Chapter 25
The Urinary System
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25
duct into the urogenital ridge, inducing the mesoderm there to
form nephrons. Te distal ends of the ureteric buds form the re-
nal pelves, calyces, and collecting ducts, and their unexpanded
proximal parts, now called the
ureteric ducts
, become the ure-
ters (Figure 25.22d).
Because the kidneys develop in the pelvis and then ascend to
their final position, they receive their blood supply from succes-
sively higher sources. Although the lower blood vessels usually
degenerate, they sometimes persist, and for this reason multiple
renal arteries are common. Te metanephric kidneys excrete
urine by the third month of fetal life, and most of the amniotic
fluid that surrounds a developing fetus is fetal urine. Nonethe-
less, the fetal kidneys do not work nearly as hard as they will
a±er birth because exchange through the placenta allows the
mother’s urinary system to clear most of the undesirable sub-
stances from the fetal blood.
As the metanephros is developing, the cloaca subdivides to
form the future rectum and anal canal and the
urogenital sinus
,
into which the urinary and genital ducts empty. Te urinary
bladder and the urethra then develop from the urogenital sinus
(Figure 25.22b–d).
As the kidneys develop in a young embryo, it almost seems as if
they are unable to “make up their mind” how to go about it. Tree
different sets of kidneys develop from the
urogenital ridges
, paired
elevations of the intermediate mesoderm that give rise to both the
urinary organs and reproductive organs
(Figure 25.22)
. Only
the last set persists to become adult kidneys.
During the fourth week of development, the first tubule
system, the
pronephros
(pro-nef
9
ros; “prekidney”), forms and
then quickly degenerates as a second, lower set appears. Al-
though the pronephros never functions and is gone by the sixth
week, the
pronephric duct
that connects it to the cloaca persists
and is used by the later-developing kidneys. (Te cloaca is the
terminal part of the gut that opens to the body exterior.)
As the second renal system, the
mesonephros
(mez
0
o-nef
9
ros; “middle kidney”), claims the pronephric duct, it comes to
be called the
mesonephric duct
(Figure 25.22a, b). Te meso-
nephric kidneys degenerate (with remnants incorporated into
the male reproductive system) once the third set, the
metaneph-
ros
(met
0
ah-nef
9
ros; “a±er kidney”), makes its appearance
(Figure 25.22b, c).
Te metanephros starts to develop at about five weeks as hol-
low
ureteric buds
that push superiorly from the mesonephric
Somatic motor
nerve activity
External urethral
sphincter opens
Sympathetic
activity
Parasympathetic
activity
Urinary bladder
fills, stretching
bladder wall
Spinal
cord
Promotes micturition
by acting on all three
spinal efferents
Inhibits micturition by
acting on all three spinal
efferents
Allow or inhibit micturition
as appropriate
Brain
Simple
spinal
reflex
Spinal
cord
Inhibits
Parasympathetic activity
Sympathetic activity
Somatic motor nerve activity
Pontine micturition
center
Pontine storage
center
Higher brain
centers
Detrusor contracts;
internal urethral
sphincter opens
Afferent impulses
from stretch
receptors
Micturition
Figure 25.21
Control of micturition.
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