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  Buffers

Buffers:

A buffer might be described as a solution that resists the change in pH that will take place on addition of small quantities of acid or base to the solution. Buffers are mixtures of weak acid and the salt of it or weak base and the salt of it. The pH of the solution is described as the negative logarithm of hydrogen ion concentration. The pH of buffers are ascertained by Henderson Haselbach equation that is derived as follows:

Let us consider a weak acid that ionizes like this:
                    HA  ↔   H    A

Then equilibrium constant K will be

Ka =  [H+] [A-]
             [HA]

Rearranging the equation, we get,

Ka [HA] = [H+] [A-]

[H+] = Ka [HA]
               [A-]

Taking log on both the sides,

log [H+] = log Ka + log [HA]
                                [A-]

Multiplying by -1, we get
-log [H+] = - log Ka – log [HA]
                                 [A-]

pH = pKa + log  [A-]
                    [HA]



The pH of blood is 7.4 and it have to be kept constant. If pH increases over than the 7.5, alkalosis takes place and beyond 7.8 death takes place.

If it falls below, 7.3, acidosis takes place and below 7.0 is incompatible for life. Because of metabolism and dietary intake, large quantities of acids and bases are generated in the body and they have to be transported by blood for elimination. This should take place without any main changes in the pH. This is efficiently done in the body through means of the buffers available in the blood and by two mechanisms that are:

1. Respiratory mechanism and
2. Renal mechanism

The buffer systems of blood are like this:

2459_buffer system of blood.jpg

The denominators are salts and numerators are acid components.

Because the concentrations of phosphate and organic acids are low in plasma, they do not play a main role in regulation of pH.

The main buffer in plasma is bicarbonate buffer and the pKa of carbonic acid is 6.1. Substituting it in the Henderson Hasselbach equation,

7.4 = 6.1 + log (BHCO3/H2CO3)
                          
7.4 - 6.1 + log (BHCO3/H2CO3)
                          
1.3 = log (BHCO3/H2CO3)
                  
Because antilog of 1.3 is 20,

BHCO3/H2CO3  = 20/1
 
To efficiently preserve the pH of blood, as per to Henderson Hasselbach equation, the ratio of bicarbonate to carbonic acid have to be 20: 1. The carbon dioxide generated through metabolism is buffered through the hemoglobin buffer system like this.

 

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