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					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
                Uric acid
       Elimination - the discharge of waste products into the

Urinary System
        Kidneys – produce urine
        Ureters – transport urine to bladder
        Urinary bladder – stores urine
        Urethra – transports urine to exterior

Urinary System in Gross Dissection

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
               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
              Renal papillae (base of pyramids)
              Renal pelvis
                      Minor calyces (drain to
                      major calyx)
                      Major calyces (drain to
      Gross Vasculature:
              Renal arteries
              Branch into segmental arteries

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

Nephron – The Functional Unit of Kidney
      Nephron consists of:
              Renal corpuscle
                      Bowman’s capsule
              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

                                                     Renal Corpuscle

Renal Corpuscle
       Podocytes cover lamina densa of capillaries
               Project into the capsular space
               Pedicels of podocytes separated by filtration slits

      Proximal convoluted tubule (PCT)
              Actively reabsorbs nutrients, plasma proteins and ions from
              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

Collecting Tubules (Collecting ducts)
        Collecting tubules - Receive urine from distal convoluted tubules

                                             Uriniferous Tubule Epithelium

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

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

Summary of Nephron Function
     Concentration of urine controlled by filtrate
     Each segment of nephron and collecting system
             Renal corpuscle
             Loop of Henle
                      Thin descending limb
                      Thick ascending limb
             Collecting ducts

Urine Excretion
        Filtrate leaves Collecting System as urine
        Enters renal pelvis
        Rest of the urinary system transports, stores, and

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

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
                         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

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

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

          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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