Heal Yourself At Home
Acid buffering systems in body

Acid / Alkaline Buffering Systems

(3) Renal Mechanisms

 Compensates for acid pH by excretion of H+ by producing acidic urine


      Normal Renal Mechanisms vs. Renal Compensation



-       Normal Renal Mechanism - Distal tubules of the kidneys secrete H+ ions directly into the filtrate so that urine is acidified and the H+ ions are lost from the body.  This is a normal process that occurs at a normal rate. 


-       Renal Compensation -  occurs when other mechanisms for acid-base balance fail and the rate of kidney excretion of H+ ions increases above normal, which are then eliminated from the body.



      The kidneys are the ultimate acid-base regulatory organs - Although the body’s buffer systems can resist pH changes, by tying up excess H+ ions,  the constantly generated H+ must eventually be eliminated from the body.  The lungs can eliminate only carbonic acid (by eliminating carbon dioxide), but only the kidneys can rid the body of metabolic acids (phosphoric, uric, and lactic acids, and ketones) and prevent metabolic acidosis.


(1)    By eliminating H+ ions or retaining bicarbonate - The kidneys control H+ ion concentration in extra-cellular fluids by either –


(a)   H+ ion secretion - Excess H+ ions [secreted by kidney’s nephron tubule cells] combine with urinary buffers to be excreted as acids (i.e. acidifies the urine) and ammonium.  


or   (b)  Bicarbonate (HCO3-) ion reabsorption - Excess, secreted H+ ions and bicarbonate ions [filtered into the glomerular filtrate] combine in the kidney tubules to form carbon dioxide [which is exhaled out] and water [which is urinated out].                           


E.g.        HCl + NaHCO3  ↔ NaCl + H2CO3 ↔ CO2 + H2O + NaCl


Or (2)   By exchanging H+ ions for Na+ or K+ ions - Another compensatory method for acidosis is to exchange H+ ions for sodium or potassium ions in the kidney tubules, producing a sodium/potassium imbalance and possible hyperkalemia;


-        Effective blood buffering by the kidneys produces a urine pH of 4.6 – 8;


      Note: the kidney can correct states of excess but not states of deficiency - Correction of an acidosis due to a loss of or insufficient base (Na+ or K+  +  HCO3- ), e.g. by diarrhea, requires the administration of Na or K salts from which Na+ or K+  +  HCO3- can be formed.



Kidney Stones


      Formed in overly-alkaline conditions: are composed of magnesium and calcium phosphates, carbonates and oxalates, which are insoluble in alkaline fluids. A diet that acidifies the urine is desireable (e.g. cranberry juice) to reduce these type of stones.


      Formed in overly-acid conditions: are composed of uric acid and crystine. A therapeutic diet would alkalize the urine.

Acid/Alkaline Links



Related Links



-    Acidity health problems

-    Acid/Alkaline Food Charts

-    What does pH mean?


About Acid /Alkaline Buffering Systems

-  (1) Chemical buffer systems

-    (2) Respiratory compensation

-    (3) Renal mechanisms