HUMAN BIOLOGY

the UrInary SyStem 231

hoW do the kidneys help manage the body’s

fluid balance?

• To maintain the volume of extracellular fluid, hormones adjust
  the amount of water urine contains.


• ADH stimulates the kidneys to conserve water. It acts on distal
  tubules and collecting ducts.


• Aldosterone promotes the reabsorption of sodium, which
  indirectly increases the amount of water the body retains.

taKe-Home message

Figure 12.9 a negative feedback loop from the kidneys

to the brain helps adjust the fluid volume of the blood.

A Stimulus

C Kidneys reabsorb

more water, so less

water leaves in urine.

D The blood

volume rises.

E Response

Water loss reduces

blood volume. Sensors

in the brain trigger

release of ADH.

B ADH makes distal

tubules, collecting

ducts more permeable

to water.

Receptors in brain

detect the increase

in blood volume.

Signals for ADH

secretion stop.

pressure. Then a negative feedback loop inhibits the
release of ADH (Figure 12.9).
Reduced blood volume also affects cells in the afferent
arterioles that bring blood to nephrons. These cells release
the enzyme renin. They are part of the juxtaglomerular
apparatus (Figure 12.10A). Juxta- means “next to,” and this
“apparatus” is an area where arterioles of the glomerulus
come into contact with a nephron’s distal tubule.
Renin triggers reactions that produce a protein called
angiotensin I and then convert it to angiotensin II. Among
other effects, angiotensin II stimulates cells of the adrenal
cortex, the outer portion of a gland perched on top of each
kidney, to secrete the hormone aldosterone (Figures 12.3
and 12.10B). Aldosterone causes cells of the distal tubules
and collecting ducts to reabsorb sodium faster, so less
of it and less water are excreted. By limiting the loss of
water, this process also influences blood pressure.
What must the kidneys do to make dilute urine? Not
much. Urine is automatically dilute as long as ADH levels
are low, so little of the hormone acts on the distal tubules
and collecting ducts.
A diuretic is a substance that promotes the loss of water
in urine. For example, caffeine reduces the reabsorption of
sodium along nephron tubules, so more water is excreted.

a thirst center monitors sodium

What makes you thirsty when you don’t drink enough?
The concentration of salt in your blood has risen, and this
change reduces the amount of saliva your salivary glands
produce. A drier mouth stimulates nerve endings that
signal a thirst center in the brain. The center also receives
signals from the same sensors that stimulate the release of
ADH. In this case the signals are relayed to a part of the
brain that “tells” you to find and drink fluid.

Figure 12.10 The juxtaglomerular apparatus (JGa) and renin-secreting cells assist with the reabsorption of sodium.

(efferent arteriole

leaving the

glomerulus)

(afferent arteriole

leading into

glomerulus)

Receptors in kidneys,

elsewhere detect falling

sodium level

Angiotensinogen

converts to angiotensin I

Angiotensin II

renin-secreting cells

in juxtaglomerular

apparatus

distal

tubule

Bowman’s

capsule

proximal

tubule

Aldosterone secreted

(from adrenal glands)

Aldosterone acts on

distal tubules

Sodium (and water)

reabsorbed

A

B

Renin released from

cells in the JGA

© Cengage Learning

© Cengage Learning

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).