J1iii: Compare & contrast the body’s bicarbonate, phosphate & protein buffer systems

  • Definition – buffer: a substance with a capacity to bind/release H+ to minimise pH ∆
    • Buffers consist of a weak acid + their conjugate base
    • Buffers are most effective at their pKa, where they are 50% ionised 
  • The whole point of a buffer is to minimise pH ∆
    • Target pH ECF = 7.4
    • Target pH ICF = 6.8 
  • Buffering power determined by:
    1. pKa
    2. Concentration of the buffer
    3. Whether the buffer is ‘open-ended’

HCO3

Phosphate

Proteins

pKa

6.1

6.8

6.8

Concentration

Plasma [HCO3–] = 24mol/L

Very low in plasma

Significant in urine

Hb = 15g/dL

Other protein = 7g/dL

Area most effective

Major ECF buffer

ICF buffer & tubule fluid buffer

ICF & ECF

Buffering Mechanism

H+ + HCO3 –‎⇔ H2CO3 ⇔ H2O + CO2

H+ + HPO42- ‏⇔H2PO4–-

H+ + protein ⇔ H protein

Comments on effectiveness

pKa furthest from physiological pH but this is the most important buffer because:

1.       High substrate [HCO3–]

2.       Open-ended → CO2 & HCO3– can both be regulated by acid base control mechanisms (lungs, kidneys)

Requires presence of CA

Can only buffer metabolic acids → cannot buffer H2CO3

 Poor intracellular buffer bc usually require transporters and are slow to cross the membrane

pKa closer to ICF & urinary tubule fluid pH
[Phosphate] of tubule fluid v high & significant buffer here

Imidazole of histidine

Residues binds H+ & buffers

pKa 6.8

Free amino & carboxyl terminals can also buffer H+ but amino pKa 9.0 & carboxyl pKa 2

∴ not important physiologically

Hb most important protein because:

  1. Greater concentration 15g/dL cf. 7g/dL
  2. More histidine residues → 38 per molecule Hb

DeoxyHb has pKa 7.9 which is even closer to physiological pH

∴ dissociates more & is a better buffer → this is part of the HALDANE EFFECT (deoxygenated blood can carry more CO2