#Buffers

#What is a buffer?

Buffers are solutions that resist pH change in response to the addition of acid or base.

• 60% of buffering is intracellular (intracellular proteins > phosphate)
• 40% is extracellular (Hb > bicarbonate > plasma proteins > phosphate)

Open buffers are systems where some of the products of reaction may escape.

#What makes a good buffer?

Good buffers:

• have a high concentration in the compartment of interest
• are close to 50% ionised, which occurs when the pKa is equal to the pH
  • pKa describes the Henderson–Hasselbalch equation
  • pH = pKa + log10 (forward reaction / backward reaction)
  • i.e. = pKa + log10 ( [A-] / [HA] )
• open buffers are effective because they maintain their concentration gradient, allowing the reaction to continue

#Buffer systems

The main buffer systems are:

• bicarbonate
• phosphate
• ammonium
• proteins
  • Hb - accounts for 50% of extracellular buffering
  • intracellular proteins - accounts for most of intracellular buffering and 50% of total body buffering
  • plasma proteins
• bone

#Bicarbonate (HCO₃⁻)

Characteristics

• High concentration in plasma
• pKa = 6.1
• Open buffer system

Location

• Extracellular fluid (ECF)
• Red blood cells (RBCs)

Mechanism

• CO₂ + H₂O ↔ H₂CO₃ ↔ HCO₃⁻ + H⁺
  • carbonic anhydrase catalyses this reaction
• ↑ Acid in plasma → reaction shifts left → ↓[H⁺] (↑pH) and ↑CO₂
• Open buffering in both directions:
  • Lungs eliminate CO₂ → drives leftward reaction
  • Kidneys excrete HCO₃⁻ → drives rightward reaction
• Relatively slow buffer: HCO₃⁻ diffuses poorly across most cell membranes (except RBCs)

#Phosphate

Characteristics

• Low plasma concentration
• High concentration in urine and ICF
• pKa = 6.8
• Effective buffer in ICF and urine; minimal contribution to plasma buffering
• Can be unreliable as it may bind other compounds

Reaction

• H₂PO₄⁻ ↔ HPO₄²⁻ + H⁺

The renal phosphate buffer system is described in more detail here.

#Ammonium (NH₄⁺)

Characteristics

• pKa 9.3
• filtered glutamine is absorbed into proximal convoluted tubule (PCT) cell in the nephron
• then metabolised to NH4+ and bicarbonate in PCT cell
  • NH4+ is secreted from the PCT, mostly through the NHE3 transporter (ammonium ‘pretends’ to be H+)
  • bicarbonate is reabsorbed

Reaction

• NH₄⁺ ↔ NH₃ + H⁺
• At physiological pH most ammonium exists as NH₄⁺

The ammonium buffer is described in more detail here.

#Haemoglobin

Characteristics

• Very high concentration in RBCs
• Oxyhaemoglobin pKa ≈ 6.6
• Deoxyhaemoglobin pKa ≈ 8.2
• Is considered an extracellular buffer despite being contained within RBCs by moving CO2 and bicarbonate to and from extracellular fluid
• Accounts for approximately 50% of plasma buffering

Mechanisms

• CO₂ binding (carbamino formation)
  • Hb binds CO₂ (deoxyHb has higher affinity)
  • Removing CO₂ drives HCO₃⁻ + H⁺ → H₂CO₃ → CO₂ + H₂O; consuming H⁺
• Histidine (imidazole) residues
  • Hb contains ~38 histidine residues which are present on imidazole functional groups
    • these histidine residues can bind H+
  • Deoxygenated Hb has greater affinity for H⁺ (Bohr effect)
  • Promotes conversion of CO₂ → HCO₃⁻ (by driving the above equation leftward)
    • HCO₃⁻ exits RBC in exchange for Cl⁻ (known as the chloride shift)

#Intracellular proteins

• High concentration in ICF
  • accounts for majority of intracellular buffering, and 50% of total body buffering
• High histidine/imidazole content
  • histidine residues on imidazole groups can bind H+
  • pKa of imidazole group is 6.8

#Plasma proteins

• e.g. albumin
• Much lower concentration than intracellular proteins and hence contributes less to total body buffering
• Plasma protiens generally have lower histidine/imidazole content
• charged functional groups can bind H+

#Bone

• Acts as a reservoir of phosphate and carbonate
• Releases buffers slowly into the circulation — important for long-term compensation in chronic acid–base disorders
• Kinetics are slow compared with respiratory and renal mechanisms