changes and stimulate the posterior pituitary to secrete
ADH. ADH stimulates water reabsorption in the kidneys,
which produce concentrated urine and help to restore wa-
ter balance (Figure 15.12). The osmoreceptors also stim-
ulate thirst and cause you to drink more water.
Exactly how does ADH work? As discussed with the
countercurrent multiplier earlier in this chapter, the kid-
neys set up an osmotic gradient from the cortex through
the medulla due to increasing levels of NaCl in the intersti-
tial fluid. By the time the filtrate reaches the distal tubules
and collecting ducts, the fluid is hypotonic compared with
the interstitial fluid that surrounds the tubing. When ADH
is present, the apical membranes of the principal cells of
the distal tubule and collecting ducts become permeable
to water. As the hypotonic filtrate descends through the
collecting ducts, water flows out of the collecting ducts in
response to this osmotic gradient. Increased water reab-
sorption produces concentrated urine and reduces water
loss from the body. If no ADH is present, the membranes
are impermeable to water, so no water is reabsorbed as
the filtrate passes through the DCT and collecting duct,
resulting in dilute urine.
Because water follows the osmotic gradients set up by
NaCl movements, its flow can also be influenced by the
regulation of reabsorption of NaCl, which is the responsi-
bility of the renin-angiotensin-aldosterone system.
Water Levels Also Depend
on Changes in NaCl Levels
You just learned that the blood water concentration changes
when you become dehydrated. Blood volume also decreases
with dehydration. These changes in blood volume lead to a
decrease in blood pressure. The juxtaglomerular cells in the
afferent arteriole of the nephron sense this decrease and re-
spond by releasing the enzyme renin. Through a cascade of
events referred to as the renin-angiotensin-aldosterone
system, renin yields angiotensin II (AII), which causes the
adrenal glands to secrete aldosterone. Aldosterone increas-
es sodium reabsorption in the kidneys, which indirectly
increases water reabsorption to restore blood volume and
blood pressure (Figure 15.13). Aldosterone regulates re-
absorption of only about 1-4% of the filtered sodium, which
amounts to about 5.8 g, on a daily basis.
If blood volume and blood pressure were to increase
(due to excessive fluid intake, for example), the kidneys
would need to excrete sodium and water to restore blood
volume. To accomplish this, atrial cells of the heart release
the hormone atrial natriuretic peptide (ANP), which acts
on the kidney (directly and indirectly).
N e g a t i v e f e e d b a c k r e g u l a t i o n o f w a t e r
r e a b s o r p t i o n in t h e k i d n e y •
F i g u r e 1 5 . 1 2
The hypothalamus is responsible for monitoring blood water
concentrations. ADH can help restore water homeostasis when
blood becomes too concentrated.
The level of ADH in the blood would
if the blood water concentration increased.
456 CHAPTER 15
The Urinary System and Fluid, Electrolyte, and Acid-Base Balance