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.
By eliminating H+ ions
or retaining bicarbonate - The kidneys
control H+ ion concentration in extra-cellular fluids by
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)
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].
HCl + NaHCO3 ↔ NaCl + H2CO3 ↔ CO2 + H2O + NaCl
exchanging H+ ions for Na+ or K+ ions -
compensatory method for acidosis is to exchange H+ ions for sodium or
potassium ions in the kidney tubules, producing a sodium/potassium imbalance and
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.
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
are composed of uric acid and crystine. A therapeutic diet would
alkalize the urine.