Important role of distal tubule of the kidney in acid-base balance -

Ans. is 'a' i.e., Secretion of Ammonia o The H+ ions secreted into the tubule are mopped up by urinary buffers present in the tubular fluid. The three important buffers are : - Bicarbonate buffer: - Present in proximal segments (Proximal tubules. Thick ascending limb). It is nothing else but filtered HCO3- ions which react with H+ to form H2CO3- This is the most important buffer in proximal tubules. Phosphate buffer : - Especially important in distal tubules. Ammonia buffers : - Bind to H+ and produce ammonium ion (H+ + NH3 = NH4). The reaction with NH3 occurs in the proximal and distal tubules but the main buffering mechanism is active in distal tubule and this is the most important buffer in distal tubule. o Thus, these buffers neutralize H+ in the tubules and in exchange HCO3- is reabsorbed. In case of bicarbonate buffer all filtered bicarbonates are reabsorbed. In case of ammonia and phosphate buffers, new HCO3 is generated due to secretion of H+ So, ammonia and phosphate buffers are excreted in exchange of HCO3- absorption.