BIOL 2304
The Urinary System
Functions of the Urinary System
Homeostatic regulation of blood plasma
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
Kidneys – produce urine
Ureters – transport urine to bladder
Urinary bladder – stores urine
Urethra – transports urine to exterior
Urinary System in Gross Dissection
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Location and Size of Kidneys
Located retroperitoneally –
Within the abdominal cavity, behind the peritoneum
Lateral to T12 – L3 vertebrae
Average kidney
12 cm tall, 6 cm wide, 3 cm thick
External Anatomy of Kidneys
Hilum
A cleft on concave surface
Vessels, ureters, and nerves enter and exit
Renal capsule – a fibrous capsule of dense CT that surrounds the kidney
Anatomy and Vasculature of the Kidneys
Internal Gross Anatomy
Renal cortex
Renal medulla
Renal pyramids
Renal columns (between
pyramids)
Renal papillae (base of pyramids)
Renal pelvis
Minor calyces (drain to
major calyx)
Major calyces (drain to
ureter)
Gross Vasculature:
Renal arteries
Branch into segmental arteries
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Blood Flow Through the Kidneys
Anatomy of the Kidneys
Superficial outer cortex and inner medulla
The medulla consists of 6-18 renal pyramids
The cortex is composed of roughly 1.25 million nephrons
Two or three minor calyces converge to form a major calyx
Major calyces along with the pelvis drain urine to the ureters
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Nephron – The Functional Unit of Kidney
Nephron consists of:
Renal corpuscle
Bowman’s capsule
Glomerulus
Renal tubule:
Proximal convoluted
tubule (PCT)
Loop of Henle
Distal convoluted
tubule (DCT)
Nephron empties tubular fluid into a
system of collecting ducts and
papillary ducts
Two Types of Nephron
Cortical nephrons
~85% of all nephrons
Located in the cortex
Juxtamedullary nephrons
Closer to renal medulla
Loops of Henle extend deep into renal pyramids
Renal Corpuscle
Consists of:
Glomerulus (Glomerular capillaries) – tuft of fenestrated capillaries
Bowman’s capsule (Glomerular 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
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Renal Corpuscle
Renal Corpuscle
Podocytes cover lamina densa of capillaries
Project into the capsular space
Pedicels of podocytes separated by filtration slits
Nephron
Proximal convoluted tubule (PCT)
Actively reabsorbs nutrients, plasma proteins and ions from
filtrate
Released into peritubular fluid
Loop of Henle
Descending limb
Ascending limb
Each limb has a thick and thin section
Distal convoluted tubule (DCT)
Actively secretes ions, toxins, drugs
Reabsorbs sodium ions from tubular fluid
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Collecting Tubules (Collecting ducts)
Collecting tubules - Receive urine from distal convoluted tubules
Uriniferous Tubule Epithelium
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Types Of Capillary Beds In Nephron
Glomerular capillaries – fed and drained by afferent and efferent arterioles
Peritubular capillaries (juxtamedullary and cortical nephrons)
Arise from efferent arterioles
Low-pressure, porous capillaries
Absorb solutes
Vasa recta (juxtamedullary nephrons)
Thin-walled looping vessels
Part of the kidney’s urine-concentrating mechanism
Overview of Nephron
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Mechanisms of Urine Production
Filtration – filtrate of blood leaves kidney capillaries
Reabsorption – most nutrients, water, and essential ions reclaimed
Secretion – active process of removing undesirable molecules
JGA and Macula Densa
Juxtaglomerular apparatus (JGA)
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
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Summary of Nephron Function
Concentration of urine controlled by filtrate
modification
Each segment of nephron and collecting system
contributes
Renal corpuscle
PCT
Loop of Henle
Thin descending limb
Thick ascending limb
DCT
Collecting ducts
Urine Excretion
Filtrate leaves Collecting System as urine
Enters renal pelvis
Rest of the urinary system transports, stores, and
eliminates
Ureters
Bladder
Urethra
The Ureters
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
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Histology of Ureter
Mucosa – transitional epithelium
Muscularis – two layers
Inner longitudinal layer
Outer circular layer
Adventitia – typical connective tissue
Urinary Bladder
A collapsible muscular sac
Stores and expels urine
Full bladder – spherical
Expands into the abdominal cavity
Empty bladder – lies entirely within the pelvic cavity
Urinary Bladder
Wall of bladder:
Mucosa – covered in transitional epithelium; folds form
rugae
Transitional epithelium - ranges from appearing cuboidal when bladder is empty to appearing
squamous when bladder is full
Rugae – large folds in the mucosa that allow the bladder to expand as needed
Muscular layer – detrusor muscle (smooth muscle)
Adventitia – connective tissue layer
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The Urethra
Extends from the urinary bladder to the exterior of the body
Passes through urogenital diaphragm
Differs in length and function in males and females
Internal urethral sphincter – involuntary smooth muscle
External urethral sphincter – voluntary skeletal muscle
Urinary Bladder and Urethra – Male
Males – average 22 cm in length
Three 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
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Urinary Bladder and Urethra – Female
Females – length of 3–4 cm
The smooth triangular region of the base is called the trigone – many bladder infections persist in this region
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Urethra
Epithelium of urethra:
Transitional epithelium at the proximal end (nearest the bladder in both sexes)
Stratified and pseudostratified columnar – mid urethra (in males)
Stratified squamous epithelium at the distal end (near the urethral opening in both sexes)
Micturition (Urination)
Micturition – the discharge of urine from the body
Bladder can hold 250 – 400 mL
Urination coordinated by micturition reflex
Reflex initiated by stretch receptors in wall of bladder
Detrusor muscle in bladder wall contracts
Internal sphincter relaxes
Urination also requires voluntary relaxation of external urethral sphincters
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