Chemical Pathology 3 - Acid Base handling

Question 1

What two electrolytes are involved in short-term H+/pH buffering in the ECF?

Answer 1

Either bicarbonate or phosphate:
HCO3- _ H+ –> H2CO3
HPO4- + H+ –> H2PO4

Question 2

How is pH control achieved in the proximal convuluted tubule?

Answer 2

1. H+ and HCO3- combine in tubule lumen to form H2CO3
2. Carbonic anhydrase on tubule tumen membrane converts H2CO3 to H2O and CO2 and absorbs it into the tubule wall cell
3. H2O + CO2 –> H2CO3 again inside the cell, via carbonic anhydrase II
4. Bicarbonate is exchanged with chloride ions, releasing into the capillary
5. H+ ions can be actively secreted into the tubule lumen, or transported via a sodium-proton exchanger

Question 3

Recall 3 broad mechanisms of aetiology of metabolic acidosis

Answer 3

1. H+ prodution (eg DKA)
2. Decreased H+ excretion (eg renal tubular acidosis)
3. Bicarbonate loss (eg intestinal fistula)

Question 4

Describe the change in the acid-base equilibrium in a metabolic acidosis

Answer 4

Equilibrium =
HCO3- + H+ H2CO3 H2O + CO2
Extra H+ produced by acidosis pushes reaction RIGHT
CO2 production increases –> blown out by increased ventilation

Question 5

Describe the change in the acid-base equilibrium in a respiratory acidosis

Answer 5

Equilibrium =
HCO3- + H+ H2CO3 H2O + CO2
Excess CO2 produced by reduced ventilation pushes reaction LEFT, so more H+ and HCO3- is produced
Chronically - CO2 remains raised (due to reduced ventilation), and HCO3- remains raised to maintain physiological pH

Question 6

Describe the change in the acid-base equilibrium in a metabolic alkalosis

Answer 6

Equilibrium =
HCO3- + H+ H2CO3 H2O + CO2
Pathology = decreased H+ / increased HCO3-
Either way - need to regenerate H+
Therefore, reaction moves LEFT
To do this: resp rate decreases (to increase CO2)

Question 7

Describe the acute change in the acid-base equilibrium in a respiratory alkalosis

Answer 7

Equilibrium =
HCO3- + H+ <> H2CO3 <> H2O + CO2
Hyperventilation –> reduced CO2
Reaction moves RIGHT to restore CO2

Question 8

What are the possible causes of metabolic alkalosis?

Answer 8

H+ loss: pyloric stenosis, hypokalaemia

HCO3- excess: lots of Rennies

Question 9

Describe the chronic change in the acid-base equilibrium in a respiratory alkalosis

Answer 9

Equilibrium =
HCO3- + H+ H2CO3 H2O + CO2

Acutely, reaction moves RIGHT to restore CO2 (so you get low H+ and HCO3-)

Chronically, kidneys compensate by reducing H+ excretion - so H+ returns to normal, but HCO3- and CO2 remain low