24 PART 1
The Urinary System
The Urinary System
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Important functions of the kidneys
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Maintain the chemical consistency of blood
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Filter many liters of fluid from blood
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Send toxins, metabolic wastes, and excess water out of the body
• Main waste products are three nitrogenous compounds
• Urea
• Uric acid
• Creatinine
Organs of the Urinary System
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Kidneys
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Ureters
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Urinary bladder
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Urethra
Location and External Anatomy of Kidneys
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Kidneys are red-brown in color
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Located retroperitoneally
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Behind the peritoneum
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Lateral to T12–L3 vertebrae
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Average kidney is 12 cm tall, 6 cm wide, 3 cm thick
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Hilum
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Is the concave surface
• Vessels and nerves enter and exit
Location and External Anatomy of Kidneys
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Fibrous capsule
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Capsule of dense connective tissue surrounds the kidney
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Inhibits spread of infections
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Perirenal fat capsule
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External to renal capsule
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Renal fascia
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External to perirenal fat capsule
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Contains fat
Internal Gross Anatomy of the Kidneys
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Frontal section through the kidney
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Renal cortex
• Superficial region, granular appearance
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Renal medulla consists of
• Cone-shaped renal pyramids
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Renal pelvis
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Major calices
Minor calices
Internal Gross Anatomy of the Kidneys
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Gross vasculature
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Renal arteries branch into segmental arteries
• Segmental arteries branch into interlobar arteries
• Arcuate arteries branch from interlobar arteries
Internal Gross Anatomy of the Kidneys
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Nerve supply—renal plexus
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A network of autonomic fibers
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An offshoot of the celiac plexus
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Supplied by sympathetic fibers from
• Lowest thoracic splanchnic nerve
• First lumbar splanchnic nerve
Microscopic Anatomy of the Kidneys
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Nephron is the functional unit of the kidney
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Over 1 million nephrons in each kidney
Mechanisms of Urine Production
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Filtration
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Filtrate of blood leaves kidney capillaries
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Resorption
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Most nutrients, water, and essential ions reclaimed
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Secretion
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Active process of removing undesirable molecules
Nephron Structure
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Nephron is composed of
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Renal tubule
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Renal corpuscle
24 PART 2
The Urinary System
Nephron Structure
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Renal corpuscle—first part of nephron
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Glomerulus and glomerular capsule
• Glomerulus—tuft of capillaries
• Capillaries of glomerulus are fenestrated
• Glomerular (Bowman’s) capsule
• Parietal layer—simple squamous epithelium
• Visceral layer—consists of podocytes
Filtration Membrane
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The filtration membrane
• Filter that lies between blood in the glomerulus and capsular space
• Consists of three layers
• Fenestrated endothelium of the capillary
• Filtration slits between foot processes of podocytes
• Basement membrane
Filtration Membrane
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Basement membrane and slit diaphragm
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Hold back most proteins
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Allow passage of
• Water
• Ions
• Glucose
• Amino acids
• Urea
Renal Tubule
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Filtrate proceeds to renal tubules from glomerulus
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Proximal convoluted tubule
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Nephron loop
• Descending limb
• Descending thin limb (DTL)
• Ascending thin limb (ATL)
• Thick ascending limb (TAL)
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Distal convoluted tubule
Renal Tubule
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Collecting ducts
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Receive urine from several nephrons
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Play an important role in conserving body fluids
• Posterior pituitary secretes ADH
• Increases permeability of collecting ducts and distal convoluted tubules to
water
Classes of Nephron
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Cortical nephrons
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85% of nephrons
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Juxtamedullary nephrons
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15% of nephrons
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Contribute to kidney’s ability to concentrate urine
Blood Vessels Associated with Nephrons
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Nephrons associate closely with two capillary beds
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Glomeruli
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Peritubular capillaries in cortical nephrons or vasa recta in juxtamedullary
nephrons
Blood Vessels Associated with Nephrons
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Glomeruli
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Produce filtrate that becomes urine
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Fed and drained by arterioles
• Afferent glomerular arteriole
• Efferent glomerular arteriole
Blood Vessels Associated with Nephrons
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Glomeruli
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Efferent arteriole has a smaller diameter than afferent arteriole
