The Urinary System
Chapter 23
Introduction
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The kidneys are perfect examples of
homeostatic organs
Maintain constancy of fluids in our
internal environment
Filter 200 liters of fluid a day
Remove toxins, metabolic wastes, and
excess ions to leave the body in urine
Return needed substances to the blood
A primary organ of excretion
Kidney Functions
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Kidneys regulate volume and chemical
makeup of the blood
Maintain the proper balance between water
and salts as well as between acids and bases
Gluconeogenesis - supply glucose during
fasting
Produce enzyme renin which helps regulate
blood pressure and kidney function
Produce hormone erythropoietin which
stimulates red blood cell production
Urinary System Organs
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Structures of the
urinary system
include;
–
–
–
–
Kidneys
Urinary bladder
Ureters
Urethra
Kidney Location
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The kidneys extend
approximately from
the level of the 12th
thoracic vertebra to
the third lumbar
vertebra
They receive some
protection from ribs
Kidney Location
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The right lies
somewhat lower
than left as it is
positioned under
liver
The lateral surface
of each kidney is
convex, while the
medial is concave
External Anatomy
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The adult kidney
weights about
150 g (5 oz.)
Dimensions are
12 cm long, 6 cm
wide, 3 cm thick
External Anatomy
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Medial surface
has a vertical
cleft called the
renal hilus that
leads into the
space within the
kidney called the
renal sinus
Atop each kidney
is an adrenal
gland which is
unrelated to
kidney function
External Anatomy
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Structures such
as the ureters,
the renal blood
vessels,
lymphatics, and
nerves enter the
kidney at the
hilus
These structures
occupy the renal
sinus
Position of the Kidneys
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The kidneys are retroperitoneal, or behind the
peritoneum
Position of the Kidneys
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Kidneys supported by three layers of supportive tissue
The renal capsule adheres directly to the kidney
surface and isolates it from surrounding region
Position of the Kidneys
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The adipose capsule attaches the kidney to the
posterior body wall and cushions it against trauma
Position of the Kidneys
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The renal fascia is dense fibrous connective tissue
which surrounds the kidney and anchors these organs
to the surrounding structures
Internal Anatomy
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The kidney has
three distinct
regions
– Cortex
– Medulla
– Pelvis
Internal Anatomy
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The most
superficial
region
The renal cortex
is light in color
and has a
granular
appearance
Internal Anatomy
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Deep to the
cortex is the
renal medulla
Darker tissue
which exhibits
cone shaped
tissue masses
called medullary
or renal
pyramids
Medullary
pyramids
Internal Anatomy
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Each renal
pyramid has a
base which is
convex, and a
apex which
tapers toward its
apex or papilla
Medullary
base
Medullary
apex
Internal Anatomy
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The apex, or
papilla, points
internally
The pyramids
appear striped
because they are
formed almost
entirely of
roughly parallel
bundles of urine
collecting
tubules
Pyramidal
stripes
Internal Anatomy
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Inward
extensions of
cortical tissue
called renal
columns
separate the
pyramids
Each medullary
pyramid is
surrounded by a
capsule of
cortical tissue to
form a lobe
Internal Anatomy
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Within the
renal sinus is
the renal pelvis
This flat, funnel
shaped tube is
continuous with
the ureter
leaving the
hilus
Internal Anatomy
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Branching
extensions of the
renal pelvis form
2-3 major calyces,
each of which
sub-divides to
form several
minor calyces
These cup shaped
areas collect the
urine which drain
continuously
from the papillae
Internal Anatomy
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Urine flows
through the
renal pelvis into
the ureter, which
transports it to
the bladder
The walls of the
calyces, pelvis,
and ureter
contain smooth
muscle which
contract to move
urine
Blood Supply
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The kidney continuously cleanse the blood
and adjust its composition
Kidneys possess an extensive blood supply
Under normal resting conditions, the renal
arteries deliver approximately one-fourth of
the total systemic cardiac output (1200 ml)
to the kidneys each minute
Blood Supply
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The renal arteries
issue at right
angles from the
abdominal aorta
Each renal artery
divides into five
segmental arteries
that enter the hilus
Each segmental
artery divides into
lobar and
interlobar arteries
Anatomy of the Kidneys
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The main
structural and
functional unit of
the kidneys is the
uriniferous tubule
The unit consists
of a nephron and
its collecting duct
or tubule
Anatomy of the Kidneys
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Uriniferous
tubules are
separated from
one another by
small amounts of
loose areolar
connective called
interstitial
connective tissue
Anatomy of the Kidneys
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The urine forming
nephron is composed
of
– Renal corpuscle
– Proximal convoluted
tubule
– Loop of Henle
– Distal convoluted
tubule
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A collecting duct
(collecting tubule)
– Concentrating urine
Anatomy of the Kidneys
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Throughout its length
the uriniferous tubule
is lined by a simple
squamous epithelium
that is adapted for
various aspects of the
production of urine
Mechanisms of Urine Production
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The uriniferous
tubule produces
urine through three
interacting
mechanisms
– Filtration
– Reabsorption
– Secretion
