Physiological buffers: a brief introduction

Physiological Buffers

A "buffer system," which minimizes pH changes on addition of acid or base, consists of a solution containing a weak acid together with one of its soluble salts e.g. H₂CO₃ and NaHCO₃.

The acid, being weak, is slightly ionized but the soluble salt is ionized to a large extent. The mixture thus provides a reservoir of base (anions) which can combine with added H⁺ (neutralize added acid) and a reservoir of acid (undissociated acid molecules) which can donate hydrogen ions to neutralize added base.

Efficacy of the buffer depends on the pH of the buffer i.e. the concentration of weak acid and base added to constitute the buffer. This relationship is expressed by the Henderson-Hasselbalch equation.

pH= pK+ log [salt]/[acid]

A buffer is most effective when the concentration of its salt and acid are equal. pK of a buffer is that pH at which its salt and acid forms are in equal concentration. Hence a buffer is most effective in a solution the pH of which is equal to the pK of the buffer. The most effective physiological buffers are those whose pK is around 7.4.

In a solution which has various buffer systems then they should be in mutual equilibrium. If the acid:base is known for any one of the buffer systems, the pH of the mixture can be known. This can be described in terms of isohydric principle:

[H⁺] = K1*[acid 1]/[salt 1] = K2*[acid 2]/[salt 2]= K3*[acid 3]/[salt 3]

In plasma, H₂CO₃-NaHCO₃ buffer is the easiest to measure and most important quantitatively. Therefore total CO₂ content of plasma (sum of HCO₃^- and H₂CO₃ + CO₂) can represent the total buffering capacity.