CO2 formation -> exhaled

·     pKa 6.1 is distant to pH 7.4, however…

·     High concentration - 24mM

·     Rapid enzyme - carbonic acid

·     Open ended - can exhale CO2, can urinate H+

Haemoglobin

·     Second most important ECF buffer

·     Compared with plasma proteins:

o ~2x concentration (140g/L cf. 70g/L)

o ~3x imidazole groups per molecule (38 cf. 13)

·     Imidazole groups on histidine residues

o HHb + K+ <-> KHb + H+

o pKa = pH of ICF in RBCs = 6.8

·     Carbamino compounds

o CO2 + Hb-NH2 <-> Hb-NHCOO- + H+

o CO2 binds terminal amino groups, and amino groups on side chains

o Released H+ is buffered by imidazole groups as above

Plasma proteins

·     Imidazole groups of histidine residues

o AH <-> A- + H+

·     pKa 6.8 is moderately close to pH 7.4

 

Respiratory compensation: (minutes)

Mechanism

·     Acidosis -> stimulate peripheral chemoreceptors -> ↑ MV -> ↓ PaCO2 -> ↓[H+]

·     From HH equation: pH = 6.1 + [HCO3] / 0.03 x PaCO2

Extent

·     Winter’s formula: PaCO2 = 8 + 1.5 x HCO3-

·     ECF pH approaches but does not reach or exceed 7.4

·     *Note respiratory compensation is ineffective for fixed acid, because both H+ and HCO3- are being removed as CO2*

 

Renal compensation: (hours-days)

Anion exchange

·     ↑HCO3-/Cl- exchange in proximal > distal tubule

o HH approach: hence ↑HCO3- reabsorption

o Stewart approach: hence ↑Cl- excretion -> ↑SID

o KN: Stewart correct, but kidneys respond to pH and HCO3-, not SID
(N.B. the vast majority of HCO3- is absorbed regardless)

·     Multiple structures involved:

o Carbonic anhydrase – cellular & tubular

o Basolateral Na+K+ATPase

o Apical H+ATPases

o Na+3HCO3- symporter

↑Excretion of titratable acid

·     Buffered: as phosphoric and sulfuric acid

·     ↑Activity of H+ATPase and H+K+ATPase in type A intercalated cell

·     Minimal HCO3- in distal tubule; hence H+ combines with PO43- and SO42-

·     Usual 30mmol/day, max 60mmol/day

↑Excretion of ammonium

·     ↓pH -> ↑uptake and oxidation of glutamine in proximal tubule

·     Glutamine + H+ -> α-ketoglutarate + NH4+

o Occurs when titratable acid exhausted

·     50% NH4+ reabsorbed and recycled, other 50% excreted

o HH approach: hence net ↑HCO3- reabsorption

o Stewart approach: hence ↑NH4+ excretion (strong ion) -> ↓SID

·     Up to 300mmol/day, i.e. high capacity system

 

Ion exchange:

Cells (minutes)

·  H+/K+ exchange

o ↓pH 0.1 -> ↑K+ 0.6mM

o Can conceal K+ depletion e.g. DKA

Bone
(days-weeks)

·  H+/Ca2+ exchange

 

Correction:

NAGMA

·  Treatment of the underlying disease

o N.B. in hyperchloraemic acidosis, renal compensation = correction

HAGMA

·  Treatment of underlying disease

o E.g. Lactic acidosis: tissue oxygenation

o E.g. ketoacidosis: insulin

 

 

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