Ganong's Review of Medical Physiology, 23rd Edition

120
100
80
60
40
20
0
Glucose
Proximal
tubule
FIGURE 38–14 Changes in the percentage of the filtered
amount of substances remaining in the tubular fluid along the
length of the nephron in the presence of vasopressin. (Modified
from Sullivan LP, Grantham JJ: Physiology of the Kidney, 2nd ed. Lea & Febiger, 1982.)
LOOP OF HENLE
Fraction remaining in
tubular fluid
Osmoles
Na +
Loop of
Henle
Water
Distal
tubule
Creatinine
Urea
Inulin
Collecting
tubule
As noted above, the loops of Henle of the juxtamedullary
nephrons dip deeply into the medullary pyramids before
draining into the distal convoluted tubules in the cortex, and
all the collecting ducts descend back through the medullary
pyramids to drain at the tips of the pyramids into the renal pelvis.
There is a graded increase in the osmolality of the interstitium
of the pyramids in humans: The osmolality at the tips of
the papillae can reach about 1200 mOsm/kg of H 2 O, approximately
four times that of plasma. The descending limb of the
loop of Henle is permeable to water, due to the presence of
aquaporin-1 in both the apical and basolateral membrane, but
the ascending limb is impermeable to water (Table 38–8). Na + ,
K + , and Cl – are cotransported out of the thick segment of the
ascending limb. Therefore, the fluid in the descending limb of
the loop of Henle becomes hypertonic as water moves out of
the tubule into the hypertonic interstitium. In the ascending
limb it becomes more dilute because of the movement of Na +
and Cl – out of the tubular lumen, and when fluid reaches the
top of the ascending limb (called the diluting segment) it is
now hypotonic to plasma. In passing through the descending
loop of Henle, another 15% of the filtered water is removed, so
approximately 20% of the filtered water enters the distal tubule,
and the TF/P of inulin at this point is about 5.
In the thick ascending limb, a carrier cotransports one Na + ,
one K + , and 2Cl – from the tubular lumen into the tubular
cells. This is another example of secondary active transport;
the Na + is actively transported from the cells into the interstitium
by Na, K ATPase in the basolateral membranes of the
cells, keeping the intracellular Na + low. The Na–K–2Cl transporter
has 12 transmembrane domains with intracellular
amino and carboxyl terminals. It is a member of a family of
CHAPTER 38 Renal Function & Micturition 653
TABLE 38–8 Permeability and transport
in various segments of the nephron. a
Loop of Henle
transporters found in many other locations, including salivary
glands, the gastrointestinal tract, and the airways.
The K + diffuses back into the tubular lumen and back into the
interstitium via ROMK and other K + channels. The Cl – moves
into the interstitium via ClC-Kb channels (Figure 38–15).
DISTAL TUBULE
The distal tubule, particularly its first part, is in effect an extension
of the thick segment of the ascending limb. It is relatively
impermeable to water, and continued removal of the solute in
excess of solvent further dilutes the tubular fluid.
COLLECTING DUCTS
Permeability
H 2 O Urea NaCl
Active
Transport
of Na +
Thin descending limb 4+ + ± 0
Thin ascending limb 0 + 4+ 0
Thick ascending limb 0 ± ± 4+
Distal convoluted
tubule
Collecting tubule
± ± ± 3+
Cortical portion 3+* 0 ± 2+
Outer medullary
portion
Inner medullary
portion
3+* 0 ± 1+
3+* 3+ ± 1+
aData are based on studies of rabbit and human kidneys. Values indicated by
asterisks are in the presence of vasopressin. These values are 1+ in the absence of
vasopressin.
Modified and reproduced with permission from Kokko JP: Renal concentrating and
diluting mechanisms. Hosp Pract [Feb] 1979;110:14.
The collecting ducts have two portions: a cortical portion and
a medullary portion. The changes in osmolality and volume in
the collecting ducts depend on the amount of vasopressin acting
on the ducts. This antidiuretic hormone from the posterior
pituitary gland increases the permeability of the collecting
ducts to water. The key to the action of vasopressin on the collecting
ducts is aquaporin-2. Unlike the other aquaporins, this
aquaporin is stored in vesicles in the cytoplasm of principal
cells. Vasopressin causes rapid insertion of these vesicles into
the apical membrane of cells. The effect is mediated via the