Transcript 25 - Jackson County School District

PowerPoint® Lecture Slides
prepared by
Barbara Heard,
Atlantic Cape Community
College
CHAPTER
25
The Urinary
System
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
Kidney Functions
• Regulating total water volume and total
solute concentration in water
• Regulating ECF ion concentrations
• Ensuring long-term acid-base balance
• Removal of metabolic wastes, toxins,
drugs
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Kidney Functions
• Endocrine functions
– Renin - regulation of blood pressure
– Erythropoietin - regulation of RBC
production
• Activation of vitamin D
• Gluconeogenesis during prolonged fasting
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Urinary System Organs
• Kidneys - major excretory organs
• Ureters - transport urine from kidneys to
urinary bladder
• Urinary bladder - temporary storage
reservoir for urine
• Urethra transports urine out of body
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Figure 25.1 The urinary system.
Hepatic veins (cut)
Esophagus (cut)
Inferior vena cava
Adrenal gland
Renal artery
Renal hilum
Aorta
Renal vein
Kidney
Iliac crest
Ureter
Rectum (cut)
Uterus (part of female
reproductive system)
Urinary
bladder
Urethra
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Kidney Anatomy
• Retroperitoneal, in the superior lumbar
region; ~ T12 to L5
• Right kidney crowded by liver  lower
than left
• Adrenal (suprarenal) gland atop each
kidney
• Convex lateral surface, concave medial
surface; vertical renal hilum leads to renal
sinus
• Ureters, renal blood vessels, lymphatics,
and nerves enter and exit at hilum
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Figure 25.2b Position of the kidneys against the posterior body wall.
12th rib
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Kidney Anatomy
• Layers of surrounding supportive tissue
– Renal fascia
• Anchoring outer layer of dense fibrous connective
tissue
– Perirenal fat capsule
• Fatty cushion
– Fibrous capsule
• Prevents spread of infection to kidney
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Internal Anatomy
• Renal cortex
– Granular-appearing superficial region
• Renal medulla
– Composed of cone-shaped medullary (renal)
pyramids
– Pyramids separated by renal columns
• Inward extensions of cortical tissue
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Internal Anatomy
• Papilla
– Tip of pyramid; releases urine into minor calyx
• Lobe
– Medullary pyramid and its surrounding cortical
tissue; ~ 8/kidney
• Renal pelvis
– Funnel-shaped tube continuous with ureter
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Internal Anatomy
• Minor calyces
– Drain pyramids at papillae
• Major calyces
– Collect urine from minor calyces
– Empty urine into renal pelvis
• Urine flow
– Renal pyramid  minor calyx  major calyx
 renal pelvis  ureter
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Homeostatic Imbalance
• Pyelitis
– Infection of renal pelvis and calyces
• Pyelonephritis
– Infection/inflammation of entire kidney
• Normally - successfully treated with
antibiotics
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Figure 25.2a Position of the kidneys against the posterior body wall.
Anterior
Inferior
vena cava
Aorta
Peritoneum
Peritoneal cavity
(organs removed)
Supportive
tissue layers
• Renal fascia
anterior
posterior
Renal
vein
Renal
artery
• Perirenal
fat capsule
• Fibrous
capsule
Body of
vertebra L2
Body wall
Posterior
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Figure 25.3 Internal anatomy of the kidney.
Renal
hilum
Renal cortex
Renal medulla
Major calyx
Papilla of
pyramid
Renal pelvis
Minor calyx
Ureter
Renal pyramid in
renal medulla
Renal column
Fibrous capsule
Photograph of right kidney, frontal section
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Diagrammatic view
Blood and Nerve Supply
• Kidneys cleanse blood; adjust its
composition  rich blood supply
• Renal arteries deliver ~ ¼ (1200 ml) of
cardiac output to kidneys each minute
• Arterial flow into and venous flow out of
kidneys follow similar paths
• Nerve supply via sympathetic fibers from
renal plexus
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Figure 25.4a 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
Frontal section illustrating major blood vessels
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Figure 25.4b 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 artery
Afferent arteriole
Cortical radiate vein
Peritubular
capillaries
or vasa recta
Efferent arteriole
Glomerulus (capillaries)
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Nephron-associated blood vessels
(see Figure 25.7)
(b) Path of blood flow through renal blood vessels
Nephrons
• Structural and functional units that form
urine
• > 1 million per kidney
• Two main parts
– Renal corpuscle
– Renal tubule
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Renal Corpuscle
• Two parts of renal corpuscle
– Glomerulus
• Tuft of capillaries; fenestrated endothelium 
highly porous  allows filtrate formation
– Glomerular capsule (Bowman's capsule)
• Cup-shaped, hollow structure surrounding
glomerulus
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Figure 25.5 Location and structure of nephrons.
Renal cortex
Renal medulla
Glomerular capsule: parietal layer
Renal pelvis
Ureter
Kidney
Renal corpuscle
• Glomerular capsule
• Glomerulus
Distal
convoluted
tubule
Basement
membrane
Podocyte
Fenestrated endothelium
of the glomerulus
Glomerular capsule: visceral layer
Apical
microvilli
Mitochondria
Highly infolded basolateral
membrane
Proximal convoluted tubule cells
Proximal
convoluted
tubule
Cortex
Apical side
Medulla
Thin segment
Nephron loop
• Descending limb
• Ascending limb
Thick
segment
Basolateral side
Distal convoluted tubule cells
Nephron loop (thin-segment) cells
Collecting
duct
Principal
cell
Intercalated cell
Collecting duct cells
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Renal Corpuscle
• Glomerular capsule
– Parietal layer - simple squamous epithelium
– Visceral layer - branching epithelial
podocytes
• Extensions terminate in foot processes that cling
to basement membrane
• Filtration slits between foot processes allow
filtrate to pass into capsular space
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Figure 25.5 Location and structure of nephrons. (2 of 7)
Glomerular capsule: parietal layer
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Figure 25.5 Location and structure of nephrons. (3 of 7)
Basement
membrane
Podocyte
Fenestrated endothelium
of the glomerulus
Glomerular capsule: visceral layer
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Renal Tubule
• Three parts
– Proximal convoluted tubule
• Proximal  closest to renal corpuscle
– Nephron loop
– Distal convoluted tubule
• Distal  farthest from renal corpuscle
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Renal Tubule
• Proximal convoluted tubule (PCT)
– Cuboidal cells with dense microvilli (brush
border  surface area); large mitochondria
– Functions in reabsorption and secretion
– Confined to cortex
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Figure 25.5 Location and structure of nephrons. (4 of 7)
Apical microvilli
Mitochondria
Highly infolded
basolateral membrane
Proximal convoluted tubule cells
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Renal Tubule
• Nephron loop
– Descending and ascending limbs
– Proximal descending limb continuous with
proximal tubule
– Distal descending limb = descending thin
limb; simple squamous epithelium
– Thick ascending limb
• Cuboidal to columnar cells; thin in some nephrons
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Figure 25.5 Location and structure of nephrons. (6 of 7)
Nephron loop (thin-segment) cells
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Renal Tubule
• Distal convoluted tubule (DCT)
– Cuboidal cells with very few microvilli
– Function more in secretion than reabsorption
– Confined to cortex
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Figure 25.5 Location and structure of nephrons. (5 of 7)
Apical side
Basolateral side
Distal convoluted tubule cells
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Collecting Ducts
• Two cell types
– Principal cells
• Sparse, short microvilli
• Maintain water and Na+ balance
– Intercalated cells
• Cuboidal cells; abundant microvilli; two types
– A and B; both help maintain acid-base balance of blood
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Figure 25.5 Location and structure of nephrons. (7 of 7)
Principal cell
Collecting duct cells
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Intercalated cell
Collecting Ducts
• Receive filtrate from many nephrons
• Run through medullary pyramids 
striped appearance
• Fuse together to deliver urine through
papillae into minor calyces
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Figure 25.5 Location and structure of nephrons.
