J1iv: H-H & Stewart Approach to Acid Base

Henderson-Hasslebank Approach

  • LAW OF MASS ACTION → the velocity of a chemical reaction is proportional to the active concentrations of the reaction
  • The H-H equation describes the relationship between:
    • pH
    • Dissociation constant
    • Weak acid & its base salt
H-H Equation
  • In solution, an acid dissociates:
Acid Dissociates
  • The proportions of acid & base depend on the dissociation constants
  • If K1 > K2 then there will be more H+ & A– in solution
  • pKa = the negative log of the dissociation constant & is equal to the pH at which the substance is 50% dissociated

pKa = – log [KA] ← acid dissociation constant

NB: pH is temp related → ↓temp = ↓ionic dissociation H2O = ↑pH

Stewart Approach

Introduction

  • AKA Physiochemical Approach
  • Differs from HH which is determined by H+ & HCO3–
  • 3 independent variables which control acidity

Independent Variables → these ∆ pH (= 1° cause)

  • pCO2
  • ATOT (total weak non-volatile acids)
  • SID

Dependent Variables (= 2° effect)

  • These values depend on values of independent variables
  • If these ∆, then independent variables must have ∆
    • H+
    • OH–
    • CO32-
    • HA (weak acid)
    • A– (weak anions)

Physical Laws Of Physiochemical Methods

  • Interaction of dependent & independent variables must obey the laws of aqueous solution:
    1. ELECTRONEUTRALITY – in any aqueous solution the sum of all +vely charged ions must equal the sum of all -vely charged ions
    2. DISSOCIATION EQUILIBRIA – derived from Law of Mass Action. The velocity of a chemical reaction is proportional to the active [ ] of the reaction
    3. CONSERVATION OF MASS – the amount of substance remains constant unless added, removed, generated, or destroyed

Strong Ions

  • Definition = ions that completely dissociation in a solution

SID of H2O

  • H2O → H+ + OH–

Determined by SID

SID
    • SID = [strong cations] – [strong anions]

↑OH– = alkalosis

↓OH– = acidosis

  • Recall: H2O → OH– + H+ because altering OH– will alter H+ (dissociated)

SID in Plasma

  • All solutions of body have H2O
  • [H+] of these solutions depends on the dissociation of H2O
  • H2O dissociation depends on the VARIABLES!
    • SID, PCO2, ATOT
  • Strong cations = Na, K, Ca, Mg
  • Strong anions = Cl, SO42-
  • SID = strong cations – strong anions
  • Normal plasma SID = 42 mEq/L
  • But you must have Electroneutrality 
  • Whereby Anions = cations so that the SID = 0
  • But it’s not, it’s 42mEq/L and slightly alkaline
  • ∴somewhere in plasma is unmeasured anions (the OH- in the above gamblegram)
  • ↑SID =  alkalosis (the unmeasured anions have increased further)
  • ↓SID = acidosis (there are less unmeasured anions)

(see Gamblegram)

Effect of Independent Variables in pH

  • SID = STRONG CATION – STRONG ANION

       = (Na + K + Ca + Mg) – {Cl + lactate + urate}

       = Na – Cl (mainly)

  • ∴SID is affected by:
  •  H2O deficit / excess

Dehydration = concentrates alkaline = ↑SID

H2O excess  = dilutes acidosis = ↓SID

  • ↑/↓ Na
  • ↑/↓ Cl –
  • ↑ organic acids with pKa < 4 (Strong acids)e. lactate, ketones
  • ATOT
    • Non-volatile weak acids (non-CO2) exist in all body fluid compartments
    • In plasma: PO42-, serum proteins, albumin
    • Controlled by metabolic state & liver
    • Non-volatile weak acids dissociate:

HA ↔ H+ + A–

  • Total concentration of weak acid = ATOT
  • ↑Alb/PO4 = ↓ SID = acidosis
  • ↓Alb/PO4 = ↑ SID = alkalosis
  • PCO2
    • Controlled by respiratory system
    • ↑CO2 = ↑H+ (adding)

Classification According to Stewart’s

Resp Causes

  • ↑/↓ PCO2

Non-Resp Causes

  • ABNORMAL SID ↑/↓ H2O

Water excess = ↓SID

Water deficit = ↑ SID

  • STRONG ION IMBALANCE ↓/↑ SID

↑Cl = ↓SID

↓Cl = ↑SID

Anion excess = ↓SID

  • ATOT ABNORMALITY ∆ ATOT

↓/↑ Phosphate

↑/↓ Albumin