Chemical Buffer Systems and Acid-Base Balance

Chemical buffers resist pH changes and are the body's first line of defense. 

Ability of an acid-base mixture to resist sudden changes in pH is called its buffer action. 

Tissue cells and vital organs of the body are extremely sensitive to even the slightest change in the pH environment 

In high concentrations, both acids and bases can be extremely damaging to living cells

Essentially every biological process within the body is disrupted

Buffers work against sudden and large changes in the pH of body fluids by

1. Releasing hydrogen ions (acting as acids) when the pH increases, and

2. Binding hydrogen ions (acting as bases) when the pH decreases.

Three major chemical buffer systems in the body are the:

Carbonic acid-bicarbonate buffer system

Phosphate buffer system

Protein buffer system

Carbonic Acid-Bicarbonate Buffer System and Acid-Base Balance

The carbonic acid-bicarbonate buffer system plays an extremely important role in maintaining pH homeostasis of the blood

Carbonic acid (H2CO3) dissociates reversibly and releases bicarbonate ions (HCO3) and protons (H+) as follows:

Response to an increase in pH - H+ proton donor

  H2CO3 -> HCO3  +  H+

Response to a decrease in pH  - H+ proton acceptor 

H2CO3 <- HCO3  +  H+

Under normal conditions, the ratio between the HCO3 and H2CO3 in the blood is 20:1

co2 to Hco3.jpg

Chemical equilibrium between carbonic acid (weak acid) and bicarbonate ion (weak base) works to resist sudden changes in blood pH.

For example, when the blood pH increases (i.e., becomes more alkaline from the addition of a strong base), the equilibrium shifts to the right.

A right shift forces more carbonic acid to dissociate, which in turn causes the pH to decrease.

In contrast, when the blood pH decreases (i.e., becomes more acidic from the addition of a strong acid), the equilibrium moves to the left. 

A left shift forces more bicarbonate to bind with protons.

Carbonic acid-bicarbonate buffer system converts:

1. Strong bases to a weak base (bicarbonate ion), and

2. Strong acids to a weak acid (carbonic acid) 

Blood pH changes are much less than they would be if this buffering system did not exist.