Mechanisms of Urine Production
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In filtration a filtrate
of the blood leaves
the kidney capillaries
and enters the
nephron
This filtrate
resembles tissue fluid
in that it contains all
the small molecules
of blood plasma
Mechanisms of Urine Production
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As filtrate proceeds
through the
uriniferous tubule,
the filtrate is
processed into urine
by the mechanisms of
reabsorption and
secretion
Mechanisms of Urine Production
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During reabsorption,
most of the nutrients,
water, and essential
ions are reclaimed
from the filtrate, and
returned to the blood
of capillaries in the
surrounding
connective tissue
99% of the volume of
renal filtrate is
reabsorbed
Mechanisms of Urine Production
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As the essential
molecules are
reclaimed from the
filtrate, the
remaining wastes
and needed
substances
contribute to the
urine that ultimately
leaves the body
A passive process
Mechanisms of Urine Production
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Secretion is an active
process which moves
undesirable
molecules into the
tubule from the
blood of surrounding
capillaries
Nephrons
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Each kidney contains over 1 million tiny
blood processing units called nephrons,
which carry out the processes that form
urine
In addition, there are thousands of
collecting ducts, each of which collects
urine from several nephrons and conveys
it to the renal pelvis
Nephron - Renal Corpuscle
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The first part of the nephron, where the
filtration occur, is the spherical renal
corpuscle
Nephron - Renal Corpuscle
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Renal corpuscles
consist of a tuft of
capillaries called a
glomerulus
surrounded by a cup
shaped, hollow
glomerular capsule
(Bowman’s capsule)
Nephron - Renal Corpuscle
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The glomerulus lies in
the glomerular
capsule like an under
inflated ballon
This tuft of capillaries
is supplied by an
afferent arteriole and
drained by an efferent
arteriole
Nephron - Renal Corpuscle
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The endothelium of the capillaries in the
glomerulus is fenestrated (pores) and
thus these capillaries are highly porous,
allowing large quantities of fluid and
small molecules to pass from the capillary
blood
Nephron - Renal Corpuscle
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The fluid passes from
the capillary into the
hollow interior of the
glomerular capsule, the
capsular space
This fluid is the filtrate
that is ultimately
processed into urine
20% enters the space
while 80% remains in
the blood within the
capillary
Nephron - Renal Corpuscle
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The external parietal
layer of the glomerular
capsule, which is simple
squamous epithelium,
simply contributes to
the structure of the
capsule
It plays no part in the
formation of filtrate
Nephron - Renal Corpuscle
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The capsule’s visceral
layer clings to the
glomerulus and consists
of unusual, branching
epithelial cells called
podocytes
Nephron - Renal Corpuscle
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The podocytes have
many branches which
end in foot processes or
pedicels
The processes inter
digitate with one another
as they surround the
glomerular capillaries
The filtrate passes
through the thin
filtration slits into the
capsular space
Nephron - Renal Corpuscle
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The external parietal
layer of the glomerular
capsule, which is simple
squamous epithelium,
simply contributes to
the structure of the
capsule
It plays no part in the
formation of filtrate
Nephron - Renal Corpuscle
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The external parietal
layer of the glomerular
capsule, which is simple
squamous epithelium,
simply contributes to
the structure of the
capsule
It plays no part in the
formation of filtrate
Nephron
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Each nephron
consists of a
glomerulus, a tuft of
capillaries associated
with a renal tubule
The end of the renal
tubule is a blind,
enlarged, and cupshaped and
completely surround
the glomerulus
Glomerulus
Nephron
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The renal corpuscle refers to the enclosed glomerulus
and the capsule of the glomerulus referred to as
Bowman’s capsule
Nephron
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The glomerulus endothelium is fenestrated,
(penetrated by many pores), which make
these capillaries exceptionally porous
The capillaries allow large amounts of
solute-rich, virtually protein free fluid to
pass from the blood into the glomerulus
capsule
This plasma-derived fluid or filtrate is the
raw material that is processed by the renal
tubules to form urine
Nephron
Nephron
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The external parietal layer of the glomerular
capsule is simple squamous epithelium
This layers contributes to the structure of the
capsule and plays no part in forming filtrate
The visceral layer that clings to the
glomerulus consists of highly modified,
branching epithelial cells called podocytes
Nephrons
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Podocytes terminate in foot processes, which
intertwine and form filtration silts or slit pores
The silts allow filtrate to pass to the interior of capsule
Nephrons
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The filtration
membrane is the
actual filter that
lies between the
blood and the
interior of the
glomerular
capsule
It is a porous
membrane that
allows free
passage of water
and solutes
Nephrons
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It is a porous
membrane that
allows free
passage of water
and solutes
smaller that
plasma proteins
The capillary
pores prevent
passage of blood
cells, but plasma
components are
allowed to pass
Nephron
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Once filtered out of
the plasma the urine
enters the collecting
duct
Urine passes into
larger ducts until it
reaches the ureters
It leaves the kidneys
and moves toward
the bladder in the
ureters
Glomerulus
Renal Physiology
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Skip to sections on Ureters located
