Chemical acid base buffer systems
When there is a change in the H+ ion concentration, the body fluids react immediately to minimize the change. Chemical buffers act by converting either strong acids or bases into weaker acids or bases. The various chemical buffers are-
Bicarbonate
This is the most important buffer system in the body. Bicarbonate combines with H+ ions to form carbonic acid in the tubular fluid, which then dissociates to CO2 and H20. The CO2 formed is removed by the lungs and the HCO3– formed in the cells is reabsorbed from the filtrate to the blood.
Phosphate buffer
It plays a major role in buffering renal tubular fluid and ICF. The two main elements of the phosphate buffer are HPO42- (base) and H2PO4– (weak acid). Hydrogen ions from strong acids are captured by converting a weak base to a weak acid and strong base captured by conversion of a weak acid to a weak base.
Protein buffer
It is an intracellular buffer present in high concentrations in the blood. Hemoglobin molecule forms the second most important blood buffer and is present in the form of proteinate ions (Hb–). These basic ions, with their weak acids (HHb) form a buffer pair. When an acid is added to the blood,
H+ + Hb– → HHb
This reaction shifts to the right and the ratio of base to acid is decreased. Hemoglobin is a powerful buffer and it binds with protein, CO2 and H+ ions. In the tissues CO2 passes into the RBC where it combines with water to form carbonic acid which is catalyzed by the enzyme carbonic anhydrase. Carbonic acid then dissociates into HCO3– and H+ ions. The H+ ions binds to hemoglobin to form HHb and the HCO3– ions passes back to the plasma in exchange for chloride ions. In the lungs this process is reversed where in H+ ions bind to hemoglobin and recombine with bicarbonate to form CO2 which passes into the alveoli.
Ammonia buffer system
Ammonia is formed by the hydrolysis of glutamine in the presence of enzyme glutaminase in the tubular cells which freely diffuses into luminal fluid and continues with H+ ions to form NH4+ ions. This NH4+ ions combines with chloride ions and is excreted as ammonium chloride in the urine.