Renal cortex
Renal medulla
Glomerular capsule: parietal layer
Renal pelvis
Ureter
Kidney
Renal corpuscle
• Glomerular capsule
• Glomerulus
Distal
convoluted
tubule
Basement
membrane
Podocyte
Fenestrated endothelium
of the glomerulus
Glomerular capsule: visceral layer
Apical
microvilli
Mitochondria
Highly infolded basolateral
membrane
Proximal convoluted tubule cells
Proximal
convoluted
tubule
Cortex
Apical side
Medulla
Thin segment
Nephron loop
• Descending limb
• Ascending limb
Thick
segment
Basolateral side
Distal convoluted tubule cells
Nephron loop (thin-segment) cells
Collecting
duct
Principal
cell
Intercalated cell
Collecting duct cells
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Classes of Nephrons
• Cortical nephrons—85% of nephrons;
almost entirely in cortex
• Juxtamedullary nephrons
– Long nephron loops deeply invade medulla
– Ascending limbs have thick and thin
segments
– Important in production of concentrated urine
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Figure 25.7a Blood vessels of cortical and juxtamedullary nephrons.
Cortical nephron
• Short nephron loop
• Glomerulus further from the cortex-medulla junction
• Efferent arteriole supplies peritubular capillaries
Glomerulus
Renal
corpuscle (capillaries)
Glomerular
capsule
Efferent
arteriole
Proximal
convoluted
tubule
Juxtamedullary nephron
• Long nephron loop
• Glomerulus closer to the cortex-medulla junction
• Efferent arteriole supplies vasa recta
Cortical radiate vein
Cortical radiate artery
Afferent arteriole
Collecting duct
Distal convoluted tubule
Afferent
Efferent
arteriole
arteriole
Peritubular
capillaries
Ascending
limb of
nephron loop
Kidney
Cortex-medulla
junction
Arcuate vein
Arcuate artery
Vasa recta
Nephron loop
Descending
limb of
nephron loop
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Nephron Capillary Beds
• Renal tubules associated with two
capillary beds
– Glomerulus
– Peritubular capillaries
• Juxtamedullary nephrons also associated
with
– Vasa recta
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Nephron Capillary Beds
• Glomerulus - specialized for filtration
• Different from other capillary beds – fed
and drained by arteriole
– Afferent arteriole  glomerulus  efferent
arteriole
• Blood pressure in glomerulus high
because
– Afferent arterioles larger in diameter than
efferent arterioles
– Arterioles are high-resistance vessels
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Nephron Capillary Beds
• Peritubular capillaries
– Low-pressure, porous capillaries adapted for
absorption of water and solutes
– Arise from efferent arterioles
– Cling to adjacent renal tubules in cortex
– Empty into venules
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Nephron Capillary Beds
• Vasa recta
– Long, thin-walled vessels parallel to long
nephron loops of juxtamedullary nephrons
– Arise from efferent arterioles serving
juxtamedullary nephrons
• Instead of peritubular capillaries
– Function in formation of concentrated urine
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Figure 25.7a Blood vessels of cortical and juxtamedullary nephrons.
Cortical nephron
• Short nephron loop
• Glomerulus further from the cortex-medulla junction
• Efferent arteriole supplies peritubular capillaries
Glomerulus
Renal
corpuscle (capillaries)
Glomerular
capsule
Efferent
arteriole
Proximal
convoluted
tubule
Juxtamedullary nephron
• Long nephron loop
• Glomerulus closer to the cortex-medulla junction
• Efferent arteriole supplies vasa recta
Cortical radiate vein
Cortical radiate artery
Afferent arteriole
Collecting duct
Distal convoluted tubule
Afferent
Efferent
arteriole
arteriole
Peritubular
capillaries
Ascending
limb of
nephron loop
Kidney
Cortex-medulla
junction
Arcuate vein
Arcuate artery
Vasa recta
Nephron loop
Descending
limb of
nephron loop
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Juxtaglomerular Complex (JGC)
• One per nephron
• Involves modified portions of
– Distal portion of ascending limb of nephron
loop
– Afferent (sometimes efferent) arteriole
• Important in regulation of rate of filtrate
formation and blood pressure
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Juxtaglomerular Complex (JGC)
• Three cell populations
– Macula densa, granular cells, extraglomerular
mesangial cells
• Macula densa
– Tall, closely packed cells of ascending limb
– Chemoreceptors; sense NaCl content of
filtrate
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Juxtaglomerular Complex (JGC)
• Granular cells (juxtaglomerular, or JG
cells)
– Enlarged, smooth muscle cells of arteriole
– Secretory granules contain enzyme renin
– Mechanoreceptors; sense blood pressure in
afferent arteriole
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Juxtaglomerular Complex (JGC)
• Extraglomerular mesangial cells
– Between arteriole and tubule cells
– Interconnected with gap junctions
– May pass signals between macula densa and
granular cells
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Figure 25.8 Juxtaglomerular complex (JGC) of a nephron.
