The Urinary System
Ch 24
C. Babaian
Human Anatomy
Sonya Schuh-Huerta, Ph.D.
Outline
• Historical perspective
• Kidney overview
• Nephron structure & function
• Ureters, Bladder & Urethra
• Hot research
• Disorders/diseases
Organs of the Urinary System
•
•
•
•
Kidneys
Ureters
Urinary bladder
Urethra
Why study the kidney?
What does it do?
L. Da Vinci, 1508
The kidney: a vital organ with many
functions
 Filters the blood
-Removal of wastes & toxins
-Waste products: urea, uric acid, creatinine
-Production of urine
 Homeostasis
-Body fluids & pressure
Kidney
-Ions (Na+, K+, Cl-, Ca2+, HCO3-)
-Acid-base balance
Ureter
-Blood sugar
 Production of hormones
-Blood volume & pressure
-Calcium metabolism
-Red blood cell production
Bladder
What can urine tell us?
• Historical interest in resolving urine’s
composition & relation to health
• Alchemists of Medieval Europe
thought urine contained gold!
• Urea isolated in 1773; 1st organic
compound to be artificially synthesized
Gerrit Dou, 1617
Urine color wheel
What can urine tell us?
• Yellow color: breakdown products of
hemoglobin
Modern-day urine color wheel
Urine color & components tell
a lot about a person’s health
Salt & water balance
• Huge variation in amount & concentration of urine
• Kidney’s job: keep salt & water balanced (input = output)
• Maintain osmolarity of ~300 mOsmol/L
Matt Stonie, competitive eater
The kidneys
Renal artery
Renal hilum
Renal vein
Kidney
Ureter
Urinary
bladder
Urethra
Location & gross anatomy
 In almost all animals including humans
 Paired, “kidney” bean-shaped organs
 Located in the abdominal cavity: T12 – L3
 Within the retroperitoneum
 Left kidney behind spleen
 Top of right kidney behind liver
 Hilum  opening where renal artery,
vein, & ureter join kidney
 Surrounded by fibrous capsule, fat & fascia
Dorsal view
Position of the kidneys
Anterior
Inferior
vena cava
Aorta
Peritoneum
Peritoneal cavity
(organs removed)
Renal
vein
Renal
artery
Renal fascia
anterior
posterior
Perirenal
fat capsule
Body of
vertebra L2
Fibrous
capsule
Body wall
Posterior
12th rib
Supportive
tissue layers
Gross anatomy of the kidney
Renal cortex
Renal
medulla
Major calyx
Papilla of
pyramid
Renal pelvis
Minor calyx
Ureter
Renal pyramid
in renal medulla
Renal column
Fibrous capsule
Gross anatomy of the kidney
renal
capsule
medulla
renal
pelvis
hilum
Blood vessels of the kidney
Cortical radiate vein
Cortical radiate artery
Arcuate vein
Arcuate artery
Interlobar vein
Interlobar artery
Segmental arteries
Renal vein
Renal artery
Renal pelvis
Ureter
Renal medulla
Renal cortex
(a) Frontal section, posterior view, illustrating
major blood vessels
Blood vessels of the kidney
Aorta
Inferior vena cava
Renal artery
Renal vein
Segmental artery
Interlobar vein
Interlobar artery
Arcuate vein
Arcuate artery
Cortical radiate vein
Cortical radiate artery
Afferent arteriole
Peritubular capillaries
and vasa recta
Efferent arteriole
Glomerulus (capillaries)
Nephron-associated blood vessels
(see Figure 24.9)
(b) Path of blood flow through renal
blood vessels
The kidney
• Contains ~1 million tubular
structures = nephrons
• Carry out functions of the
kidney
At the microscopic level
Nephron – the functional unit
• Glomerulus & Bowman’s
capsule
• Proximal tubule
• Loop of Henle
• Peritubular capilaries &
Vasa recta
• Distal tubule
• Collecting duct
cortex
Edward Sales
First part of the nephron
• Renal corpuscle  Bowman’s capsule +
glomerulus
• Glomerulus = tuft of porous capillaries
• Bowman’s capsule = surrounds glomerulus
Glomerulus
Madelaine Dela Cruz
Glomerulus
Glomerular filtration
• Holds back proteins & RBCs
• Allows small molecules through:
Water
Ions (Na+, Cl-, K+,
Ca2+, HCO3-)
Glucose
Amino acids
Urea & uric acid
Efferent
arteriole
Glomerular
capsular space
Afferent
arteriole
Glomerular capillary
covered by podocytes
Proximal
convoluted
tubule
Outer layer
of Bowman’s
capsule
Glomerular filtration
• Glomeruli
– Porous capillaries
– Fed & drained by
afferent & efferent
arterioles
• Efferent arteriole has smaller diameter
– Filter 1 liter of fluid every 8 minutes!
