The Urinary System
Chapter 26
Introduction
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
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
Structures of the
urinary system
include;
– Kidneys
– Urinary bladder
– Ureters
– Urethra
Kidney Location
The kidneys extend
approximately from
the level of the 12th
thoracic vertebra to
the third lumbar
vertebra
Receive some
protection from ribs
Right lies somewhat
lower than left as it is
positioned under
liver
External Antomy
The adult kidney
weights about
150 g (5 oz.)
Dimensions are
12 cm long, 6 cm
wide, 3 cm thick
Lateral surface
is convex while
the medial
surface is
concave
External Antomy
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
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
The kidneys are retroperitoneal, or behind the
peritoneum
Position of the Kidneys
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
The adipose capsule attaches the kidney to the
posterior body wall and cushions it against trauma
Position of the Kidneys
The renal fascia is dense fibrous connective tissue
which surrounds the kidney and anchors these organs
to the surrounding structures
Internal Anatomy
The kidney has
three distinct
regions
– Cortex
– Medulla
– Pelvis
Internal Anatomy
The most
superficial
region
The renal cortex
is light in color
and has a
granular
appearance
Internal Anatomy
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
Each renal
pyramid has a
base which is
convex, and a
apex which
tapers toward its
apex or papilla
Medullary Medullary
base apex
Internal Anatomy
The apex, or Pyramidal
papilla, points stripes
internally
The pyramids
appear striped
because they are
formed almost
entirely of
roughly parallel
bundles of urine
collecting
tubules
Internal Anatomy
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
Within the
renal sinus is
the renal pelvis
This flat, funnel
shaped tube is
continuous with
the ureter
leaving the
hilus
Internal Anatomy
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
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
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
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
Nephrons
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
Each nephron Glomerulus
consists of a
glomerulus, a tuft of
capillaries associated
with a renal tubule
The end of the renal
tubule is a blind,
enlarged, and cup-
shaped and
completely surround
the glomerulus
Nephron
The renal corpuscle refers to the enclosed glomerulus
and the capsule of the glomerulus referred to as
Bowman’s capsule
Nephron
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
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
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
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
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
Once filtered out of Glomerulus
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
Renal Physiology
Skip to sections on Ureters located
on page 1029
Ureters
The ureters are
slender tubes
that convey urine
from the kidneys
to the bladder
Ureters
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
The ureters are
protected from a
backflow of urine
because any
increase within
the bladder
compresses and
closes the ends of
the ureters
Ureters
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
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
The urinary bladder is a smooth, collapsible,
muscular sac that stores urine
Urinary Bladder
In males, the bladder lies immediately anterior
to the rectum
Urinary Bladder
In females, the bladder is anterior to the vagina
and uterus
Urinary Bladder
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
The bladder wall has three layers
– A mucosa containing transitional epithelium
– A thick muscular layers
– A fibrous adventitia
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
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
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
The urethra is
a thin
muscular tube
that drains
urine from the
bladder and
conveys it out
of the body
Urethra
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
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
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
Its external
opening, the
external
urethral
orifice,
anterior to
the vaginal
opening and
posterior to
the clitoris
Urethra
In males the
urethra is 20
cm long with
three regions
– Prostatic
urethra
– Membranous
urethra
– Spongy or
penile urethra
Urethra
The male urethra has two basic functions
– It carries urine out of the body
– It carries semen into the female reproductive
tract
Micturition
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
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
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
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
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
End of material from chapter 26