Glomerular
capsule
Efferent
arteriole
Afferent
arteriole
Glomerulus
Parietal layer
of glomerular
capsule
Capsular
space
Foot
processes
of podocytes
Podocyte cell body
(visceral layer)
Red blood cell
Proximal
tubule cell
Efferent
arteriole
Juxtaglomerular
complex
• Macula densa
cells
of the ascending
limb of nephron loop
• Extraglomerular
mesangial cells
• Granular
cells
Afferent
arteriole
Lumens of
glomerular
capillaries
Endothelial cell
of glomerular
capillary
Glomerular mesangial
cells
Juxtaglomerular complex
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Renal corpuscle
Kidney Physiology: Mechanisms of Urine
Formation
• 180 L fluid processed daily; only 1.5 L 
urine
• Three processes in urine formation and
adjustment of blood composition
– Glomerular filtration
– Tubular reabsorption
– Tubular secretion
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Kidney Physiology: Mechanisms of Urine
Formation
• Glomerular filtration – produces cell- and
protein-free filtrate
• Tubular reabsorption
– Selectively returns 99% of substances from
filtrate to blood in renal tubules and collecting
ducts
• Tubular secretion
– Selectively moves substances from blood to
filtrate in renal tubules and collecting ducts
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Kidney Physiology: Mechanisms of Urine
Formation
• Kidneys filter body's entire plasma volume
60 times each day; consume 20-25%
oxygen used by body at rest; produce
urine from filtrate
• Filtrate (produced by glomerular filtration)
– Blood plasma minus proteins
• Urine
– <1% of original filtrate
– Contains metabolic wastes and unneeded
substances
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Figure 25.9 A schematic, uncoiled nephron showing the three major renal processes that adjust plasma composition.
Afferent
arteriole
Glomerular
capillaries
Efferent arteriole
Cortical
radiate
artery
1
Glomerular capsule
Renal tubule and
collecting duct
containing filtrate
2
Peritubular
capillary
3
To cortical radiate vein
Three major
renal processes: Urine
Glomerular filtration
1
Tubular reabsorption
2
Tubular secretion
3
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Glomerular Filtration
• Passive process
• No metabolic energy required
• Hydrostatic pressure forces fluids and
solutes through filtration membrane
• No reabsorption into capillaries of
glomerulus
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The Filtration Membrane
• Porous membrane between blood and
interior of glomerular capsule
– Water, solutes smaller than plasma proteins
pass; normally no cells pass
• Three layers
– Fenestrated endothelium of glomerular
capillaries
– Basement membrane (fused basal laminae
of two other layers)
– Foot processes of podocytes with filtration
slits; slit diaphragms repel macromolecules
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Figure 25.10a The filtration membrane.
Efferent
arteriole
Glomerular
capsular space
Cytoplasmic extensions
of podocytes
Filtration slits
Podocyte
cell body
Afferent
arteriole
Glomerular
capillary covered by
podocytes that form
the visceral layer of
glomerular capsule
Proximal
convoluted
tubule
Parietal layer
Fenestrations
of glomerular
(pores)
capsule
Glomerular capillaries and the
visceral layer of the glomerular
capsule
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Glomerular
capillary endothelium
(podocyte covering
and basement
membrane removed)
Foot
processes
of podocyte
Figure 25.10b The filtration membrane.
Filtration slits
Podocyte
cell body
Foot
processes
Filtration slits between the podocyte foot processes
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Figure 25.10c The filtration membrane.
Capillary
Filtration membrane
• Capillary endothelium
• Basement membrane
• Foot processes of podocyte
of glomerular capsule
Filtration
slit
Plasma
Fenestration
(pore)
Filtrate
in capsular
space
Slit
diaphragm
Foot
processes
of podocyte
Three layers of the filtration membrane
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The Filtration Membrane
• Macromolecules "stuck" in filtration membrane
engulfed by glomerular mesangial cells
• Allows molecules smaller than 3 nm to pass
– Water, glucose, amino acids, nitrogenous wastes
• Plasma proteins remain in blood  maintains
colloid osmotic pressure  prevents loss of all
water to capsular space
– Proteins in filtrate indicate membrane problem
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Pressures That Affect Filtration
• Outward pressures promote filtrate
formation
– Hydrostatic pressure in glomerular
capillaries = Glomerular blood pressure
• Chief force pushing water, solutes out of blood
• Quite high – 55 mm Hg (most capillary beds ~ 26
mm Hg)
– Because efferent arteriole is high resistance vessel with
diameter smaller than afferent arteriole
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Pressures That Affect Filtration
• Inward forces inhibiting filtrate formation
– Hydrostatic pressure in capsular space (HPcs)
• Pressure of filtrate in capsule – 15 mm Hg
– Colloid osmotic pressure in capillaries (OPgc)
• "Pull" of proteins in blood – 30 mm Hg
• Sum of forces  Net filtration pressure
(NFP)
– 55 mm Hg forcing out; 45 mm Hg opposing =
net outward force of 10 mm Hg
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Net Filtration Pressure (NFP)
• Pressure responsible for filtrate formation
(10 mm Hg)
• Main controllable factor determining
glomerular filtration rate (GFR)
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Figure 25.11 Forces determining net filtration pressure (NFP).