• 180 liters of fluid per day
• Total blood volume is filtered ~60X/day!
Filtration membrane
• Filter between blood in glomerulus & space of
Bowman’s capsule
1. Endothelium of capillary
2. Basement membrane
3. Slits between foot
processes of podocytes
Cytoplasmic extensions
of podocytes
Filtration slits
Podocyte
cell body
Fenestrations
(pores)
Glomerular
capillary endothelium
Foot processes
of podocyte
How does the nephron work?
1) Filtration
Afferent arteriole
Glomerulus
– Blood filtrate leaves glomeruli
Efferent arteriole
2) Reabsorption
– Nutrients (glucose), H2O, &
essential ions taken back into blood
Bowman’s capsule
Renal tubule
3) Secretion
– Active process of removing
undesirable chemicals from blood
– Dumping them into tubules
4) Excretion
– Filtrate leaves as urine
Peritubular
capillary
Filtration
Reabsorption
Secretion
Urine
Different cell types & functions
Renal cortex
Renal medulla
Glomerular capsule: parietal layer
Renal pelvis
Basement
membrane
Ureter
Kidney
Podocyte
Renal corpuscle
Glomerular capsule
Glomerulus
Fenestrated
endothelium of
the glomerulus
Distal
convoluted
tubule
Proximal
convoluted
tubule
Glomerular capsule: visceral layer
Microvilli
Mitochondria
Highly infolded plasma
membrane
Proximal convoluted tubule cells
Cortex
Medulla
Distal convoluted tubule cells
Thick segment
Thin segment
Loop of Henle
Descending limb
Ascending limb
Collecting
duct
Nephron loop (thin-segment) cells
Principal cell
Collecting duct cells
Intercalated
cell
Glomerular Filtration Rate
• Glomerular Filtration Rate (= GFR)
Volume of fluid filtered by the glomeruli per unit
time  180 L/day or 125 ml/min
– Can only be measured by compounds that are
not reabsorbed or secreted
Coupling of water reabsorption to
Na+ reabsorption
Lumen
Tubule Cell
Peritubular
Capillary
H2O
Na+
H2O
H2O
H2O
Na+
Na+
Na+
H2O
Na+
Na+
low Na+
H2O
Na+
Na+
H2O
Na+
Na+ Na+ Na+
NaK
H2O
ATP ATPase
+
Na
K+
H2O
Na+
H2O
Na+
K+
Na+
H2O
Na+
H2O
H2O
Proximal convoluted tubule
• Most solutes are reabsorbed here
• All glucose is reabsorbed
• Drugs/toxins are secreted
• Filtrate is isoosmotic
Loop of Henle
• Ascending limb permeable to Na+, not H2O
• Descending limb permeable to H2O, not Na+
• Concentrates medulla
• Part of urine-concentrating system
 Countercurrent multiplier system
Friedrich Henle, 1873
Countercurrent Multiplier System
• Fluids in adjacent tubes running in opposite
directions – promotes the exchange of material
– Build-up of concentration
– Movement of solutes
– Examples in nature & mimicked in industry
– Loop of Henle!
