Renal Control of
Acid-Base Balance
Most effective regulator of pH
• The kidneys control acid-base balance by
Excreting an acidic urine.
Reduces the amount of
acid in extracellular
fluid.
Excreting a basic urine
Removes base from the
extracellular fluid.
Reabsorption of bicarbonate
4320 mEq/day HCO3-
4320 milliequivalents of H+ must be
secreted each day just to reabsorb the
filtered bicarbonate.
80 milliequivalents of H+ must be
secreted to rid the body of the
nonvolatile acids produced each day
Na–H exchange
(secondary active transport)
FILTERED
HCO3-
T
U
B
U
NEW
L
E
• In ALKALOSIS
• In ACIDOSIS
• The kidneys regulate extracellular fluid H+
concentration through three fundamental
mechanisms
1. Secretion of H+
2. Reabsorption of filtered HCO3
3. Production of new HCO3
Secretion of Hydrogen Ions
In PCT
Late DCT and
collecting tubules.
Intercalated cells
Only 5 percent
But maximally acidic urine pH 4.5
Fate of H+ in the Urine : Limiting pH
• The maximal H+ gradient against which the
transport mechanisms can secrete in humans
corresponds to a urine pH of about 4.5
• Nephron cannot produce a urine pH < 4.5.
pH 4.5 is the Limiting pH.
Limiting pH
• At urinary pH 4.5 can excrete 0.03 mEq/L of
H+ ions.
• To excrete the 80 milliequivalents of
nonvolatile acid formed by metabolism each
day, about 2667 liters of urine
would have to be excreted if the H+ remained
free in solution.
Fate of H+ in the Urine
This pH of 4.5 would be reached
rapidly, and H+ secretion would stop.
Three important reactions in the
tubular fluid remove free H+
Buffer Systems
1. Reactions with HCO3– to form CO2 and H2O,
2. Reactions with HPO42– to form H2PO4–,
3. Reactions with NH3 to form NH4+.
Reactions with HCO3– to form CO2 and H2O
HCO3_
No net gain
Phosphate Buffer System
• HPO4 2- and H2PO4
• Effective buffer in the tubular fluid.
Concentrated in the tubular fluid because
of their relatively poor reabsorption
pK of this system is about 6.8 (close to
urinary pH)
• Comes into action after bicarbonates get
depleted.
• 30 to 40 mEq/day is available for buffering H+
Phosphate Buffer System
Net gain
Of HCO3–
Excreted as a
sodium salt
(NaH2PO4)
Ammonia Buffer System
Renal ammonium production
• About 60% of the ammonium in urine is
obtained by the deamidation and deamination
of glutamine.
• The rest is obtained from alanine, glycine and
from the arterial blood perfusing the kidney.
In PCT
In Collecting tubules
An increase in extracellular
fluid H+ concentration
stimulates renal glutamine
metabolism
Cannot cross tubular epithelium
Gets stuck in the lumen ones formed
Normal conditions : 50% H+ ion Excretion
Chronic acidosis: upto 500 mEq/day excretion
•Lipid soluble
•Can move across
membranes
•Rapidly combines
with H+ at lower
pH
Titratable acid
• It is a measure of non-bicarbonate, non-NH4+
buffer excreted in the urine.
• It includes H+ added to the tubular fluid that
combined with phosphate and other organic
buffers.
Generation of New Bicarbonate
• Phosphate Buffer System
• Ammonia Buffer System
Factors effecting H+ Secretion and HCO3Reabsorption by the Renal Tubules