Efferent
arteriole
Glomerular
capsule
HPgc = 55 mm Hg
OPgc = 30 mm Hg
Afferent
arteriole
HPcs = 15 mm Hg
NFP = Net filtration pressure
= outward pressures – inward pressures
= (HPgc) – (HPcs + OPgc)
= (55) – (15 + 30)
= 10 mm Hg
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Glomerular Filtration Rate (GFR)
• Volume of filtrate formed per minute by
both kidneys (normal = 120–125 ml/min)
• GFR directly proportional to
– NFP – primary pressure is hydrostatic
pressure in glomerulus
– Total surface area available for filtration –
glomerular mesangial cells control by
contracting
– Filtration membrane permeability – much
more permeable than other capillaries
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Regulation of Glomerular Filtration
• Constant GFR allows kidneys to make
filtrate and maintain extracellular
homeostasis
– Goal of intrinsic controls - maintain GFR in
kidney
• GFR affects systemic blood pressure
–  GFR  urine output   blood pressure,
and vice versa
– Goal of extrinsic controls - maintain systemic
blood pressure
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Regulation of Glomerular Filtration
• Intrinsic controls (renal autoregulation)
– Act locally within kidney to maintain GFR
• Extrinsic controls
– Nervous and endocrine mechanisms that
maintain blood pressure; can negatively affect
kidney function
– Take precedence over intrinsic controls if
systemic BP < 80 or > 180 mm Hg
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Regulation of Glomerular Filtration
• Controlled via glomerular hydrostatic
pressure
– If rises  NFP rises  GFR rises
– If falls only 18% GFR = 0
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Intrinsic Controls
• Maintains nearly constant GFR when MAP
in range of 80–180 mm Hg
– Autoregulation ceases if out of that range
• Two types of renal autoregulation
– Myogenic mechanism
– Tubuloglomerular feedback mechanism
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Intrinsic Controls: Myogenic Mechanism
• Smooth muscle contracts when stretched
•  BP  muscle stretch  constriction of
afferent arterioles  restricts blood flow
into glomerulus
– Protects glomeruli from damaging high BP
•  BP  dilation of afferent arterioles
• Both help maintain normal GFR despite
normal fluctuations in blood pressure
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Intrinsic Controls: Tubuloglomerular
Feedback Mechanism
• Flow-dependent mechanism directed by
macula densa cells; respond to filtrate
NaCl concentration
• If GFR  filtrate flow rate  
reabsorption time  high filtrate NaCl
levels  constriction of afferent arteriole
  NFP & GFR  more time for NaCl
reabsorption
• Opposite for  GFR
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Extrinsic Controls: Sympathetic Nervous
System
• Under normal conditions at rest
– Renal blood vessels dilated
– Renal autoregulation mechanisms prevail
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Extrinsic Controls: Sympathetic Nervous
System
• If extracellular fluid volume extremely low
(blood pressure low)
– Norepinephrine released by sympathetic
nervous system; epinephrine released by
adrenal medulla 
• Systemic vasoconstriction  increased blood
pressure
• Constriction of afferent arterioles   GFR 
increased blood volume and pressure
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Extrinsic Controls: Renin-AngiotensinAldosterone Mechanism
• Main mechanism for increasing blood
pressure – see Chapters 16 and 19
• Three pathways to renin release by
granular cells
– Direct stimulation of granular cells by
sympathetic nervous system
– Stimulation by activated macula densa cells
when filtrate NaCl concentration low
– Reduced stretch of granular cells
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Extrinsic Controls: Other Factors Affecting
GFR
• Kidneys release chemicals; some act as
paracrines that affect renal arterioles
– Adenosine
– Prostaglandin E2
– Intrinsic angiotensin II – reinforces effects of
hormonal angiotensin II
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Tubular Reabsorption
• Most of tubular contents reabsorbed to
blood
• Selective transepithelial process
– ~ All organic nutrients reabsorbed
– Water and ion reabsorption hormonally
regulated and adjusted
• Includes active and passive tubular
reabsorption
• Two routes
– Transcellular or paracellular
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Tubular Reabsorption
• Transcellular route
– Apical membrane of tubule cells 
– Cytosol of tubule cells 
– Basolateral membranes of tubule cells 
– Endothelium of peritubular capillaries
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Tubular Reabsorption
• Paracellular route
– Between tubule cells
• Limited by tight junctions, but leaky in proximal
nephron
– Water, Ca2+, Mg2+, K+, and some Na+ in the PCT
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Figure 25.13 Transcellular and paracellular routes of tubular reabsorption.
Slide 1
The paracellular route
Transport across the
The transcellular route 3
involves:
basolateral membrane. (Often
involves:
•
Movement
through leaky
involves the lateral intercellular
1
Transport across the
tight junctions, particularly in
spaces because membrane
apical membrane.
the PCT.
transporters transport ions into
these spaces.)
• Movement through the inter2 Diffusion through the
stitial fluid and into the
4 Movement through the intercytosol.
capillary.
stitial fluid and into the capillary.
Filtrate
Tubule cell
Interstitial fluid
in tubule
PeriLateral
Tight junction
lumen
tubular
intercellular capillary
space
3
H2O and
solutes
Apical
membrane
H2O and
solutes
1
2
4
3
4
Transcellular Capillary
endothelial
route
cell
Paracellular route
Basolateral
membranes
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Tubular Reabsorption of Sodium
• Na+ - most abundant cation in filtrate
– Transport across basolateral membrane
• Primary active transport out of tubule cell by
Na+-K+ ATPase pump  peritubular capillaries
– Transport across apical membrane
• Na+ passes through apical membrane by
secondary active transport or facilitated diffusion
mechanisms
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Reabsorption of Nutrients, Water, and Ions
• Na+ reabsorption by primary active
transport provides energy and means for
reabsorbing most other substances
• Creates electrical gradient  passive
reabsorption of anions
• Organic nutrients reabsorbed by
secondary active transport; cotransported
with Na+
– Glucose, amino acids, some ions, vitamins
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Passive Tubular Reabsorption of Water
• Movement of Na+ and other solutes
creates osmotic gradient for water
• Water reabsorbed by osmosis, aided by
water-filled pores called aquaporins
– Aquaporins always present in PCT 
obligatory water reabsorption
– Aquaporins inserted in collecting ducts only if
ADH present  facultative water
reabsorption
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Passive Tubular Reabsorption of Solutes
• Solute concentration in filtrate increases
as water reabsorbed  concentration
gradients for solutes 
• Fat-soluble substances, some ions and
urea, follow water into peritubular
capillaries down concentration gradients
–  Lipid-soluble drugs, environmental
pollutants difficult to excrete
© 2013 Pearson Education, Inc.
Figure 25.14 Reabsorption by PCT cells.
Slide 1
1
At the basolateral membrane, Na+ is
pumped into the interstitial space by the
Na+-K+ ATPase. Active Na+ transport
creates concentration gradients that drive:
Nucleus
Filtrate
in tubule
lumen
Tubule cell
Interstitial
fluid
Peritubular
capillary
2 “Downhill” Na+ entry at the
apical membrane.
3 Reabsorption of organic
nutrients and certain ions by
cotransport at the apical
membrane.
4
Reabsorption of water by
osmosis through
aquaporins. Water
reabsorption increases the
concentration of the
solutes that are left behind.
These solutes can then be
reabsorbed as they move
down their gradients:
5
Lipid-soluble substances
diffuse by the transcellular
route.
2
Glucose
Amino
acids
Some
ions
Vitamins
1
3
4
Lipidsoluble 5
substances
6
Various
Ions
and urea
Tight junction
Primary active transport
Secondary active transport
Passive transport (diffusion)
© 2013 Pearson Education, Inc.
Paracellular
route
Transport protein
Ion channel
Aquaporin
6 Various ions (e.g., Cl−,
Ca2+, K+) and urea diffuse
by the paracellular route.
Tubular Reabsorption
• Most of tubular contents reabsorbed to
blood
• Selective transepithelial process
– ~ All organic nutrients reabsorbed
– Water and ion reabsorption hormonally
regulated and adjusted
• Includes active and passive tubular
reabsorption
• Two routes
– Transcellular or paracellular
© 2013 Pearson Education, Inc.
Transport Maximum
• Transcellular transport systems specific
and limited
– Transport maximum (Tm) for ~ every
reabsorbed substance; reflects number of
carriers in renal tubules available
– When carriers saturated, excess excreted in
urine
• E.g., hyperglycemia  high blood glucose levels
exceed Tm  glucose in urine
© 2013 Pearson Education, Inc.
