Regulation Of Acid-Base Balance Of Acid-Base Bufer System

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Chapter: Anatomy, Physiology And Health Education : Acid-Base Balance

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+.


REGULATION OF ACID-BASE BALANCE OF ACID-BASE BUFER SYSTEM

 

DEFINITION

 

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:

 

a.       Bicarbonate buffer system

b.       Phosphate buffer system

c.       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, HCO –, which is a weak base. HCO 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.

 

Normally, when HCl is mixed with a fluid, pH of  that fluid decreases quickly because the strong HCl dissociates into H+ and Cl.

 

When bicarbonate buffer is added to the fluid with HCl, the pH is not altered much.

 

This is because the H+ dissociated from HCl combines with HCO –  of NaHCO   and forms a weak H CO  . This H2COin turn dissociates into COand H2O.

 


 

Bicarbonate buffer system also prevents the increase in pH in a fluid to which a strong base like sodium hydroxide (NaOH) is added.

 

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Normally, when a base (NaOH) is added to a fluid, pH increases. It is prevented by adding H2CO3, which dissociates into H+ and HCO . The hydroxyl group (OH)

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 of NaOH combines with H+ and forms H O. And Na+ combines with HCO and forms NaHCO . NaHCO  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 (HCO and CO ) of this buffer system is regulated separately by two different mechanisms.

 

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Concentration of HCO 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  (NaH PO    –  weak  acid)  is  formed.  This permits only a mild change in the pH of the fluid.

 


 

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.

 


 

 

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 (KH PO ) and dipotassium hydrogen phosphate

 

 

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:

 

1.                  C-terminal carboxyl group, N-terminal amino group and side-chain carboxyl group of glutamic acid

2.                         Side-chain amino group of lysine

3.                         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.

 

Protein Buffer System In Erythrocytes (Hemoglobin)

 

Hemoglobin is the most effective protein buffer and the major buffer in blood. Due to its high concentration than the plasma proteins, hemoglobin has about six times more buffering capacity than the plasma proteins. The deoxygenated hemoglobin is a more powerful buffer than oxygenated hemoglobin because of the higher pK. When a hemoglobin molecule becomes deoxygenated in the capillaries, it easily binds with H+, which are released when CO2 enters the capillaries. Thus, hemoglobin prevents fall in pH when more and more CO2 enters the capillaries.

 

REGULATION OF ACID-BASE BALANCE BY RESPIRATORY MECHANISM

 

Lungs play an important role in the maintenance of acid-base balance by removing CO2 which is produced during various metabolic activities in the body. This CO2 combines with water to form carbonic acid.

 

Since carbonic acid is unstable, it splits into H+ and  HCO 

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Entire reaction is reversed in lungs when CO diffuss from blood into the alveoli of lungs.

 


 

 

And CO2 is blown off by ventilation.

 

When metabolic activities increase, more amount of CO2 is produced in the tissues and the concentration of H+ increases as seen above. Increased H+ concentration increases the pulmonary ventilation (hyperventilation) by acting through the chemoreceptors (Chapter 126). Due to hyperventilation, the excess of CO2 is removed from the body.

 

 

REGULATION OF ACID-BASE BALANCE BY RENAL MECHANISM

 

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Kidney maintains the acid-base balance of the body by the secretion of H+ and by the retention of HCO (Fig. 5.1).

 


 

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