Human Physiology, 14th edition (2016)

596 Chapter 17

the number of aquaporins (water channels) in the plasma mem-

branes of the collecting duct epithelial cells.

The posterior pituitary secretes arginine vasopressin, the

molecule that functions as the antidiuretic hormone ( ADH ),

in response to as little as a 1% increase in plasma osmolality.

When ADH binds to its membrane receptors in the cells of

the collecting duct, it stimulates the production of cAMP as

a second messenger (chapter 11, section 11.2). This initiates

a chain of events that causes vesicles with aquaporins in their

it is transported by capillaries to the general circulation. In this
way, most of the water remaining in the filtrate is returned to the
vascular system ( fig. 17.18 ).
Note that it is the osmotic gradient created by the counter-
current multiplier system that provides the force for water reab-
sorption through the collecting ducts. Although this osmotic
gradient is normally constant, the rate of osmosis across the
walls of the collecting ducts can be varied by adjustments in their
permeability to water. These adjustments are made by regulating

Proximal

tubule

Distal

tubule

100

100

200

400 400 400

400

400

600

800

1,000 1,000 1,000

1,200 H 2 O

H 2 O

H 2 O

H 2 O

H 2 O

Collecting

duct

Cortex

Outer medulla

Inner medulla

1,200 1,200

800

800 800 800

600

600 600

300

300

300

320

Vasa recta

Descending

limb of loop

Ascending

limb of loop

Figure 17.18 The osmolality of different regions of the kidney. The countercurrent multiplier system in the nephron loop
and countercurrent exchange in the vasa recta help to create a hypertonic renal medulla. Under the influence of antidiuretic hormone
(ADH), the collecting duct becomes more permeable to water, and thus more water is drawn out by osmosis into the hypertonic renal
medulla and peritubular capillaries. (Numbers indicate osmolality.)