Reabsorptive Capabilities of Renal Tubules
and Collecting Ducts
• PCT
– Site of most reabsorption
•
•
•
•
All nutrients, e.g., glucose and amino acids
65% of Na+ and water
Many ions
~ All uric acid; ½ urea (later secreted back into
filtrate)
© 2013 Pearson Education, Inc.
Reabsorptive Capabilities of Renal Tubules
and Collecting Ducts
• Nephron loop
– Descending limb - H2O can leave; solutes
cannot
– Ascending limb – H2O cannot leave; solutes
can
• Thin segment – passive Na+ movement
• Thick segment – Na+-K+-2Cl- symporter and Na+H+ antiporter; some passes by paracellular route
© 2013 Pearson Education, Inc.
Reabsorptive Capabilities of Renal Tubules
and Collecting Ducts
• DCT and collecting duct
– Reabsorption hormonally regulated
•
•
•
•
Antidiuretic hormone (ADH) – Water
Aldosterone – Na+ (therefore water)
Atrial natriuretic peptide (ANP) – Na+
PTH – Ca2+
© 2013 Pearson Education, Inc.
Reabsorptive Capabilities of Renal Tubules
and Collecting Ducts
• Antidiuretic hormone (ADH)
– Released by posterior pituitary gland
– Causes principal cells of collecting ducts to
insert aquaporins in apical membranes 
water reabsorption
• As ADH levels increase  increased water
reabsorption
© 2013 Pearson Education, Inc.
Reabsorptive Capabilities of Renal Tubules
and Collecting Ducts
• Aldosterone
– Targets collecting ducts (principal cells) and
distal DCT
– Promotes synthesis of apical Na+ and K+
channels, and basolateral Na+-K+ ATPases
for Na+ reabsorption; water follows
–  little Na+ leaves body; aldosterone absence
 loss of 2% filtered Na+ daily - incompatible
with life
– Functions – increase blood pressure;
decrease K+ levels
© 2013 Pearson Education, Inc.
Reabsorptive Capabilities of Renal Tubules
and Collecting Ducts
• Atrial natriuretic peptide
– Reduces blood Na+  decreased blood
volume and blood pressure
– Released by cardiac atrial cells if blood
volume or pressure elevated
• Parathyroid hormone acts on DCT to
increase Ca2+ reabsorption
© 2013 Pearson Education, Inc.
Tubular Secretion
• Reabsorption in reverse; almost all in PCT
– Selected substances
– K+, H+, NH4+, creatinine, organic acids and
bases move from peritubular capillaries
through tubule cells into filtrate
– Substances synthesized in tubule cells also
secreted – e.g., HCO3-
© 2013 Pearson Education, Inc.
Tubular Secretion
• Disposes of substances (e.g., drugs)
bound to plasma proteins
• Eliminates undesirable substances
passively reabsorbed (e.g., urea and uric
acid)
• Rids body of excess K+ (aldosterone
effect)
• Controls blood pH by altering amounts of
H+ or HCO3– in urine
© 2013 Pearson Education, Inc.
Figure 25.15 Summary of tubular reabsorption and secretion.
Cortex
65% of filtrate volume
reabsorbed
• H2O
• Na+, HCO3−, and
many other ions
• Glucose, amino acids,
and other nutrients
• H+ and NH4+
• Some drugs
Outer
medulla
Regulated reabsorption
• Na+ (by aldosterone;
Cl− follows)
• Ca2+ (by parathyroid
hormone)
Regulated
secretion
• K+ (by
aldosterone)
Regulated
reabsorption
• H2O (by ADH)
• Na+ (by
aldosterone; Cl−
follows)
• Urea (increased
by ADH)
• Urea
Inner
medulla
Regulated
secretion
• K+ (by
aldosterone)
• Reabsorption or secretion
to maintain blood pH
described in Chapter 26;
involves H+, HCO3−,
and NH4+
© 2013 Pearson Education, Inc.
Reabsorption
Secretion
Regulation of Urine Concentration and
Volume
• Osmolality
– Number of solute particles in 1 kg of H2O
– Reflects ability to cause osmosis
© 2013 Pearson Education, Inc.
Regulation of Urine Concentration and
Volume
• Osmolality of body fluids
– Expressed in milliosmols (mOsm)
– Kidneys maintain osmolality of plasma at
~300 mOsm by regulating urine concentration
and volume
– Kidneys regulate with countercurrent
mechanism
© 2013 Pearson Education, Inc.
Countercurrent Mechanism
• Occurs when fluid flows in opposite
directions in two adjacent segments of
same tube with hair pin turn
– Countercurrent multiplier – interaction of
filtrate flow in ascending/descending limbs of
nephron loops of juxtamedullary nephrons
– Countercurrent exchanger - Blood flow in
ascending/descending limbs of vasa recta
© 2013 Pearson Education, Inc.
Countercurrent Mechanism
• Role of countercurrent mechanisms
– Establish and maintain osmotic gradient
(300 mOsm to 1200 mOsm) from renal cortex
through medulla
– Allow kidneys to vary urine concentration
© 2013 Pearson Education, Inc.
Figure 25.16a Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to
produce urine of varying concentration. (1 of 4)
The three key players and their
orientation in the osmotic gradient:
(c) The collecting ducts of
all nephrons use the gradient
to adjust urine osmolality.
300
300
(a) The long nephron loops of
juxtamedullary nephrons create
the gradient. They act as
countercurrent multipliers.
400
600
900
(b) The vasa recta preserve the
gradient. They act as
countercurrent exchangers.
© 2013 Pearson Education, Inc.
1200
The osmolality of the medullary
interstitial fluid progressively
increases from the 300 mOsm of
normal body fluid to 1200 mOsm
at the deepest part of the medulla.
Countercurrent Multiplier: Nephron Loop
• Descending limb
– Freely permeable to H2O
– H2O passes out of filtrate into hyperosmotic
medullary interstitial fluid
– Filtrate osmolality increases to ~1200 mOsm
© 2013 Pearson Education, Inc.
Countercurrent Multiplier: Nephron Loop
• Ascending limb
– Impermeable to H2O
– Selectively permeable to solutes
• Na+ and Cl– actively reabsorbed in thick segment;
some passively reabsorbed in thin segment
– Filtrate osmolality decreases to 100 mOsm
© 2013 Pearson Education, Inc.
The Countercurrent Multiplier
• Constant 200 mOsm difference between
two limbs of nephron loop and between
ascending limb and interstitial fluid
• Difference "multiplied" along length of loop
to ~ 900 mOsm
© 2013 Pearson Education, Inc.
