REGULATION OF ACID-BASE BALANCE BY ACID-BASE BUFFER SYSTEM
An acid-base buffer system is the
combination of a weak acid (protonated substance) and a base – the salt (unprotonated
substance). Buffer
system is the one, which acts immediately to prevent the changes in pH. Buffer
system maintains pH by binding with free H+.
Types of Buffer
Systems
Body fluids have three
types of buffer systems, which act under different conditions:
1. Bicarbonate buffer system
2. Phosphate buffer
system
3. Protein buffer
system.
1. Bicarbonate
Buffer System
Bicarbonate buffer system
is present in ECF (plasma). It consists of the protonated substance, carbonic acid (H2CO3) which is a weak
acid and the unprotonated substance, HCO3–, which
is a weak base. HCO3 – is
in the form of salt, i.e. sodium bicarbonate (NaHCO3).
Mechanism of action
of bicarbonate buffer system
Bicarbonate buffer system prevents the
fall of pH in a fluid to which a strong acid like hydrochloric
acid (HCl)
is added. Normally, when HCl is mixed with a fluid, pH of that
fluid decreases quickly because the strong HCl dissociates into H+ and
Cl–. But,
if bicarbonate buffer system (NaHCO3) is added
to the fluid with HCl, the pH is not altered much. This is because the
H+ dissociated
from HCl combines
with HCO3
– of NaHCO3 and forms a weak H2CO3. This H2CO3 in
turn dissociates into CO2 and H2O.
HCl + NaHCO3 →
H2CO3 +
NaCl
↓
CO2 + H2O
Bicarbonate buffer system also
prevents the increase in pH in a fluid to which a strong base
like
sodium hydroxide
(NaOH) is added. Normally, when a base (NaOH) is added to a fluid, pH increases. It is
prevented by adding H2CO3, which dissociates into H+ and
HCO3 –. The
hydroxyl group (OH)
of NaOH combines with
H+ and forms H2O.
And Na+ combines
with HCO3
– and forms NaHCO3. NaHCO3 is
a weak base and it prevents the
increase in pH by the strong NaOH..
As sodium bicarbonate is a very weak
base, its association with H+ is poor. So the rise in pH of the fluid
is very mild.
Importance of
bicarbonate buffer system
Bicarbonate buffer system is not powerful
like the other buffer systems because of the large difference between the
pH of ECF (7.4) and the pK of bicarbonate buffer system (6.1). But this buffer system
plays an important role in maintaining the pH of body fluids than
the other
buffer systems. It is because the concentration of two components
(HCO3
– and
CO2)
of this buffer system is regulated separately by two different mechanisms. Concentration
of HCO3
– is regulated by
kidney and the concentration of CO2 is regulated by the respiratory system. These
two regulatory mechanisms operate constantly and simultaneously, making this
system more effective.
2. Phosphate
Buffer System
This system consists of a weak acid,
the dihydrogen phosphate (H2PO4 – protonated
substance) in the
form of sodium dihydrogen phosphate
(NaH2PO4) and the
base, hydrogen phosphate (HPO4 –
unprotonated substance) in the form of disodium hydrogen phosphate (Na2HPO4).
Phosphate buffer
system is useful in the intracellular fluid (ICF), in red blood cells or other
cells, as the concentration of phosphate is more in ICF than in ECF.
Mechanism of
phosphate buffer system
When a strong acid like hydrochloric
acid is mixed with a fluid containing phosphate buffer, sodium
dihydrogen
phosphate (NaH2PO4 – weak acid) is formed. This permits
only a mild change in the pH of the fluid. HCl + Na2HPO4 →
NaH2PO4 +
NaCl
(strong acid) (weak acid) If a
strong base such as sodium hydroxide (NaOH) is added to the fluid
containing phosphate buffer, a weak base called disodium hydrogen phosphate (Na2HPO4) is
formed. This prevents the changes in
pH. NaOH + NaH2PO4 → NaHPO4 + H2O
(strong base) (weak base)
Importance of
phosphate buffer system
Phosphate buffer
system is more powerful than bicarbonate buffer system as it has a pK of 6.8,
which
is close to the pH of
the body fluids, i.e. 7.4. In addition to ICF, phosphate buffer is useful in
tubular fluids of kidneys
also. It is because more phosphate ions are found in tubular fluid. In the red blood
cells, the potassium ion concentration is higher than the sodium
ion concentration. So, the elements of phosphate buffer inside the red blood cells are in the form of
potassium dihydrogen phosphate (KH2PO4) and dipotassium
hydrogen phosphate (K2HPO4).
3. Protein Buffer
System
Protein buffer systems
are present in the blood; both in the plasma and erythrocytes.
Protein buffer
systems in plasma
Elements of proteins,
which form the weak acids in the plasma are:
i. C-terminal carboxyl
group, N-terminal amino group
and side-chain carboxyl group of glutamic
acid
ii. Side-chain amino
group of lysine
iii. Imidazole group of histidine.
Protein buffer systems
in plasma are more powerful because of their high concentration in plasma and
because of their pK being very close
to 7.4.
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