Page 337 - Color Atlas of Biochemistry

328 Tissues and organs

Electrolyte and water recycling with glucose and amino acids (see p. 322).
These two pathways are responsible for
+
60–70% of total Na resorption. In the ascend-
A. Electrolyte and water recycling
ingpartofHenle’s loop,there is another
Electrolytes and other plasma components transporter (shown at the bottom right),
with low molecular weights enter the primary which functions electroneutrally and takes
+
+
urine by ultrafiltration (right). Most of these up one Na ion and one K ion together with
–
substances are recovered by energy-depen- two Cl ions. This symport is also dependent
+
+
dent resorption (see p. 322). The extent of on the activity of Na /K ATPase [2], which
+
the resorption determines the amount that pumps the Na resorbed from the primary
+
ultimately reaches the final urine and is ex- urine back into the plasma in exchange for K .
creted. The illustration does not take into ac- The steroid hormone aldosterone (see
+
count the zoning of transport processes in the p. 55) increases Na reuptake, particularly in
kidney (physiology textbooks may be referred the distal tubule, while atrial natriuretic
to for further details). peptide (ANP) originating from the cardiac
Calcium and phosphate ions. Calcium atrium reduces it. Among other effects, aldo-
2+
+
+
(Ca ) and phosphate ions are almost com- sterone induces Na /K ATPase and various
+
pletely resorbed from the primary urine by Na transporters on the luminal side of the
active transport (i. e., inanATP-dependent cells.
fashion). The proportion of Ca 2+ resorbed is Water. Water resorption in the proximal
over 99%, while for phosphate the figure is tubule is a passive process in which water
80–90%. The extent to which these two elec- follows the osmotically active particles, par-
+
trolytes are resorbed is regulated by the three ticularly the Na ions. Fine regulation of water
hormones parathyrin, calcitonin, and calci- excretion (diuresis) takes place in the collect-
triol. ing ducts, where the peptide hormone vaso-
The peptide hormone parathyrin (PTH), pressin (antidiuretic hormone, ADH) operates.
which is produced by the parathyroid gland, This promotes recovery of water by stimulat-
stimulates Ca 2+ resorption in the kidneys and ing the transfer of aquaporins (see p. 220)
at thesametime inhibits the resorption of into the plasma membrane of the tubule cells
phosphate. In conjunction with the effects of via V 2 receptors. A lack of ADH leads to the
this hormone in the bones and intestines (see disease picture of diabetes insipidus, in which
p. 344), this leads to an increase in the plasma up to 30 L of final urine is produced per day.
level of Ca 2+ and a reductioninthe levelof
phosphate ions.
Calcitonin, a peptide produced in the C cells B. Gluconeogenesis
of the thyroid gland, inhibits the resorption of Apart from the liver, the kidneys are the only
both calcium and phosphate ions. The result is organs capable of producing glucose by
an overall reductioninthe plasma level of both neosynthesis (gluconeogenesis; see p.154).
ions. Calcitonin is thus a parathyrin antago- The main substrate for gluconeogenesis in
2+
nist relative to Ca . the cells of the proximal tubule is glutamine.
The steroid hormone calcitriol, which is In addition, other amino acids and also lac-
formed in the kidneys (see p. 304), stimulates tate, glycerol,and fructose can be used as
the resorption of both calcium and phosphate precursors. As in the liver, the key enzymes
ions and thus increases the plasma level of for gluconeogenesis are induced by cortisol
both ions. (see p. 374). Since the kidneys also have a
Sodium ions. Controlled resorption of Na + high level of glucose consumption, they only
from the primary urine is one of the most release very little glucose into the blood.
+
important functions of the kidney. Na resorp-
tion is highly effective, with more than 97%
being resorbed. Several mechanisms are in-
+
volved: some of the Na is taken up passively
in theproximal tubulethrough thejunctions
between the cells (paracellularly). In addition,
there is secondary active transport together

Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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