The Countercurrent Exchanger
• Vasa recta
• Preserve medullary gradient
– Prevent rapid removal of salt from interstitial
space
– Remove reabsorbed water
• Water entering ascending vasa recta
either from descending vasa recta or
reabsorbed from nephron loop and
collecting duct 
– Volume of blood at end of vasa recta greater
than at beginning
© 2013 Pearson Education, Inc.
Figure 25.16a Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to
produce urine of varying concentration. (2 of 4)
Long nephron loops of juxtamedullary nephrons create the gradient.
The countercurrent multiplier depends on three properties
of the nephron loop to establish the osmotic gradient.
Fluid flows in the
opposite direction
(countercurrent)
through two
adjacent parallel
sections of a
nephron loop.
The descending
limb is permeable
to water, but not
to salt.
© 2013 Pearson Education, Inc.
The ascending limb
is impermeable to
water, and pumps
out salt.
Figure 25.16a Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to
produce urine of varying concentration. (3 of 4)
Long nephron loops of juxtamedullary nephrons create the gradient.
These properties establish a positive feedback cycle that
uses the flow of fluid to multiply the power of the salt pumps.
Interstitial fluid
osmolality
Start
here
Water leaves the
descending limb
Osmolality of filtrate
in descending limb
© 2013 Pearson Education, Inc.
Salt is pumped out
of the ascending limb
Osmolality of filtrate
entering the ascending
limb
Figure 25.16a Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to
produce urine of varying concentration. (4 of 4)
(continued) As water and solutes are reabsorbed, the loop first concentrates the filtrate, then dilutes it.
Active transport
Passive transport
Water impermeable
300
300
Osmolality of interstitial fluid (mOsm)
300
100
Cortex
1
Filtrate entering the
nephron loop is isosmotic to
both blood plasma and
cortical interstitial fluid.
400
600
300
100
5
400
Filtrate is at its most dilute as it
leaves the nephron loop. At
100 mOsm, it is hypo-osmotic
to the interstitial fluid.
200
4 Na+ and Cl- are pumped out
of the filtrate. This increases the
interstitial fluid osmolality.
Outer
medulla
600
400
900
700
2
Water moves out of the
filtrate in the descending limb
down its osmotic gradient.
This concentrates the filtrate.
900
1200
© 2013 Pearson Education, Inc.
Inner
medulla
3 Filtrate reaches its highest
concentration at the bend of the
loop.
Nephron loop
1200
Figure 25.16b Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to
produce urine of varying concentration.
Vasa recta preserve the gradient.
The entire length of the vasa recta is highly permeable to water
and solutes. Due to countercurrent exchanges between each
section of the vasa recta and its surrounding interstitial fluid, the
blood within the vasa recta remains nearly isosmotic to the
surrounding fluid. As a result, the vasa recta do not undo the
osmotic gradient as they remove reabsorbed water and solutes.
Blood from
efferent
arteriole
To vein
325
300
300
400
The countercurrent
flow of fluid moves
through two adjacent
parallel sections of
the vasa recta.
400
600
600
900
900
© 2013 Pearson Education, Inc.
Vasa recta
1200
Figure 25.16c Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to
produce urine of varying concentration.
Collecting ducts use the gradient.
Under the control of antidiuretic hormone, the collecting
ducts determine the final concentration and volume of
urine. This process is fully described in Figure 25.17.
Collecting duct
400
600
900
© 2013 Pearson Education, Inc.
Urine
1200
Osmolality of interstitial fluid (mOsm)
300
Formation of Dilute or Concentrated Urine
• Osmotic gradient used to raise urine
concentration > 300 mOsm to conserve
water
– Overhydration  large volume dilute urine
• ADH production ; urine ~ 100 mOsm
• If aldosterone present, additional ions removed 
~ 50 mOsm
– Dehydration  small volume concentrated
urine
• Maximal ADH released; urine ~ 1200 mOsm
• Severe dehydration – 99% water reabsorbed
© 2013 Pearson Education, Inc.
Figure 25.17 Mechanism for forming dilute or concentrated urine.
If we were so overhydrated we had no ADH...
If we were so dehydrated we had maximal ADH...
Osmolality of extracellular fluids
Osmolality of extracellular fluids
ADH release from posterior pituitary
ADH release from posterior pituitary
Number of aquaporins (H2O channels) in collecting duct
Number of aquaporins (H2O channels) in collecting duct
H2O reabsorption from collecting duct
H2O reabsorption from collecting duct
Large volume of dilute urine
Small volume of concentrated urine
Collecting
duct
Cortex
100
600
300
400
600
100
Outer
medulla
900
700
900
1200
© 2013 Pearson Education, Inc.
300
300
100
300
300
400
600
400
600
600
900
900
Outer
medulla
Urea
700
900
Urea
100
Inner
medulla
1200
Large volume
of dilute urine
Active transport
Passive transport
150
Cortex
Urea
Inner
medulla
300
100
DCT
100
Osmolality of interstitial fluid (mOsm)
DCT
300
Descending limb
of nephron loop
300
100
1200
1200
1200
Small volume of
Urea contributes to concentrated urine
the osmotic gradient.
ADH increases its
recycling.
Osmolality of interstitial fluid (mOsm)
Descending limb
of nephron loop
Collecting duct
Urea Recycling and the Medullary Osmotic
Gradient
• Urea helps form medullary gradient
– Enters filtrate in ascending thin limb of
nephron loop by facilitated diffusion
– Cortical collecting duct reabsorbs water;
leaves urea
– In deep medullary region now highly
concentrated urea  interstitial fluid of
medulla  back to ascending thin limb 
high osmolality in medulla
© 2013 Pearson Education, Inc.
Diuretics
• Chemicals that enhance urinary output
– ADH inhibitors, e.g., alcohol
– Na+ reabsorption inhibitors (and resultant H2O
reabsorption), e.g., caffeine, drugs for
hypertension or edema
– Loop diuretics inhibit medullary gradient
formation
– Osmotic diuretics - substance not reabsorbed
so water remains in urine, e.g., high glucose
of diabetic patient
© 2013 Pearson Education, Inc.
Clinical Evaluation of Kidney Function
• Urine examined for signs of disease
• Assessing renal function requires both
blood and urine examination
© 2013 Pearson Education, Inc.
Renal Clearance
• Volume of plasma kidneys clear of
particular substance in given time
• Renal clearance tests used to determine
GFR
– To detect glomerular damage
– To follow progress of renal disease
© 2013 Pearson Education, Inc.