on page 1029
Ureters
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The ureters are
slender tubes
that convey urine
from the kidneys
to the bladder
Ureters
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Each leaves the
renal pelvis,
decends behind
the peritoneum
to the base of the
bladder, turns
and then runs
obliquely
through the
medial bladder
wall
Ureters
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The ureters are
protected from a
backflow of urine
because any
increase within
the bladder
compresses and
closes the ends of
the ureters
Ureters
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Histologically, the walls of the ureter is
trilayered
– An inner layer of transitional epithelium
lines the inner mucosa
– The middle muscularis layer is composed of
a an inner longitudinal layer and an outer
circular layer
– The outer layer is composed of fibrous
connective tissue
Ureters
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The ureters play an active role in
transporting urine
Distension of the ureters by incoming
urine stimulates the muscularis layer to
contract, which propels the urine into the
bladder
The strength and frequency of peristaltic
waves are adjusted to the rate of urine
formation
Urinary Bladder
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The urinary bladder is a smooth, collapsible,
muscular sac that stores urine
Urinary Bladder
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In males, the bladder lies immediately anterior
to the rectum
Urinary Bladder
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In females, the bladder is anterior to the vagina
and uterus
Urinary Bladder
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The interior of the
bladder has
openings for both
ureters and the
urethra
The triangular
region of the
bladder base
outlined by these
openings is called
the trigone which is
a common site of
infections
Urinary Bladder
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The bladder wall has three layers
– A mucosa containing transitional epithelium
– A thick muscular layers
– A fibrous adventitia
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The muscle layer consists of smooth
muscle arranged inner and outer
longitudinal layers
Collectively the muscle layer is called the
detrusor muscle (literally to thrust out)
Urinary Bladder
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The bladder is very distensible and
uniquely suited for its function of urine
storage
It can expand for storage or collapse
when empty
Empty its walls are thick and thrown into
folds (rugae)
As it expands it becomes pear shaped and
rises in the abdominal cavity
Urinary Bladder
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The bladder can store more than 300 ml
or urine without a significant increase in
internal pressure
A moderately full bladder holds
approximately 500 ml and can about
1000 ml at capacity
Urine is held in the bladder until release
is convenient
Urethra
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The urethra is
a thin
muscular tube
that drains
urine from the
bladder and
conveys it out
of the body
Urethra
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The epithelium of its mucosal lining is
mostly pseudostratified columnar
epithelium
Near the bladder it is transitional
epithelium and near its external opening
it changes to a protective squamous
epithelium
Urethra
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At the bladder-urethra junction a
thickening of the detrusor muscle forms
the internal sphincter
This voluntary sphincter keeps the
urethra closed when urine is not being
passed
A second sphincter, the external urethral
sphincter, surrounds the urethra and is
composed of skeletal muscle and thus is
under voluntary control
Urethra
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The levator ani muscle of the pelvic floor
also serves as a voluntary constrictor of
the urethra
The length and functions of the urethra
differ in the two sexes
In females the urethra is 3-4 cm long and
is tightly bound to the anterior vaginal
wall by fibrous connective tissue
Urethra
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Its external
opening, the
external
urethral
orifice,
anterior to
the vaginal
opening and
posterior to
the clitoris
Urethra
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In males the
urethra is 20
cm long with
three regions
– Prostatic
urethra
– Membranous
urethra
– Spongy or
penile urethra
Urethra
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The male urethra has two basic functions
– It carries urine out of the body
– It carries semen into the female reproductive
tract
Micturition
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Micturition, also called voiding or
urination, is the act of emptying the
bladder
Ordinarily, as urine accumulates,
distension of the bladder walls activates
stretch receptors
Impulses are transmitted via visceral
afferent fibers to the sacral region of the
spinal cord
Micturition
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Spinal reflexes
– Initiate increased sympathetic outflow to the
bladder that inhibits the detursor muscle
and internal sphincter (temporarily)
– Stimulate contraction of the external
urethral sphincter
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When about 200 ml of urine has
accumulated, afferent impulses are
transmitted to the brain, at this point one
feels the urge to void their bladder
Micturition
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Contractions of the bladder become both
more frequent and urgent with time
If the time is convenient to empty the
bladder voiding reflexes are initiated
Visceral afferent impulses activate the
micturition center of the dorsolateral pons
Acting as an on/off switch for urination, this
center signals the parasympathetic neurons
to stimulate contraction of the detrusor
muscle and relaxation of sphincters
Micturition
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When one chooses not to void, reflex
bladder contractions subside within a
minute or so and urine continue to
accumulate
Because the external sphincter (and the
levator ani) is voluntarily controlled, we
can choose to keep it closed and postpone
bladder emptying temporarily
The urge to void eventually becomes
irresistible and micturition occurs
Chapter 26
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End of material from chapter 26