Acid-Base Balance

Question 1

What is the difference between acidosis & acidaemia and between alkalosis & alkalaemia.

Answer 1

Acidaemia/ Alkalaemia = pH of blood outside the normal range

Acidosis/ Alkalosis = underlying PROCESS that may result in the blood’s pH being outside of the normal range

e.g. DKA can result in acidaemia of the blood.

Question 2

What is the pH scale?

Answer 2

Acidity expressed as the negative log of hydrogen ion concentration.

pH = - log₁₀ [H+]

Question 3

What is the carbonic acid reaction?

Answer 3

THE CARBONIC ACID REACTION

= The chemical relationship b/w CO₂ and water.

CO₂ carried in the blood as HCO3^- & then released in the lungs after being converted back to CO₂.

CO₂ + H₂0 ⇔ H₂CO₃ ⇔ H⁺ + HCO₃^-

Question 4

What organs are involved in the control of pH and how do they do this?

Answer 4

CONTROL OF PH

• Brain & Lungs = SHORT TERM CONTROL
  
  • CSF acidotic –> respiratory centre increases resp drive to blow off CO₂
  • CSF alkalotic (bicarbonate secreted into the CSF as it is produced in the choroid plexus) –> reduced respiratory drive to conserve CO₂
• Kidney = LONG TERM CONTROL (over hrs/ days)
  
  • pH persistently low
  • –> kidneys increase excretion of H+ ions
  • This pushes to carbonic acid reaction to the right
  • CO₂ + H₂0 –> H₂CO₃ –> H⁺ + HCO₃^-
  • More bicarbonate is produced
  • this compensates for the acidosis
  • a raised compensatory bicarbonate => indicates a longer term process e.g. chronic respiratory illness

Question 5

What are the 3 independent variables that affect the pH in the body, according to Stewart’s Strong Ion Theory?

Answer 5

STEWART’S STRONG ION THEORY

3 INDEPENDENT VARIABLE AFFECT pH

1. PCO₂ (the carbonic acid rxn)
2. SID = Strong Ion Difference (the electrical charge difference b/w cations and anions)
   
   • NB electroneutrality in extracellular fluid MUST be maintained
   • strong ions = ions that exist mainly in ionic form, rather than in combined form as molecules
     
     • CATIONS = Na⁺, K⁺, Mg²⁺, Ca²⁺
     • ANIONS = HCO₃^-, Cl^-, Alb^-, phosphate^2-, lactate^-, X_A^- (unmeasured weak acids)
   • ([Na⁺] + [K⁺] + [Ca²⁺] + [Mg²⁺]) - ([Cl^-] + [Lact^-])
   • BUT K+ / Ca2+/ Mg 2+ are small numbers that do not vary much =>
   • SID = [Na⁺] - ( [Cl^-] + [Lact^-] )
3. [A_TOT] = the total concentration of NON-VOLATILE WEAK ACIDS in the body
   
   • ALBUMIN (=> albumin infusions can contribute to acidaemia)
   • phosphate
   • plasma proteins & unmeasured weak acids (X_A^-)

[A_TOT] = (Alb^- + Phos^2- + XA^-)

Acidosis and Alkalosis are the consequence of changes in these 3 parameters.

Question 6

What are the 6 dependent ion concentrations in the body? What are dependent ion concentrations? Elaborate on the role of bicarbonate.

Answer 6

DEPENDENT ION CONCENTRATIONS

= Ions whose concentrations depend on on the concentrations of OTHER ions and molecules

1. [H⁺]
2. [HCO₃^-]
   
   • both biochemical and electrical buffer
   • concentration flexes up and down to accomodate other electrolyte changes and charges
   • i.e. bicarbonate itself is not responsible for changes in pH, its value changes in response to other CAUSATIVE metabolic processes
   • this is why giving sodium bicarbonate does not often help correct metabolic acidosis - need to correct the underlying condition instead
3. [CO₃^-]
4. [OH^-]
5. [HA] (weak or poorly dissociated acids & ions)
6. [A^-] (“)

Question 7

What makes up the majority of anions and cations in the extracellular fluid?

Answer 7

Na⁺ and Cl^-

Question 8

Name the anions and cations in the extracellular fluid.

Answer 8

EXTRACELLULAR FLUID

• CATIONS
  
  • Na⁺
  • K⁺
  • Mg²⁺
  • Ca²⁺
• ANIONS
  
  • Cl^-
  • HCO₃^-
  • Alb^-
  • Phos^2-
  • Lactate^-
  • X_A^- (unmeasured weak acids) = O! KRREP!
    
    • O= Organic acids (methylmalonic/ propionic/ isolvaleric acidaemia, maple syrup urine Dz)
    • !! = Isoniazid Intoxication
    • K = Ketoacidosis (diabetic, starvation, alcoholic)
    • R = Rhabdomyolysis
    • RE= REnal failure (residual acids normally renally eliminated)
    • P= Poisons
      
      • methanol
      • ethanol
      • ethylene glycol
      • aspirin

BOLD = ANION GAP

Question 9

What is the anion gap? Give the formulas for the anion gap and corrected anion gap.

Answer 9

ANION GAP

• = the UNMEASURED anions and cations
• i.e. all the anions and cations for which measurements are NOT usually available
• Consists of the following anions:
  
  • Albumin^- (large component)
  • Phopshate^2-
  • Lactate^-
  • X_A^- (unmeasured weak acids = O! KRREP!)