Renal Clearance
• C = UV/P
– C = renal clearance rate (ml/min)
– U = concentration (mg/ml) of substance in
urine
– V = flow rate of urine formation (ml/min)
– P = concentration of same substance in
plasma
© 2013 Pearson Education, Inc.
Renal Clearance
• Inulin (plant polysaccharide) is standard used
– Freely filtered; neither reabsorbed nor secreted by
kidneys; its renal clearance = GFR = 125 ml/min
• If C < 125 ml/min, substance reabsorbed
• If C = 0, substance completely reabsorbed, or
not filtered
• If C = 125 ml/min, no net reabsorption or
secretion
• If C > 125 ml/min, substance secreted (most
drug metabolites)
© 2013 Pearson Education, Inc.
Homeostatic Imbalance
• Chronic renal disease - GFR < 60 ml/min
for 3 months
– E.g., in diabetes mellitus; hypertension
• Renal failure – GFR < 15 ml/min
– Causes uremia – ionic and hormonal
imbalances; metabolic abnormalities; toxic
molecule accumulation
– Treated with hemodialysis or transplant
© 2013 Pearson Education, Inc.
Physical Characteristics of Urine
• Color and transparency
– Clear
• Cloudy may indicate urinary tract infection
– Pale to deep yellow from urochrome
• Pigment from hemoglobin breakdown; more
concentrated urine  deeper color
– Abnormal color (pink, brown, smoky)
• Food ingestion, bile pigments, blood, drugs
© 2013 Pearson Education, Inc.
Physical Characteristics of Urine
• Odor
– Slightly aromatic when fresh
– Develops ammonia odor upon standing
• As bacteria metabolize solutes
– May be altered by some drugs and
vegetables
© 2013 Pearson Education, Inc.
Physical Characteristics of Urine
• pH
– Slightly acidic (~pH 6, with range of 4.5 to 8.0)
• Acidic diet (protein, whole wheat)   pH
• Alkaline diet (vegetarian), prolonged vomiting, or
urinary tract infections  pH
• Specific gravity
– 1.001 to 1.035; dependent on solute
concentration
© 2013 Pearson Education, Inc.
Chemical Composition of Urine
• 95% water and 5% solutes
• Nitrogenous wastes
– Urea (from amino acid breakdown) – largest
solute component
– Uric acid (from nucleic acid metabolism)
– Creatinine (metabolite of creatine phosphate)
© 2013 Pearson Education, Inc.
Chemical Composition of Urine
• Other normal solutes
– Na+, K+, PO43–, and SO42–, Ca2+, Mg2+ and
HCO3–
• Abnormally high concentrations of any
constituent, or abnormal components, e.g.,
blood proteins, WBCs, bile pigments, may
indicate pathology
© 2013 Pearson Education, Inc.
Urine transport, Storage, and Elimination:
Ureters
• Convey urine from kidneys to bladder
– Begin at L2 as continuation of renal pelvis
• Retroperitoneal
• Enter base of bladder through posterior
wall
– As bladder pressure increases, distal ends of
ureters close, preventing backflow of urine
© 2013 Pearson Education, Inc.
Ureters
• Three layers of ureter wall from inside out
– Mucosa - transitional epithelium
– Muscularis – smooth muscle sheets
• Contracts in response to stretch
• Propels urine into bladder
– Adventitia – outer fibrous connective tissue
© 2013 Pearson Education, Inc.
Figure 25.19 Cross-sectional view of the ureter wall (10x).
Lumen
Mucosa
• Transitional
epithelium
• Lamina
propria
Muscularis
• Longitudinal
Layer
• Circular
layer
Adventitia
© 2013 Pearson Education, Inc.
Homeostatic Imbalance
• Renal calculi - kidney stones in renal
pelvis
– Crystallized calcium, magnesium, or uric acid
salts
• Large stones block ureter  pressure &
pain
• May be due to chronic bacterial infection,
urine retention, Ca2+ in blood, pH of
urine
• Treatment - shock wave lithotripsy –
noninvasive; shock waves shatter calculi
© 2013 Pearson Education, Inc.
Urinary Bladder
• Muscular sac for temporary storage of
urine
• Retroperitoneal, on pelvic floor posterior to
pubic symphysis
– Males—prostate inferior to bladder neck
– Females—anterior to vagina and uterus
© 2013 Pearson Education, Inc.
Urinary Bladder
• Openings for ureters and urethra
• Trigone
– Smooth triangular area outlined by openings
for ureters and urethra
– Infections tend to persist in this region
© 2013 Pearson Education, Inc.
Urinary Bladder
• Layers of bladder wall
– Mucosa - transitional epithelial mucosa
– Thick detrusor - three layers of smooth
muscle
– Fibrous adventitia (peritoneum on superior
surface only)
© 2013 Pearson Education, Inc.
Urinary Bladder
• Collapses when empty; rugae appear
• Expands and rises superiorly during filling
without significant rise in internal pressure
• ~ Full bladder 12 cm long; holds ~ 500 ml
– Can hold ~ twice that if necessary
– Can burst if overdistended
© 2013 Pearson Education, Inc.
Figure 25.18 Pyelogram.
Kidney
Renal
pelvis
Ureter
Urinary
bladder
© 2013 Pearson Education, Inc.
Figure 25.20a Structure of the urinary bladder and urethra.
Peritoneum
Ureter
Rugae
Detrusor
Adventitia
Ureteric orifices
Trigone of bladder
Bladder neck
Internal urethral sphincter
Prostate
Prostatic urethra
Intermediate part of the urethra
External urethral sphincter
Urogenital diaphragm
Spongy urethra
Erectile tissue of penis
External urethral orifice
Male. The long male urethra has three regions:
prostatic, intermediate, and spongy.
© 2013 Pearson Education, Inc.
Figure 25.20b Structure of the urinary bladder and urethra.
Peritoneum
Ureter
Rugae
Detrusor
Ureteric orifices
Bladder neck
Internal urethral
sphincter
Trigone
External urethral
sphincter
Urogenital diaphragm
Urethra
External urethral
orifice
Female.
© 2013 Pearson Education, Inc.
Urethra
• Muscular tube draining urinary bladder
– Lining epithelium
• Mostly pseudostratified columnar epithelium,
except
– Transitional epithelium near bladder
– Stratified squamous epithelium near external urethral
orifice
© 2013 Pearson Education, Inc.
Urethra
• Sphincters
– Internal urethral sphincter
• Involuntary (smooth muscle) at bladder-urethra
junction
• Contracts to open
– External urethral sphincter
• Voluntary (skeletal) muscle surrounding urethra as
it passes through pelvic floor
© 2013 Pearson Education, Inc.