Loop of Henle
Cortical nephron
• Cortical nephrons
Juxtamedullary nephron
Efferent
arteriole
Cortex
• Juxtamedullary nephrons
– Loop of Henle plunges deep
into medulla
Afferent arteriole
Efferent
arteriole
Peritubular
capillaries
– Vasa recta
Corticomedullary
junction
Vasa recta
– Set up the concentration
gradient of medulla
– Create driving force for
maximal H2O reabsorption
from collecting ducts
Collecting
duct
Medulla
high osmolarity
The kidney cortex & medulla
cortex
Edward Sales
medulla
Benjamin Yates
Blood vessels of nephrons
• Peritubular capillaries
– Capillary network of cortical nephrons
– Adapted for absorption
• Low-pressure, porous capillaries
• Vasa recta
– Capillary network of juxtamedullary nephrons
– Thin-walled looping vessels
• Descend deep into the medulla
– Part of the kidney’s urine-concentrating mechanism
Adaptations of the kidney
• Desert animals  conserve H2O
– Many long Loops of Henle
– Highly concentrated urine (uric acid)
– Urine “pellet”
medulla
Benjamin Yates
Kangaroo rat
Distal convoluted tubule
• Permeable to (actively pumps) Na+ out of tubule
• Impermeable to H2O
• Filtrate hypoosmotic
Collecting duct
• Reabsorption of Na+ under hormonal control
–Aldosterone (from adrenal gland)
• Reabsorption of H2O under hormonal control
–Antidiuretic hormone, ADH (from posterior pituitary)
• The nephrons fine-tune the final concentration
of urine based on body’s needs
How does the nephron fine tune
the reabsorption of Na+
and water?
Juxtaglomerular Apparatus
• Regulates salt & fluid balance
• Special cells: granular cells & the macula densa
Red blood cell
Efferent
arteriole
Proximal
tubule
Macula densa
cells of the
ascending limb
Lumens of
glomerular
capillaries
Extraglomerular
mesangial cells
Granular cells
Afferent arteriole
Mesangial cells
between capillaries
Juxtaglomerular
apparatus
Juxtaglomerular Apparatus
• Granular cells  Modified
smooth muscle cells of aff. arteriole
–Make renin
• Macula densa  Cells of distal ascending limb
–Monitors solute concentration (Na+)
–When BP is low (GFR low)  Na+ low  signals granular
cells to secrete renin
•Renin-Angiotensin System (RAS) 
Regulates sodium & fluid balance (& BP)
Plasma Volume /
Blood Pressure
Macula densa
Vascular & atrial pressure
GFR /
Flow /
Posterior pituitary
Antidiuretic hormone
NaCl
Granular cells
Renin
Collecting ducts
from Liver
Angiotensin II
Aquaporin channels
H2O Reabsorption
Adrenal gland
Aldosterone
Excretion of salt & H2O
Fluid Volume / BP
Collecting ducts
Sodium Reabsorption
Renin-Angiotensin System
• Many drugs interrupt steps of this system
• Used to treat high BP (hypertension), congenital
heart disease, kidney disease, etc.  diuretics
• Caffeine & alcohol also diuretics
Rest of the urinary system: Ureters
• Carry urine from the kidneys to the urinary
bladder
• Oblique entry into bladder prevents backflow
of urine
• Histology of ureter
– Mucosa  transitional epithelium
– Muscularis  2 layers
• Inner longitudinal layer
• Outer circular layer
– Adventitia  typical connective tissue
Transitional Epithelium – Review…
• Description:
– Has characteristics of stratified cuboidal &
stratified squamous
– Superficial cells dome-shaped when bladder
is relaxed, squamous when full
• Function: permits distension of urinary organs
by urine
• Location: epithelium of urinary bladder, ureters,
proximal urethra
Transitional Epithelium
(h) Transitional epithelium
Description: Resembles both
stratified squamous and stratified
cuboidal; basal cells cuboidal or
columnar; surface cells dome shaped
or squamous-like, depending on
degree of organ stretch.
