The Urinary
System
The Urinary System
 Paired kidneys
 A ureter for each kidney
 Urinary bladder
 Urethra
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Main Functions of Urinary System
 Kidneys filter blood to keep it pure




Toxins
Metabolic wastes
Excess water
Excess ions
 Dispose of nitrogenous wastes from blood
 Urea
 Uric acid
 Creatinine
 Regulate the balance of water and electrolytes,
acids and bases
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 Kidneys are retroperitoneal organs (see next slide)
 Superior lumbar region of posterior abdominal wall
 Lateral surface is convex
 Medial surface is concave
 Hilus* is cleft: vessels, ureters and nerves enter and leave
 Adrenal glands* lie superior to each kidney
(the yellow blob in pic)
*
*
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5
6
Note layers of
adipose (fat),
capsule, fascia
Transverse sections
show retroperitoneal
position of kidneys
Note also: liver,
aorta muscles
on CT
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8
 Kidney has two regions
 Cortex: outer
 Columns of cortex divide medulla into “pyramids”
 Medulla: inner
 Darker, cone-shaped medullary or renal pyramids
 Parallel bundles of urine-collecting tubules
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
The human kidney has lobes
 Pyramid and cortical tissue surrounding it
 5-11 per kidney

Renal pelvis (=basin)
 Expanded, funnel shaped, superior part of ureter
 Branches to form two or three major calices (seen best on right pic below)
 Each of these divides again, minor calices: collect urine from papillae of
pyramids
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The Arteries
Aorta gives off right and left renal arteries
Renal arteries divides into 5 segmental arteries as enters hilus
of kidney
Segmentals branch into
lobar arteries
Lobars divide into
interlobars
Interlobars into arcuate in
junction of medulla and
cortex
Arcuates send interlobular
arteries into cortex
Cortical radiate arteries
give rise to glomerular
arterioles
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Vasculature of the kidney
 The glomerular capillary bed is unusual in
having arterioles going both to it and away from
it (afferent and efferent), instead of a vein going
away as most
 It is also unusual in having two capillary beds in
series (one following the other)
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 Uriniferous tubule is the main structural and
functional unit
To left is a single, generalized
uriniferous tubule
More than a million of these
tubules act together to form the
urine
Three main mechanisms
a. Glomerular filtration
b. Tubular reabsorption
c. Tubular secretion
Two major parts
1. A urine-forming nephron
2. A collecting duct which
concentrates urine by removing
water from it
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Outline
 Uriniferous tubule (anatomical unit for
forming urine)
 Nephron
 Renal corpuscle (in cortex)
– Glomerulus (tuft of capillaries)
– Glomerular (Bowman’s) capsule
 Tubular section
– Proximal convoluted tubule
– Loop of Henle
– Distal convoluted tubule
 Collecting duct
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Understand at least this much:
Filtration
a. Fluid is squeezed out of
the glomerular capillary
bed
Resorption
b. Most nutrients, water ad
essential ions are
returned to the blood of
the peritubular
capillaries
Secretion
c. Moves additional
undesirable molecules
into tubule from blood of
peritubular capillaries
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Nephron
 Renal corpuscle
 Tubular section
Uriniferous tubule (anatomical unit for forming urine)
Nephron
Renal corpuscle (in cortex)
Glomerulus (tuft of
capillaries)
Glomerular (Bowman’s)
capsule
Tubular section
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct
 Renal corpuscle: only in
cortex
 Tuft of capillaries called
glomerulus
 Surrounded by cup-shaped,
hollow glomerular
(Bowman’s) capsule
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(refer to this
pic as we go)
-------------------Visceral
layer of
capsule has
podocytes
 Unusual branching
epithelial cells
 Foot processes with
slit processes
between them
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Scanning EM of podocytes clinging to
capillaries (left) and filtration membrane
The capillary pores (fenestrations)
diagram (right)
restrict the passage of the largest
elements such as blood cells
The basement membrane and slit
diaphragm hold back all but the smallest
proteins while letting through small
molecules such as water, ions, glucose,
amino acids, and urea
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Uriniferous tubule (anatomical unit for forming urine)
Nephron
Nephron
 Renal corpuscle
 Tubular section
Renal corpuscle (in cortex)
Glomerulus (tuft of capillaries)
Glomerular (Bowman’s) capsule
Tubular section
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct
(processes the filtrate)
 Proximal convoluted
tubule
 Loop of Henle
 Distal convoluted
tubule
(ends by joining
collecting duct)
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Uriniferous tubule (anatomical unit for forming urine)
Proximal convoluted
tubule
 Confined to renal
cortex
 Cuboidal
epithelial cells
with long
microvilli (fuzzy
appearance in
pics)
 Resorption of
water, ions and
solutes
Nephron
Renal corpuscle (in cortex)
Glomerulus (tuft of capillaries
Glomerular (Bowman’s) capsu
Tubular section
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct
*
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Uriniferous tubule (anatomical unit for forming urine)
Nephron
Loop of Henle
 Descending limb
Renal corpuscle (in cortex)
Glomerulus (tuft of capillaries)
Glomerular (Bowman’s) capsule
Tubular section
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct
 Thin segment
 Thick ascending
limb
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Uriniferous tubule (anatomical unit for forming urine)
Nephron
Distal convoluted tubule
 Confined to the
renal cortex
 Simple cuboidal
epithelium
 Selective
secretion and
resorption of ions
Renal corpuscle (in cortex)
Glomerulus (tuft of capillaries
Glomerular (Bowman’s) caps
Tubular section
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct
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Classes of
nephrons
 Cortical nephrons
 85% of all
nephrons
 Almost entirely
within cortex
 Juxtamedullary
nephrons
 Renal corpuscles
near cortexmedulla junction
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Uriniferous tubule (anatomical unit for forming urine)
Nephron
Renal corpuscle (in cortex)
Glomerulus (tuft of capillaries)
Glomerular (Bowman’s) capsule
Tubular section
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct
Collecting Ducts
Collecting Duct
 Each receives
urine from
several
nephrons
 Run straight
through cortex
into the deep
medulla 24
Uriniferous tubules
1. Nephron
2. Collecting ducts
Collecting Ducts
*
 At papilla of
pyramid* ducts join
to form larger
papillary ducts
 Empty into minor
calices
 Role: conserve
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body fluids
The collecting ducts
 The most important role is to conserve body
fluids
 When the body must conserve water, the
posterior pituitary gland secretes ADH
(antidiuretic hormone)
 ADH increases the permeability of the collecting
tubules and distal tubules to water so more is
reabsorbed
 This decreases the total volume of urine
 Alcohol inhibits the release of ADH, so less
water is reabsorbed producing copious amounts
of dilute urine (can cause dehydration)
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Vessels

