REGULATION OF ACID-BASE BALANCE BY ACID-BASE BUFFER SYSTEM

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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