Page 187 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

IV
The Kidney and Acid–Base Balance CO 2 and H 2O (! B). CA anchored in the mem-
brane catalyzes this reaction. CO 2 readily dif-
+
Main functions of renal H secretion (! A): fuses into the cell, perhaps via aquaporin 1
II
— reabsorption of filtered bicarbonate (! B), (! p. 166). CA then catalyzes the transforma-
+
–
+
— excretion of H ions measurable as titratable tion of CO 2 + H 2O to H + HCO 3 within the cell
+
acidity (! C), and (! B). The H ions are again secreted, while
–
+
— nonionic transport of NH 4 , i.e. in the form HCO 3 exits through the basolateral membrane
of NH 3 (! D1, 2). of the cell via an electrogenic carrier (hNBC =
+
+
1. Very large quantities of H ions are secreted human Na -bicarbonate co-transporter; ! B). – –
Kidneys, Salt, and Water Balance electroneutral Na /H -antiporter (NHE3 car- – – in the form of CO 2 (driving force: ∆P CO 2 ), and +
into the lumen of the proximal tubule (! A1)
+
The hNBC co-transports 1 Na with 3 HCO 3
2 –
–
by (a) primary active transport via H -ATPase
+
(and/or with 1 HCO 3 + 1 CO 3 ?) Thus, HCO 3
is transported through the luminal membrane
and (b) by secondary active transport via an
+
+
exits the cell across the basolateral membrane
rier, ! p. 162). The luminal pH then decreases
–
–
from 7.4 (filtrate) to about 6.6. One OH ion re-
(main driving force: membrane
as HCO 3
+
mains in the cell for each H ion secreted; OH
potential).
–
reacts with CO 2 to form HCO 3 (accelerated by
Hypokalemia leads to a rise in membrane potential
carbonic anhydrase-II, see below). HCO 3
(Nernst equation, ! p. 32) and thus to a rise in baso-
leaves the cell for the blood, where it binds one
–
+
+
secretion and, ultimately, in hypokalemic alkalosis.
lumen (and excreted) results in the elimina-
Urinary acid excretion. If the dietary protein
+
tion of one H ion from the body, except the
intake is 70 g per day (! p. 226), a daily load of
7 H ion. Thus, each H ion secreted into the lateral HCO 3 transport. This results in increased H
+
secreted H is accompanied by a secreted NH 3 about 190 mmol of H occurs after the amino
+
(see below).
2. In the connecting tubule and collecting acids of the protein have been metabolized.
duct (! A2) type A intercalated cells secrete H + HCl (from arginine, lysine and histidine),
+
ions via H /K -ATPase and H -ATPase, allowing H 2SO 4 (from methionine and cystine), H 3PO 4,
+
+
+
the luminal pH to drop as far as 4.5. In meta- and lactic acid are the main sources of H ions.
bolic alkalosis, type B intercalated cells can They are “fixed” acids which, unlike CO 2, are
–
secrete HCO 3 (! A3). not eliminated by respiration. Since about
+
Carbonic anhydrase (CA) is important in all 130 mmol H /day are used to break down or- –
–
–
cases where H ions exit from one side of a cell ganic anions (glutamate , aspartate , lactate ,
+
+
–
and/or HCO 3 exits from the other, e.g., in renal etc.), the net H production is about 60 (40–80)
+
mmol/day. Although the H ions are buffered
tubule cells, which contain CA in the cytosol
II
IV
and CA on the outside of the luminal mem- at their production site, they must be excreted
brane; ! A, B, D), as well as in the stomach, to regenerate the buffers.
In extreme cases, the urinary pH can rise to
small intestine, pancreatic duct and erythro- about pH 8 (high HCO 3 excretion) or fall to
–
cytes, etc. CA catalyzes the gross reaction about pH 4.5 (maximum H + conc. is
+
–
H 2O + CO 2 H + HCO 3 .
0.03 mmol/L). At a daily urine output of 1.5 L,
Carbonic acid (H 2CO 3) is often considered to be the the kidneys will excrete ! 1% of the produced
–
+
intermediate product of this reaction, but OH (not H ions in their free form.
H 2O) probably combines with CA. Therefore, the re- Titratable acids (80% phosphate, 20% uric
–
+
–
actions H 2O OH + H and OH + CO 2 HCO 3 – acid, citric acid, etc.) comprise a significant
underlie the aforementioned gross reaction.
fraction (10–30 mmol/day) of H + excretion
+
Reabsorption of HCO 3 (! B). The amount of (! C1). This amount of H ions can be deter-
–
HCO 3 filtered each day is 40 times the quan- mined by titrating the urine with NaOH back to
–
tity present in the blood. HCO 3 must therefore the plasma pH value, which is normally pH 7.4
–
be reabsorbed to maintain acid–base balance (! C2). Around 80% of phosphate (pK a = 6.8) in
+
2 –
(! p. 183ff.). The H ions secreted into the the blood occurs in the form of HPO 4 ,
174 lumen of the proximal convoluted tubule react whereas about all phosphate in acidic urine
–
–
with about 90% of the filtered HCO 3 to form occurs as H 2PO 4 (! p. 380), i.e., the secreted
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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