One of the main buffering systems found in blood consists of carbonic acid (H2CO

Question

One of the main buffering systems found in blood consists of carbonic acid (H2CO3) and bicarbonate ion (HCO3-). Most of the carbonic acid is actually dissolved carbon dioxide. Write an equation showing how this buffering system would neutralize glycol acid (HOCH2COOH) that might enter the blood from ethylene glycol poisoning. Suppose a cat has 0.15 mole of HCO3- and 0.15 mole of H2CO3 in its bloodstream. How many grams of glycolic acid could be neutralized before the buffering system in the blood is overwhelmed?

Explanation / Answer

In the liver, ethylene glycol is oxidized to glycolic acid (HOCH2COOH), which enters the bloodstream. The acidity of blood is critically important and tightly regulated because many proteins only function in a narrow pH range. In human blood, for example, pH is held between 7.36 and 7.42. This nearly constant blood pH is maintained by buffers. We discuss buffers more carefully later in this chapter, but for now know that a buffer is a chemical system that resists pH changes by neutralizing added acid or base. An important buffer in blood is a mixture of carbonic acid (H2CO3) and the bicarbonate ion (HCO3 -). The carbonic acid neutralizes added base:

H2CO3(aq) + OH-(aq) = H2O(l) + HCO3 -(aq)

The bicarbonate ion neutralizes added acid:

HCO3 -(aq) + H+(aq) = H2CO3(aq)

This buffering system is very effective because of the ability to convert carbonic acid to carbon dioxide (through the enzyme carbonic anhydrase) then remove CO2 from the body through respiration. For example, adding enough acid to lower the serum bicarbonate by half would normally drop the pH from 7.4 to 6.0 — but instead all the extra H2CO3 is removed by conversion to CO2. The drop in pH stimulates extra respirations so CO2 (and subsequently more H2CO3) is removed. The pH therefore falls only to 7.3 or 7.2.

The ratio of bicarbonate to carbonic acid determines the pH of the blood. Normally the ratio is about 20:1 bicarbonate to carbonic acid.
      This relationship is described in the Henderson-Hasselbach equation:

           pH = pK + log (HCO3-/H2CO3)

      (pK is the dissociation constant of the buffer, 6.10 at body temperature. The change in pK with temperature is the reason pH determinations must be adjusted for patients with abnormal temperatures.)

As carbon dioxide is directly proportional to the carbonic acid (H2CO3), and can be directly measured, it will be substituted into the H-H equation.
           PaCO2 = 33 x H2CO3       or       H2CO3 = .03 x PaCO2

      By substituting,

           pH = pK + log (HCO3-/(PaCO2 x 0.03))

      Thus by measuring serum pH and PaCO2, the serum bicarbonate can be calculated:
           log (HCO3-) = pH + log (PaCO2) - 7.604

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