Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 38Renal Function & Micturition 653

LOOP OF HENLE

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 pel-
vis. There is a graded increase in the osmolality of the intersti-
tium of the pyramids in humans: The osmolality at the tips of
the papillae can reach about 1200 mOsm/kg of H 2 O, approxi-
mately 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 tu-
bule, 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 intersti-
tium by Na,^ K ATPase in the basolateral membranes of the
cells, keeping the intracellular Na+ low. The Na–K–2Cl trans-
porter has 12 transmembrane domains with intracellular
amino and carboxyl terminals. It is a member of a family of

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 exten-

sion 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

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 act-

ing on the ducts. This antidiuretic hormone from the posteri-

or pituitary gland increases the permeability of the collecting

ducts to water. The key to the action of vasopressin on the col-

lecting 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

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.)

120

100

80

60

40

20

0

Glucose

Fraction remaining in

tubular fluid

Osmoles

Water

Na+

Creatinine

Inulin

Urea

Proximal

tubule

Loop of

Henle

Distal

tubule

Collecting

tubule

TABLE 38–8 Permeability and transport

in various segments of the nephron.a

Permeability

H 2 O Urea NaCl

Active

Transport

of Na+

Loop of Henle

Thin descending limb 4+ + ± 0

Thin ascending limb 0 + 4+ 0

Thick ascending limb 0 ± ± 4+

Distal convoluted

tubule

± ± ± 3+

Collecting tubule

Cortical portion 3+* 0 ± 2+

Outer medullary

portion

3+* 0 ± 1+

Inner medullary

portion

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.