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Example 2BUFFER ACTION AND pH OF BLOOD

The normal pH of blood is 7.4 and is maintained at this value by buffer action in

particular by the action of HCO 3 and CO 2 resulting from gaseous CO 2 dissolved

in blood and the resulting ionisation of carbonic acid:

CO 2 þH 2 OÐH 2 CO 3

H 2 CO 3 ÐHþþHCO 3

It is possible to calculate an overall equilibrium constant (Keq) for these two consecutive

reactions and to incorporate the concentration of water (55.6 M) into the value:

Keq¼

½HþŠ½HCO 3 Š

½CO 2 Š ¼^7 :^95 ^10

 (^7) hence pKeq¼ 6 : 1
Rearranging:
pH¼pKeqþlog

½HCO 3 Š

½CO 2 Š

When the pH of blood falls due to the metabolic production of Hþ, these equilibria shift

in favour of increased production of H 2 CO 3 that in turn ionises to give increased CO 2

that is then expired. When the pH of blood rises, more HCO 3 is produced and breathing

is adjusted to retain more CO 2 in the blood thus maintaining blood pH. Some disease

states may change this pH causing either acidosis or alkalosis and this may cause

serious problems and in extreme cases, death. For example, obstructive lung disease

may cause acidosis and hyperventilation alkalosis. Clinical biochemists routinely

monitor patient’s acid–base balance in blood, in particular the ratio of HCO 3 and CO 2.

Reference ranges for these at pH 7.4 are½HCO 3 Š¼ 18 : 0  26 : 0 mM and

pCO 2 ¼4.6–6.9 kPa, which gives½CO 2 Šin the range of 1.20 mM.

Question A patient suffering from acidosis had a blood pH of 7.15 and½CO 2 Šof 1.15 mM. What
was the patient’s½HCO 3 Šand what are the implications of its value to the buffer
capacity of the blood?

Answer Applying the above equation we get:

pH¼pKeqþlog

½HCO 3 Š

½CO 2 Š

7 : 15 ¼ 6 : 10 þlog

½HCO 3 Š

1 : 15

1 : 05 ¼log½HCO

 3 Š

1 : 15

Taking the antilog of this equation we get 11 : 22 ¼½HCO 3 Š= 1 : 15

Therefore½HCO 3 Š¼ 12 : 90 mM indicating that the bicarbonate concentration in

the patient’s blood had decreased by 11.1 mM i.e. 47% thereby severely reducing the

buffer capacity of the patient’s blood so that any further significant production of

acid would have serious implications for the patient.

12 Basic principles