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Generate 1 liter of fluid every 8 minutes
• 99% of filtrate is resorbed by tubules
Blood Vessels Associated with Nephrons
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Peritubular capillaries
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Arise from the efferent arterioles draining cortical glomeruli
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Are adapted for absorption
• Low-pressure, porous capillaries
• All molecules secreted by nephrons into urine are from peritubular capillaries
Blood Vessels Associated with Nephrons
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Vasa recta
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Continue from efferent arterioles of juxtamedullary nephrons
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Are thin-walled looping vessels
• Descend into the medulla
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Are part of the kidney’s urine concentrating mechanism
24 PART 3
The Urinary System
Juxtaglomerular Complex
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Juxtaglomerular complex
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Functions in regulating blood pressure
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An area of specialized contact between terminal end of the ascending limb and
afferent arteriole
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Granular cells—modified smooth muscle cells with secretory granules
• Contain the hormone renin
• Renin—secreted in response to falling blood pressure in afferent arteriole
Juxtaglomerular Complex
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Macula densa—end of nephron loop
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Adjacent to granular cells
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Tall, closely packed epithelial cells
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Monitor solute concentration in the filtrate
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Signal granular cells to secrete renin
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Initiates renin-angiotensin mechanism
Juxtaglomerular Complex
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Mesangial cells
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Located around base of the glomerulus
• Regulate blood flow within the glomerulus
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Extraglomerular mesangial cells
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Interact with macula densa and granular cells
• Help regulate blood pressure
Ureters
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Carry urine from the kidneys to the urinary bladder
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Oblique entry into bladder prevents backflow of urine
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Histology of ureter
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Mucosa—transitional epithelium
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Muscularis—two layers
• Inner longitudinal layer
• Outer circular layer
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Adventitia—typical connective tissue
Urinary Bladder
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A collapsible muscular sac
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Stores and expels urine
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Full bladder—spherical
• Expands into the abdominal cavity
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Empty bladder—lies entirely within the pelvis
Urinary Bladder
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Urachus—closed remnant of the allantois
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Prostate
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In males
• Lies directly inferior to the bladder
• Surrounds the urethra
Urinary Bladder
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Urinary bladder is composed of three layers
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Mucosa—transitional epithelium
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Thick muscular layer—detrusor
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Fibrous adventitia
Urethra
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Epithelium of urethra
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Transitional epithelium
• At the proximal end (near the bladder)
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Stratified and pseudostratified columnar—mid urethra (in males)
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Stratified squamous epithelium
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At the distal end (near the urethral opening)
Urethra
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Internal urethral sphincter
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Involuntary smooth muscle
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External urethral sphincter
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Voluntarily inhibits urination
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Relaxes when one urinates
Urethra
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In females
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Length of 3–4 cm
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In males—20 cm in length; three named regions
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Prostatic urethra
• Passes through the prostate gland
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Intermediate part of urethra
• Through the urogenital diaphragm
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Spongy (penile) urethra
• Passes through the length of the penis
Disorders of the Urinary System
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Urinary tract infections
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More common in females
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Burning sensation during micturition
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Renal calculi
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Kidney stones
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Bladder cancer
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3% of cancers—more common in men
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Kidney cancer
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Arises from epithelial cells of uriniferous tubules
The Urinary System Throughout Life
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Embryo develops three pairs of kidneys
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Pronephros
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Mesonephros
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Metanephros
• Only metanephros persists to become the adult kidneys
• Metanephric kidney produces urine by fetal month 3
• Contributes to the volume of amniotic fluid
The Urinary System Throughout Life
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Kidney and bladder function declines with advancing age
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Nephrons decrease in size and number
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Tubules are less efficient at secretion and resorption
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Filtration declines
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Recognition of desire to urinate is delayed
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Loss of muscle tone in the bladder