Transitional
epithelium
Function: Stretches readily and
permits distension of urinary organ
by contained urine.
Location: Lines the ureters, bladder,
and part of the urethra.
Basement
membrane
Connective
tissue
Photomicrograph: Transitional epithelium lining the bladder,
relaxed state (390); note the bulbous, or rounded, appearance
of the cells at the surface; these cells flatten and become
elongated when the bladder is filled with urine.
Histology of the ureter
Lumen
Longitudinal
layer
Transitional
epithelium
Lamina
propria
Mucosa
Circular
layer
Muscularis
Adventitia
Urinary bladder
• A collapsible muscular sac
• Stores & expels urine
– Full bladder  spherical
• Expands into the abdominal cavity
– Empty bladder  lies entirely within the
pelvis
Urinary bladder
• Urachus  closed remnant of the
allantois
• Prostate gland
– In males (not females!)
• Lies directly inferior to the bladder
• Surrounds the urethra
Urinary bladder
Ureter not
illustrated in (b)
Uterus
Urachus
Urinary bladder
Ductus deferens
Pubic symphysis
Prostate gland
Vagina
Urethra
(a) Sagittal section through male pelvis,
urinary bladder shown in lateral view
(b) Sagittal section through
female pelvis
Urinary bladder
•
Urinary bladder is composed of 3 layers
1. Mucosa  transitional epithelium
2. Thick muscular layer  detrusor muscle
3. Fibrous adventitia
Histology of the urinary bladder
Lumen of bladder
Transitional
epithelium
Lamina
propria
Muscular
layer
(detrusor)
Transitional
epithelium
Basement
membrane
Adventitia
(with fat
cells)
(a) Micrograph of the bladder wall (17X)
Lamina
propria
(b) Epithelium lining the lumen
of the bladder (360X)
Urethra
• Epithelium of urethra
– Transitional epithelium
• At the proximal end (near the bladder)
– Stratified & pseudostratified columnar 
mid-urethra (in males)
– Stratified squamous epithelium
• At the distal end (near the urethral opening)
Urethra
• Internal urethral sphincter
– Involuntary smooth muscle
• External urethral sphincter
– Voluntarily inhibits urination
– Relaxes when one urinates
Urethra
• In females
– Length: 3–4 cm
• In males  20 cm in length; 3 named
regions:
– Prostatic urethra
• Passes through the prostate gland
– Membranous urethra
• Through the urogenital diaphragm
– Spongy (penile) urethra
• Passes through the length of the penis
Structure of urinary bladder & urethra
Peritoneum
Ureter
Rugae
Detrusor muscle
Adventitia
Ureteric orifices
Trigone of bladder
Bladder neck
Internal urethral sphincter
Prostate
Prostatic urethra
Membranous urethra
External urethral sphincter
Urogenital diaphragm
Spongy urethra
Erectile tissue of penis
(a) Male. The long male
urethra has three regions:
prostatic, membranous,
and spongy.
External urethral orifice
Structure of urinary bladder & urethra
Peritoneum
Ureter
Rugae
Detrusor muscle
Ureteric orifices
Bladder neck
Internal urethral
sphincter
Trigone
External urethral
sphincter
Urogenital diaphragm
(b) Female
Urethra
External urethral
orifice
Micturition
Pons
Pontine
micturition
center
(+)
2
2 Integration in pontine micturition
center initiates the micturition
response. Descending pathways
carry impulses to motor neurons in
the spinal cord.
Lower thoracic
or upper lumbar
spinal cord
4
(–)
Inferior
hypogastric
ganglion
Hypogastric
nerve
3 Parasympathetic efferents
stimulate contraction of the
detrusor muscle and open the
internal urethral sphincter.