Urethra
• Female urethra (3–4 cm)
– Tightly bound to anterior vaginal wall
– External urethral orifice
• Anterior to vaginal opening; posterior to clitoris
© 2013 Pearson Education, Inc.
Figure 25.20b Structure of the urinary bladder and urethra.
Peritoneum
Ureter
Rugae
Detrusor
Ureteric orifices
Bladder neck
Internal urethral
sphincter
Trigone
External urethral
sphincter
Urogenital diaphragm
Urethra
External urethral
orifice
Female.
© 2013 Pearson Education, Inc.
Urethra
• Male urethra carries semen and urine
– Three named regions
• Prostatic urethra (2.5 cm)—within prostate
• Intermediate part of the urethra (membranous
urethra) (2 cm)—passes through urogenital
diaphragm from prostate to beginning of penis
• Spongy urethra (15 cm)—passes through penis;
opens via external urethral orifice
© 2013 Pearson Education, Inc.
Figure 25.20a Structure of the urinary bladder and urethra.
Peritoneum
Ureter
Rugae
Detrusor
Adventitia
Ureteric orifices
Trigone of bladder
Bladder neck
Internal urethral sphincter
Prostate
Prostatic urethra
Intermediate part of the urethra
External urethral sphincter
Urogenital diaphragm
Spongy urethra
Erectile tissue of penis
External urethral orifice
Male. The long male urethra has three regions:
prostatic, intermediate, and spongy.
© 2013 Pearson Education, Inc.
Micturition
• Urination or voiding
• Three simultaneous events must occur
– Contraction of detrusor by ANS
– Opening of internal urethral sphincter by ANS
– Opening of external urethral sphincter by
somatic nervous system
© 2013 Pearson Education, Inc.
Micturition
• Reflexive urination (urination in infants)
– Distension of bladder activates stretch
receptors
– Excitation of parasympathetic neurons in
reflex center in sacral region of spinal cord
– Contraction of detrusor
– Contraction (opening) of internal sphincter
– Inhibition of somatic pathways to external
sphincter, allowing its relaxation (opening)
© 2013 Pearson Education, Inc.
Micturition
• Pontine control centers mature between
ages 2 and 3
– Pontine storage center inhibits micturition
• Inhibits parasympathetic pathways
• Excites sympathetic and somatic efferent
pathways
– Pontine micturition center promotes
micturition
• Excites parasympathetic pathways
• Inhibits sympathetic and somatic efferent pathways
© 2013 Pearson Education, Inc.
Figure 25.21 Control of micturition.
Brain
Higher brain
centers
Urinary bladder
fills, stretching
bladder wall
Allow or inhibit micturition
as appropriate
Pontine micturition
center
Afferent impulses
from stretch
receptors
Inhibits micturition by
acting on all three
Spinal efferents
Promotes micturition
by acting on all three
spinal efferents
Simple
spinal
reflex
Pontine storage
center
Spinal
cord
Spinal
cord
Parasympathetic
activity
Sympathetic
activity
Detrusor contracts;
internal urethral
sphincter opens
External urethral
sphincter opens
Micturition
© 2013 Pearson Education, Inc.
Somatic motor
nerve activity
Inhibits
Parasympathetic activity
Sympathetic activity
Somatic motor nerve activity
Homeostatic Imbalance
• Incontinence usually from weakened
pelvic muscles
– Stress incontinence
• Increased intra-abdominal pressure forces urine
through external sphincter
– Overflow incontinence
• Urine dribbles when bladder overfills
© 2013 Pearson Education, Inc.
Homeostatic Imbalance
• Urinary retention
– Bladder unable to expel urine
– Common after general anesthesia
– Hypertrophy of prostate
– Treatment - catheterization
© 2013 Pearson Education, Inc.
Developmental Aspects
• Three sets of embryonic kidneys form in
succession
– Pronephros degenerates but pronephric
duct persists
– Mesonephros claims this duct; becomes
mesonephric duct
– Metanephros develop by fifth week, develops
into adult kidneys and ascends
© 2013 Pearson Education, Inc.
Figure 25.22a Development of the urinary system in the embryo.
Degenerating
pronephros
Developing
digestive tract
Urogenital
ridge
Duct to
yolk sac
Mesonephros
Allantois
Cloaca
Mesonephric duct
(initially, pronephric duct)
Hindgut
Week 5
© 2013 Pearson Education, Inc.
Ureteric bud
Figure 25.22b Development of the urinary system in the embryo.
Degenerating
pronephros
Duct to yolk sac
Allantois
Body stalk
Mesonephros
Mesonephric
duct
Week 6
© 2013 Pearson Education, Inc.
Urogenital sinus
Rectum
Ureteric bud
Metanephros
Developmental Aspects
• Metanephros develops as ureteric buds
that induce mesoderm of urogenital ridge
to form nephrons
– Distal ends of ureteric buds form renal pelves,
calyces, and collecting ducts
– Proximal ends become ureters
• Kidneys excrete urine into amniotic fluid by
third month
• Cloaca subdivides into rectum, anal canal,
and urogenital sinus
© 2013 Pearson Education, Inc.
Figure 25.22c Development of the urinary system in the embryo.
Gonad
Metanephros
(kidney)
Week 7
© 2013 Pearson Education, Inc.
Urogenital
sinus
(developing
urinary
bladder)
Rectum
Figure 25.22d Development of the urinary system in the embryo.
Urinary bladder
Gonad
Urethra
Kidney
Anus
Ureter
Week 8
© 2013 Pearson Education, Inc.
Rectum
Homeostatic Imbalance
• Three common congenital abnormalities
• Horseshoe kidney
– Two kidneys fuse across midline  single Ushaped kidney; usually asymptomatic
• Hypospadias
– Urethral orifice on ventral surface of penis
– Corrected surgically at ~ 12 months
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Homeostatic Imbalance
• Polycystic kidney disease
– Many fluid-filled cysts interfere with function
• Autosomal dominant form – less severe but more
common
• Autosomal recessive – more severe
– Cause unknown but involves defect in
signaling proteins
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Developmental Aspects
• Frequent micturition in infants due to small
bladders and less-concentrated urine
• Incontinence normal in infants: control of
voluntary urethral sphincter develops with
nervous system
• E. coli bacteria account for 80% of all urinary
tract infections
• Untreated childhood streptococcal infections
may cause long-term renal damage
• Sexually transmitted diseases can also inflame
urinary tract
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Developmental Aspects
• Most elderly people have abnormal
kidneys histologically
– Kidneys shrink; nephrons decrease in size
and number; tubule cells less efficient
– GFR ½ that of young adult by age 80
• Possibly from atherosclerosis of renal arteries
• Bladder shrinks; loss of bladder tone 
nocturia and incontinence
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