J1iii / 18A06: Define buffer. Acid Base shifts in blood buffered

18A06: Exam Report

Define a buffer (25% of marks). Describe how acid and base shifts in the blood are buffered (75% of marks).

45% of candidates passed this question.

Few candidates defined a buffer making it difficult to award 25% of the marks for this question.

The three main buffers in blood should have been described: bicarbonate system, haemoglobin and proteins. The pKa, the buffering mechanism and the capacity of the system should have been described. The enderson Hasselbach equation was sometimes incorrect. Marks were only awarded for buffers in blood and unfortunately some candidates described non-blood buffers.

J1iii / 18A06: Define a buffer (25 marks). Describe how acid base shifts in the blood are buffered (75 marks)

Definition: buffer = substance which has the capacity to bind/release H+ to minimise pH Δ

  • Buffer mixtures are a weak acid & its conjugate base
    • H2PO4– (weak acid) ↔ HPO42- (conjugate base) + H+
  • Buffers are most effective at their pKa, where they are 50% ionised
  • Buffer power determined by:
    1. pKa
    2. Concentration of the buffer

Buffering in Blood

HCO3

  • pKa 6.1
  • Concentration: highest with plasma [HCO3–] = 24 mol/L
  • HCO3 – production occurs in RBC in presence CA:
    • CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3– → diffuses out & buffers ECF
  • Although pKa very different from target pH, so effective because:
    1. High concentration
    2. Open-ended
  • Considered “open ended” because its components; CO2 & HCO3– are regulated independently
  • CO2 → lungs → exhaled
  • HCO3– → kidneys → reabsorbed
  • HCO3– buffer system can only buffer metabolic acids

Hb

  • pKa 6.8
  • Concentration = 15g/dL
  • Actually intracellular protein, but because it’s present in ‘blood’, not tissue → can be considered ECF buffer
  • Each Hb → 68 histidine residues
  • Hb high concentration in plasma
  • DeoxyHb = pKa 7.9
  • OxyHb = pKa 6.6
  • ∴ DeoxyHb dissociates greater at pH 7.4 cf. oxyHb
  • H+ taken up by histidine residues → buffered
  • Some CO2 binds to terminal amine groups → forms carbamino compounds

Plasma Proteins

  • pKa 6.8 (of imidazole of histidine residues)
  • Concentration = 7g / dL
  • The histidine residue binds & buffers H+