Kidney plays an important role in maintenance of acid-bases balance by excreting hydrogen ions and retaining bicarbonate ions.
Normally, urine is acidic in nature with a pH of 4.5-6. The metabolic activities in the body produce lot of acids (with lot of hydrogen ions) which threaten to push the body towards acidosis. However, kidneys prevent this by excreting hydrogen ions (H+) and conserving bicarbonate ions (HCO3)

Conservation or reabsorption of HCO3 is an important process because large quantity of HCO3 is filtered and all are likely to be exreted through urine. About 4320 mEq of HCO3 IS FILTERED BY THE GLOMERULI EVERYDAY. IT IS CALLED FILTERED LOAD OF hco3. Excreted of this much HCO3 through unine will affect the acid-base balance of body fluids. So HCO3 must be taken back from the renal tubele by reabsorption.

The reabsorption of filtered HCO3 ocure by the secretion of H+ in the renal tubules. About 4380 mEq of H+ appear everyday in the renal tubule by means of filtration and secretion. Not all the H+ are excreted through urine. Out of 4380 mEq, about 4280-4330 mEq of H+ is utilized for the reabsorption of filtered HCO3. Only the remaining 50-100 mEq is excreted. It results in the acidification of urine.
Regulation of kidney function-changes in the arterial blood pressure, causes changes in urinary out put produce small alteration in glomerular filteration rate or renal blood flow. The ability of glomerular filteration rate and renal blood flow to resist changes in arterial blood pressure is knowen as auto regulation .
Autoregulationinvolve the juxtaglomerular apparatus. Some of the smooth muscle cellsof the afferent arteriole secrete renin enzyme. In blood renin act on alpha globulin renin substrate and a decapeptide angiotensin I is produced angiotensin I is acted upon in lungs by the converting enzyme to produceoctapeptide angiotensin II, it is a powerfull vasoconstrictor it causes construction of afferent arteriole constriction and reduces blood flow and decrease GFR.