Chapter 25
The Urinary System
971
25
Table 25.1
Reabsorption Capabilities of Different Segments of the Renal Tubules and Collecting Ducts
TUBULE SEGMENT
SUBSTANCE REABSORBED
MECHANISM
Proximal Convoluted Tubule (PCT)
 
Sodium ions (Na
1
)
Primary active transport via basolateral Na
1
-K
1
pump; crosses apical
membrane through channels, symporters, or antiporters
 
Virtually all nutrients (glucose, amino
acids, vitamins, some ions)
Secondary active transport with Na
1
 
Cl
2
, K
1
, Mg
2
1
, Ca
2
1
, and other ions
Passive paracellular diffusion driven by electrochemical gradient
 
HCO
3
2
Secondary active transport linked to H
1
secretion and Na
1
reabsorption (see Chapter 26)
 
Water
Osmosis; driven by solute reabsorption (obligatory water
reabsorption)
 
Lipid-soluble solutes
Passive diffusion driven by the concentration gradient created by
reabsorption of water
 
Urea
Primarily passive paracellular diffusion driven by chemical gradient
Nephron Loop
Descending limb
Water
Osmosis
Ascending limb
Na
1
, Cl
2
, K
1
Secondary active transport of Cl
2
, Na
1
, and K
1
via Na
1
-K
1
-2Cl
2
cotransporter in thick portion; paracellular diffusion; Na
1
-H
1
antiport
 
Ca
2
1
, Mg
2
1
Passive paracellular diffusion driven by electrochemical gradient
Distal Convoluted Tubule (DCT)
 
Na
1
, Cl
2
Primary active Na
1
transport at basolateral membrane; secondary
active transport at apical membrane via Na
1
-Cl
2
symporter and
channels; aldosterone-regulated at distal portion
 
Ca
2
1
Passive uptake via PTH-modulated channels in apical membrane;
primary and secondary active transport (antiport with Na
1
) in
basolateral membrane
Collecting Duct
 
Na
1
, K
1
, HCO
3
2
, Cl
2
Primary active transport of Na
1
(requires aldosterone); passive
paracellular diffusion of some Cl
2
; cotransport of Cl
2
and HCO
3
2
; K
1
is both reabsorbed and secreted (aldosterone dependent), usually
resulting in net K
1
secretion
 
Water
Osmosis; controlled (facultative) water reabsorption; ADH required
to insert aquaporins
 
Urea
Facilitated diffusion in response to concentration gradient in the
deep medulla region; recycles and contributes to medullary osmotic
gradient
by the time the filtrate reaches the nephron loop. Nearly all of
the uric acid and about half of the urea are reabsorbed in the
proximal tubule, but both are later secreted back into the filtrate.
Nephron Loop
Beyond the PCT, the permeability of the tu-
bule epithelium changes dramatically. Here, for the first time,
water reabsorption is not coupled to solute reabsorption. Water
can leave the descending limb of the nephron loop but
not
the
ascending limb, where aquaporins are scarce or absent in the
tubule cell membranes. For reasons that we will explain shortly,
these permeability differences play a vital role in the kidneys’
ability to form dilute or concentrated urine.
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