AG = cations - anions

AG = (Na⁺ + K⁺ + Mg²⁺ + Ca²⁺) - (Cl^- + HCO₃^-)

But Ca²⁺ & Mg²⁺ concentrations are small and vary minimally =>

AG = (Na⁺ + K⁺ ) - (Cl^- + HCO₃^-)

Corrected anion gap

= tries to correct for dramatic changes in albumin (largest component of anion gap) which may occur in acute injury or illness, and might otherwise mask an acidotic process

NB

• Albumin only has a quarter of unit charge per molecule
• Add a quarter of the albumin deficit to uncorrected AG
• Normal Serum Albumin = 42 (the answer to the universe)

Corrected AG =

(Na⁺ + K⁺ ) - (Cl^- + HCO₃^-) + 0.25 x (42 - Alb)

Question 10

What are the causes of a raised anion gap?

Answer 10

RAISED ANION GAP

Anion gap =

• albumin
• phosphate
• LACTATE <– LACTIC ACIDOSIS
• WEAK ACIDS (X_A^-) <– O! KRREP!
  
  • O= Organic acids (methylmalonic/ propionic/ isolvaleric acidaemia, maple syrup urine Dz)
  • !! = Isoniazid Intoxication
  • K = Ketoacidosis (diabetic, starvation, alcoholic)
  • R = Rhabdomyolysis
  • RE= REnal failure (residual acids normally renally eliminated)
  • P= Poisons
    
    • methanol
    • ethanol
    • ethylene glycol
    • aspirin

Question 11

What is the Henderson-Hesselbach equation?

Answer 11

HENDERSON - HESSELBACH EQUATION

= an equation that can be used to calculate the pH of a solution

The hydrogen concentration is described in in terms of the ratio between base + acid.

pH = - log₁₀ [H⁺]

CO₂ + H₂O ⇔ H₂CO₃ ⇔ H⁺ + HCO₃^-

Moving the carbonic acid equation around we get:

K x ( [CO₂] / [HCO₃^- ] ) = [H⁺]

K = dissociation constant = ~ 6.1

So expressing this in logarithmic terms we get the

Henderson-Hesselbach equation:

pH = pK + log ( [HCO₃^- ] / [CO₂] )

REMEMBER: It’s all about that BASE! => NUMERATOR

Question 12

What are the 2 ways that HCO₃^- can be calculated?

Answer 12

CALCULATING HCO₃^-

1. Henderson Hesselbach equation:
   
   • pH = pK + log ( [HCO₃^-] / PCO₂)
2. Using SID and A_TOT
   
   • SID = cations - anions
     
     • remember that K⁺/ Mg2+/ Ca2+ are small no.s and do not change much so they are omitted
     • SID = [Na⁺] - ( [Cl^-] + [Lact^-] )
   • [A_TOT​]
     
     • ​the total concentration of NON-VOLATILE WEAK ACIDS in the body
     • [A_TOT]_​ = (Alb^- + Phos^2- + X_A^-)
   • Consider all the extracellular ions (image)
   • If you want to find HCO3- conc. then take away everything else
   • i.e. (cations - anions) - total non-volatile weak acids — i.e. albumin + phosphate + unmeasured weak acids
   • i.e. [HCO₃^-] = SID - [A_TOT]
   • inserting this into the H-H equation
   • pH = pK + log ( [HCO₃^-] / PCO₂)
   • we get
   • pH = pK + log

[Na⁺ - ( Cl^- + Lact^- + Alb^- + Phos^2- + X_A^- )]

PCO_2​

• ​=> if the numerator i.e. HCO₃^- increases, increases the pH (alkalosis)
  
  • ​increase in sodium
  • drop in chloride, lactate, albumin, phosphate or weak acids
• => if the numerator i.e. HCO₃^- decreases, pH decreases (acidosis)
  
  • ​drop in sodium
  • increase in chloride, lactate, albumin, phosphate or weak acids

Question 13

What is base excess?

Answer 13

BASE EXCESS

= the amount of acid or base that needs to be added to a blood sample to return the pH to 7.40

at 37 degrees

at a PCO₂ of 5.3 kPa

Calculated using SBC (standard bicarbonate)

Question 14

How are HCO₃^- and base excess related?

Answer 14

RELATIONSHIP B/W BASE EXCESS AND HCO₃^-

• acidosis
• fall in HCO₃^-
• mirrored by fall in base excess
• i.e. there is less base excess, it becomes more -ve
• i.e. there is a greater base deficit the more -ve this number becomes

Question 15

What components affect base excess and what equations can be used to work out which of these contributes most to the base excess?

Answer 15

COMPONENTS AFFECTING BASE EXCESS

• base excess reflects changes in HCO₃^- i.e.
  
  • in acidosis a fall in HCO₃^- is reflected by a fall in base excess i.e. it becomes more -ve
  • in alklalosis a increase in HCO₃^- is reflected by a rise in base excess i.e. it becomes more +ve
• the concentration of bicarbonate i.e. [HCO₃^-]] can also be expressed as:
  
  • Na⁺ - ( Cl^- + Lact^- + Alb^- + Phos^2- + X_A^- )
  • i.e. a change in any of these components will also affect base excess
• Components affecting base excess and equations to work out their individual contribution = SLAX
  
  • BE change due to Na and Cl = (Na⁺ - Cl^-) - 38
  • BE change due to lactate = 2 - lactate
  • BE change due to albumin = 0.25 x (42 - Alb)
  • BE change due to other acids (X_A^-) = total BE - sum of the others