The Urinary System
Functions of the urinary system
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Homeostatic regulation of blood plasma
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Regulating blood volume and pressure
Regulating plasma ion concentrations
Stabilizing blood pH
Conserving nutrients
Filter many liters of fluid from blood
Excretion - The removal of organic waste products
from body fluids
• Urea
• Uric acid
• Creatinine
Elimination - The discharge of waste products into
the environment
Urinary System
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Kidneys – produce
urine
Ureters –transport
urine to bladder
Urinary bladder stores urine
Urethra transports
urine to exterior
Figure 26.1
Location and External Anatomy of Kidneys
• Located retroperitoneally
• Lateral to T12–L3 vertebrae
• Average kidney
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Hilus
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12 cm tall, 6 cm wide, 3
cm thick
On concave surface
Vessels and nerves enter
and exit
Renal capsule surrounds
the kidney
Urinary System in Gross Dissection
Figure 26.3
Internal Gross Anatomy of the Kidneys
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Frontal section through the
kidney
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Renal cortex
Renal pyramids
Renal pelvis
• Major calicies
• Minor calicies
Gross vasculature
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Renal arteries
Branch into segmental
arteries
Anatomy of the kidneys
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Superficial outer cortex and inner medulla
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The medulla consists of 6-18 renal pyramids
The cortex is composed of roughly 1.25 million nephrons
• Major and minor calyces along with the pelvis drain urine to
the ureters
Nephron – The Functional Unit of Kidney
• Nephron consists of:
• Renal corpuscle
• Renal tubule:
• Proximal
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convoluted tubule
(PCT)
• Loop of Henle
• Distal convoluted
tubule (DCT)
Nephron empties
tubular fluid into a
system of collecting
ducts and papillary
ducts
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Renal Corpuscle
Consists of:
• Glomerulus – tuft of fenestrated capillaries
• Glomerular (Bowman’s) capsule
• Parietal layer – simple squamous epithelium
• Visceral layer – consists of podocytes
• Blood travels from efferent arteriole to peritubular capillaries
• Blood leaves the nephron via the efferent arteriole
Renal Corpuscle and the Filtration Membrane
Figure 23.6a
Glomerulus anatomy
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Podocytes cover lamina densa of
capillaries
• Project into the capsular space
• Pedicels of podocytes separated by
filtration slits
Two types of nephron
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Cortical nephrons
• ~85% of all
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nephrons
Located in the cortex
Juxtamedullary
nephrons
• Closer to renal
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medulla
Loops of Henle
extend deep into
renal pyramids
Nephron
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Proximal convoluted
tubule (PCT)
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Actively reabsorbs
nutrients, plasma proteins
and ions from filtrate
Released into peritubular
fluid
Loop of Henle
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Descending limb
Ascending limb
Each limb has a thick and
thin section
Nephron
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Distal convoluted
tubule (DCT)
• Actively secretes
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ions, toxins, drugs
Reabsorbs sodium
ions from tubular
fluid
Collecting Tubules (Collecting ducts)
• Collecting tubules - Receive urine from
distal convoluted tubules
Figure 23.8
Uriniferous Tubule
Figure 23.5a
Types Of Capillary Beds In Nephron
• Glomerulus - Fed and
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drained by afferent and
efferent arterioles
Peritubular capillaries
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Arise from efferent
arterioles
Low-pressure, porous
capillaries
Absorb solutes
• Vasa recta
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Thin-walled looping
vessels
Part of the kidney’s urineconcentrating mechanism
Mechanisms of Urine Production
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Filtration - filtrate of
blood leaves kidney
capillaries
Reabsorption – most
nutrients, water, and
essential ions reclaimed
Secretion - active process
of removing undesirable
molecules
Microscopic Anatomy of the Kidney
• Juxtaglomerular apparatus
• Functions in the regulation of blood pressure
• Juxtaglomerular cells – secrete renin
• Macula densa
• A portion of distal convoluted tubule
• Tall, closely packed epithelial cells
• Act as chemoreceptors
Summary of Nephron Function
• Each segment of nephron
and collecting system
contribute
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Glomerulus
PCT
Descending limb
Thick ascending limb
DCT and collecting
ducts
• Concentrated urine
produced after
considerable
modification of filtrate
Urine Excretion
• Leaves Collecting System
• Enters renal pelvis
• Rest of urinary system transports,
stores and eliminates
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Ureters
Bladder
Urethra
The Ureters
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Pair of muscular tubes
Extend from renal pelvis to the bladder
Peristaltic contractions force urine from the kidneys to the
urinary bladder
Oblique entry into bladder prevents backflow of urine
Histology of Ureter
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Mucosa – transitional
epithelium
Muscularis – two layers
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Inner longitudinal layer
Outer circular layer
• Adventitia – typical
connective tissue
Urinary Bladder
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A collapsible muscular
sac
Stores and expels urine
• Full bladder – spherical
• Expands into the
abdominal cavity
• Empty bladder – lies
entirely within the pelvis
Figure 23.13
Urinary Bladder
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Wall of bladder
• Mucosa - transitional epithelium
• Muscular layer - detrusor muscle
• Adventitia
The urethra
• Extends from the urinary bladder to the exterior of the body
• Passes through urogenital diaphragm (external urinary sphincter)
• Differs in length and function in males and females
• Internal urethral sphincter - involuntary smooth muscle
• External urethral sphincter - voluntarily inhibits urination, relaxes when
one urinates
Urinary Bladder and Urethra - Male
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Males – 20 cm in
length
Three named regions
• Prostatic urethra •
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passes through the
prostate gland
Membranous urethra through the urogenital
diaphragm
Spongy (penile)
urethra passes through
the length of the penis
Figure 23.16a
Urinary Bladder and Urethra - Female
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In females - length of 3–4 cm
The smooth triangular region of the base is is called the
trigone - many bladder infections persist in this region
Urethra
• Epithelium of urethra
• Transitional epithelium at the proximal
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end (near the bladder)
Stratified and pseudostratified columnar
– mid urethra (in males)
Stratified squamous epithelium at the
distal end (near the urethral opening)
Micturition
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Bladder can hold 250 400ml
Greater volumes stretch
bladder walls initiates
micturation reflex:
Urination coordinated by
micturition reflex
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Initiated by stretch receptors
in wall of bladder
Urination requires coupling
micturition reflex with
relaxation of external
urethral sphincter
Blood Flow Through the Kidney