Sacral
spinal
cord
Pelvic
nerves 1
Bladder
(+) (–)
Internal urethral
sphincter
1 Visceral afferent impulses from
stretch receptors in the bladder
wall are carried to the spinal cord
and then, via ascending tracts, to
the pontine micturition center.
3
Pelvic
splanchnic
nerves
5
External urethral sphincter
4 Sympathetic efferents to the
bladder are inhibited.
5 Somatic motor efferents to the
external urethral sphincter are
inhibited; the sphincter relaxes.
Urine passes through the urethra;
the bladder is emptied.
Visceral afferent
Sympathetic
Somatic efferent
Parasympathetic
Interneuron
Hot Research
• Adult reprogrammed cells (iPSCs) regenerate
entire kidney in vivo
(Usui et al., Amer J Path, 2012)
Disorders affecting the kidneys
• Diabetes mellitus (“flowing through” “sweet”)
– Huge sugar load  glucose in the urine & causes
osmotic diuresis; can result in low BP, coma & death
• Diabetes insipidus
– Lack antidiuretic hormone (ADH) 
massive amount of dilute urine & dehydration
• Renal calculi (kidney stones)
– Too much salt, Ca2+, uric acid  precipitates out of
solution & blocks urine flow; very painful
Disorders affecting the kidneys
• Renal Disease & Failure
–Glomerular dysfunction
–Slowed GFR, massive reabsorption of salt &
water, severe hypertension, edema
–Dialysis & Kidney Transplants
Disorders of the urinary system
• Urinary tract infections
– More common in females
– Burning sensation during micturition
• Bladder cancer
– 3% of cancers  more common in men
• Kidney cancer
– Arises from epithelial cells of nephron tubules
Disorders of the urinary system
• Congenital defects of the urogenital tract –
Hypospadias
–Abnormal placement of urethral orifice (opening); urethra opens anywhere
along the urethral groove on ventral side of penis or scrotum
–One of most common birth defects in boys (~1 out of 125 boys)
–More severe forms interfere with urination & sexual function!
–Most forms are correctable with surgery
Disorders of the urinary system
• Pelvic Floor Disorders  dropping down (prolapse) of the
bladder, urethra, small intestine, rectum, uterus, or vagina, caused by
weakness of or injury to the tissues of the pelvic floor
–Incontinence often develops
–Only occurs in women
–Incidence increases with age
(1 in 3 over age 65)
Urinary system throughout life: fetus
• Embryo develops 3 pairs of kidneys
• Only 1 persists to become kidney
• By fetal week 8  kidney & nephrons fully formed
• Produces urine by fetal month 3
• Contributes to amniotic fluid!
Urinary
bladder
Gonad
Urethra
Kidney
Anus
Ureter
8-week fetus
Rectum
Urinary system throughout life: adult
• Kidney & bladder function declines with
advancing age
–
–
–
–
–
–
Nephrons decrease in size & number
Tubules less efficient at secretion & reabsorption
Filtration declines
Recognition of desire to urinate is delayed
Loss of muscle tone in bladder & sphincters
Incontinence can develop
• And Yes,… exercise can help!  Kegels
Summary
• Importance of the kidney & role in salt & water balance
• Structure & function of the entire kidney, nephron, ureters,
bladder & urethra
Questions…?
What’s Next?
Madelaine Dela Cruz
Lab: Urinary & Reproductive Systems
Mon Lecture: Reprod & Endo Systems
Mon Lab: Student Presentations & Potluck!
& Endo Sys
Wed Lecture: Finish Endo Sys & Review
Wed Lab: Lab Exam 5!
Review: Anatomy of the Kidney
Review: Anatomy of the Nephron
(Boron & Boulpaep, Med Phys., 2003)
Renal Clearance
• Renal Clearance (= RC)
Volume of blood plasma from which a particular
substance (x) is completely cleared
RCx = Ux · V
Px
Where Ux = concentration in urine
V = flow rate of urine
Px = concentration in plasma
– RC of creatinine & inulin = GFR
– RC of glucose = 0  Why?