Afferent and efferent arterioles associated with glomerular capillaries
 Allows high pressure for forcing filtrate out of blood
 About 20% of renal plasma flow is filtered each minute (125 ml/min): this is the
glomerular filtration rate (GFR), an important clinical measure of renal function
 This is about one liter every 8 minutes (only 1% ends up as urine)

Peritubular capillaries arise from efferent arterioles
 Absorb solutes and water from tubule cells
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(vessels, continued)
The Vasa recta
The Vasa recta is a
portion of the
peritubular capillary
system which enters the
medulla where the
solute concentration in
the interstitium is high.
It acts with the loop of
Henle to concentrate
the urine by a complex
mechanism of counter
current exchange using
urea. If the vasa recta
did not exist, the high
concentration of solutes
in the medullary
interstitium would be
washed out.
____vasa recta
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Histology
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Juxtaglomerular apparatus



Regulation of blood pressure
Granule (jg cells) – modified muscle cells secreting renin in response to
falling blood pressure in afferent arteriole
Macula densa – chemoreceptors which secrete renin if solute concentration
falls
Reninangiotensin
mechanism:
Sequence of
reactions resulting
in aldosterone
secretion from
adrenal cortex:
increases sodium
resorption from
distal convoluted
tubules: water
follows, blood
volume increases
and blood pressure
increases
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For studying
Parts of the kidney:
1. Renal pyramid
2. Efferent vessel
3. Renal artery
4. Renal vein
5. Renal hilum
6. Renal pelvis
7. Ureter
8. Minor calyx
9. Renal capsule
10. Inferior renal capsule
11. Superior renal capsule
12. Afferent vessel
13. Nephron
14. Minor calyx
15. Major calyx
16. Renal papilla
17. Renal column
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The Ureters
 Slender tubes about
25 cm (10 “) long
leaving each renal
pelvis
 One for each kidney
carrying urine to the
bladder
 Descend
retroperitonealy and
cross pelvic brim
 Enter posterolateral
corners of bladder
 Run medially within
posterior bladder wall
before opening into
interior
 This oblique entry
helps prevent backflow
of urine
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Three basic layers
 Transitional epithelium
of mucosa stretches
when ureters fill
 Muscularis
Ureters play an active
role in transporting urine
(it’s not just by gravity)
 Inner longitudinal,
outer circular layers
 Inferior 3rd with extra
longitudinal layer)
 Stimulated to contract
when urine in ureter:
peristaltic waves to
propel urine to bladder
 Adventitia (external)
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Urinary Bladder
See also brief atlas
 Collapsible
muscular sac
 Stores and expels
urine
 Lies on pelvic floor
posterior to pubic
symphysis
 Males: anterior to
rectum
 Females: just
anterior to the
vagina and uterus
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37
38
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 If full: bladder is spherical and extends into
abdominal cavity (holds about 500 ml or 1 pt)
 If empty: bladder lies entirely within pelvis with
shape like upside-down pyramid
 Urine exits via the urethra
 Trigone is inside area between ureters and
urethra: prone to infection (see slide 38)
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Bladder wall has three layers (same as ureters)
 Mucosa with distensible transitional epithelium and
lamnia propria (can stretch)
 Thick muscularis called the detrusor muscle
 3 layers of highly intermingled smooth muscle
 Squeezes urine out
 Fibrous adventitia
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The Urethra
 Smooth muscle with inner mucosa
 Changes from transitional through stages to stratified squamous near end
 Drains urine out of the bladder and body
 Male: about 20 cm (8”) long
 Female: 3-4 cm (1.5”) long
 Short length is why females have more urinary tract infections than males ascending bacteria from stool contamination
urethra
Urethra____
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 Urethral sphincters
 Internal: involuntary sphincter of smooth muscle
 External: skeletal muscle inhibits urination voluntarily
until proper time (levator anni muscle also helps
voluntary constriction)
Males: urethra has
three regions (see
right)
_________trigone
1. Prostatic urethra__________
2. Membranous urethra____
3. Spongy or penile urethra_____
female
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With all the labels
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 Micturition
AKA:
 Voiding
 Urinating
 Emptying the bladder
(See book for diagram
explanation p 701)
KNOW:
Micturition center of brain:
pons
(but heavily influenced by
higher centers)
Parasympathetic: to void
Sympathetic: inhibits
micturition
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