C h a p t e r 2 4
Structure and Function of the Kidney
605
down its concentration gradient, the energy released is
used to move another substance (for instance, glucose
or an amino acid) against its concentration gradient.
Thus, the secondary active transport of a substance
such as glucose does not require energy directly from
adenosine triphosphatase (ATPase), but depends on
the energy-dependent Na
+
/K
+
-adenosine triphosphatase
(ATPase) pump on the basolateral side of renal tubular
cells. This pump maintains a low intracellular sodium
concentration that facilitates the downhill movement of
sodium across the luminal membrane; it is this downhill
diffusion of sodium to the interior of the cell that pro-
vides the energy for the simultaneous uphill transport of
glucose across the luminal membrane. A few substances,
such as hydrogen (H
+
), are secreted into the tubule.
Proximal Tubule.
Approximately 65% of all reabsorp-
tive and secretory processes that occur in the tubular
system take place in the proximal tubule. There is
almost complete reabsorption of nutritionally impor-
tant substances, such as glucose, amino acids, lactate,
and water-soluble vitamins (Fig. 24-8). Electrolytes,
such as Na
+
, K
+
, Cl
–
, and bicarbonate (HCO
3
–
), are
65% to 80% reabsorbed. As these solutes move into
the tubular cells, their concentration in the tubular
lumen decreases, providing a concentration gradient
for the osmotic reabsorption of water and urea. The
proximal tubule is highly permeable to water, and the
osmotic movement of water occurs so rapidly that the
concentration difference of solutes on either side of the
membrane seldom is more than a few milliosmoles.
Many substances, such as glucose, are freely filtered
in the glomerulus and reabsorbed by energy-dependent
cotransport carrier mechanisms. The maximum amount
of substance that these transport systems can reabsorb
per unit time is called the
transport maximum.
The trans-
port maximum is related to the number of carrier proteins
that are available for transport and usually is sufficient to
ensure that all of a filtered substance such as glucose can
be reabsorbed rather than being eliminated in the urine.
The plasma level at which the substance appears in the
urine is called the
renal threshold
. Under some circum-
stances, the amount of substance filtered in the glomeru-
lus exceeds the transport maximum. For example, when
the blood glucose level is elevated in uncontrolled diabe-
tes mellitus, the amount that is filtered in the glomerulus
often exceeds the transport maximum (approximately
320 mg/minute), and glucose spills into the urine.
Tubular
fluid
Proximal tubular cell
Glucose
Amino acids
Glucose
Amino acids
Na
+
Na
+
Na
+
H
2
O
Interstitial
fluid
H
+
Luminal cell membrane
K
+
ATP
Basolateral
cell membrane
Blood
Peritubular
capillary
Tubular
lumen
K
+
FIGURE 24-7.
Mechanism for secondary active transport or
cotransport of glucose and amino acids in the proximal tubule.
The energy-dependent sodium–potassium pump on the basal
lateral surface of the cell maintains a low intracellular gradient
that facilitates the downhill movement of sodium and glucose
or amino acids (cotransport) from the tubular lumen into the
tubular cell and then into the peritubular capillary.
Proximal tubule
• Reabsorption: Na
+
, Cl
–
,
HCO
3
–
, K
+
, H
2
O, glucose,
amino acids
• Secretion: H
+
, organic
acids and bases
Late distal tubule and
collecting duct
•
Principal cells
Reabsorption: Na
+
Cl
–
Secretion: K
+
;
ADH-mediated H
2
O reabsorption
•
Intercalated cells
Reabsorption: HCO
3
–
, K
+
Secretion: H
+
Thick ascending loop of Henle
• Reabsorption: Na
+
, Cl
–
, K
+
,
Ca
++
, HCO
3
–
, Mg
++
• Secretion: H
+
Thin descending loop of Henle
• Reabsorption: H
2
O
Early distal tubule
• Reabsorption: Na
+
,
Cl
–
, Ca
++
, Mg
++
FIGURE 24-8.
Sites of tubular water (H
2
O), glucose, amino acids, Na
+
(sodium), Cl
–
(chloride),
HCO
3
–
(bicarbonate), K
+
(potassium), Ca
++
(calcium), and Mg
++
(magnesium) reabsorption; and sites of
organic acids and bases, H
+
(hydrogen), and K
+
secretion.
