BUFFER SYSTEMS AND THEIR ROLES IN REGULATING THE pH OF BODY FLUIDS

A buffer is a mixture of a weak acid and salt of its conjugate base that resists the change in pH upon the addition of acid or base. If acid and base components of buffer are equal, the pH is equal of pK. Generally buffer works best at pH ±1 unit of its pK. Buffer work best when the ratio of acid : base is within the range of 10 : 1 to 1 : 10. Buffers are also effective at higher concentration.

Bicarbonate/carbonic acid buffer system:

This is the most important and predominant buffer system in plasma. It has pK of 6.1. The ratio of base to acid is 20:1. The effectiveness of this system is due to its high concentration (>20 mmol/L) and lung can easily dispose off and retain CO₂. In addition renal tubules can increase or decrease the rate of reclamation of bicarbonate from the glomerular filtrate. Other non bicarbonate buffer system are present at <10 mmol/L concentration. The buffer value β is the amount of base required to cause a change in pH of one unit. The buffer value of this system is 55.6 mmol/L.

Phosphate buffer system:
At physiological pH the ratio of cHPO₄^2-/cH₂PO₄^- is 4/1 (pK of 6.8). This is also found in plasma and erythrocyte but accounts only 5% of non bicarbonate buffer system in plasma. 2,3-DPG in RBC at about 4.5 mmol/l accounts for 16% non bicarbonate buffer system in erythrocyte.

 

Plasma protein buffer system:

Protein especially albumin accounts for greater proportion (95%) of non bicarbonate buffer in plasma. The most important buffer groups of proteins are imidazole groups of histidine (pK about 7.3) and each albumin contains 16 histidines.

 

The reaction to the right is the main mechanism of non bicarbonate buffer system and the component Pr^‑/HPr represents all non bicarbonate buffer system. The main purpose of non bicarbonate buffer system is to maintain constant H⁺.

Hemoglobin buffer system:

The buffer value of non bicarbonate buffers of erythrocyte fluid is about 63 mmol/L. Hemoglobin accounts for major part (53 mmol/L), with remainder being mainly caused by 2,3-DPG. The imidazole group of hemoglobin is the most important buffering groups. When oxygenated, H⁺ ions are liberated from Hb a phenomenon called the Haldane effect. Similarly release of oxygen by binding to hydrogen is called Bohr Effect.