Blood Buffering: the carbonate system

 

The pH of human blood is about 7.4

If pH falls outside range 6.8 to 7.8, you are dead.

The main blood buffer is carbonate system (but phosphate and protein systems help)

 

The pKa for carbonic acid is 6.1

 

H₂CO₃ ⇔ HCO₃^- + H⁺

 

At physiologic pH, find ratio of carbonate to carbonic acid

 

pH = pKa + log [A^-]/[HA]

 

7.4 = 6.1 + log [A^-]/[HA]

 

1.3 = log [A^-]/[HA]      Therefore: [A^-]/[HA] = 20. 

 

Given total buffer is about 28 mM, [HCO₃^-] = 26.6 mM and [H₂CO₃] = 1.4 mM

 

This seems like a poor buffer BUT infinite CO₂ reservoir gives huge buffering capacity and MOST assaults on system are metabolic acid production.

 

CO₂ (gas) ⇒ CO₂(aqueous) + H₂O ⇒ H₂CO₃ ⇒ + H⁺ + HCO₃^-

 

In clinic, conditions that effect MAINLY [HCO₃^-] are “metabolic”, and conditions that effect MAINLY [H₂CO₃] are “respiratory”

 

Metabolic acidosis means excess H+: often seen with diabetic conditions

 

Metabolic alkalosis is loss of H+: may indicate vomiting or poisoning with base

 

Respiratory acidosis:  High CO₂ yields high [H₂CO₃].  May indicate poor CO₂ clearance from lungs.

 

Respiratory alkalosis arises from very rapid removal of CO₂, which is indicative of hyperventilation.

 

 

Ex.  A patient comes in comatose.  Blood gas analyzer shows [HCO₃^-] = 50 mM and [H₂CO₃] = 1.4 mM. 

 

pH = 6.1 + log 50/1.4 = 7.67

 

metabolic alkalosis, maybe ingested too much bicarbonate (a Tums addict!)