The Urinary System

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					   Chapter 26:
The Urinary System
An Introduction to the Urinary System

                                Figure 26–1
3 Functions of the Urinary System

1. Excretion:
  –   removal of organic wastes from body
2. Elimination:
  –   discharge of waste products
3. Homeostatic regulation:
  –   of blood plasma volume and solute
• Organs that excrete urine
             Urinary Tract
• Organs that eliminate urine:
  – ureters (paired tubes)
  – urinary bladder (muscular sac)
  – urethra (exit tube)
      Urination or Micturition
• Process of eliminating urine
• Contraction of muscular urinary
  bladder forces urine through urethra,
  and out of body
 5 Homeostatic Functions of Urinary System
1. Regulate blood volume and blood pressure:
  –   by adjusting volume of water lost in urine
  –   releasing erythropoietin and renin
2. Regulate plasma ion concentrations:
  –   sodium, potassium, and chloride ions (by
      controlling quantities lost in urine)
  –   calcium ion levels (through synthesis of calcitriol)
3. Help stabilize blood pH:
  –   by controlling loss of hydrogen ions and
      bicarbonate ions in urine
The Position of the Kidneys
        • Are located either side of
          vertebral column:
          – left kidney lies superior to
            right kidney
          – superior surface capped by
            adrenal gland
        • Position is maintained by:
          – overlying peritoneum
          – contact with adjacent visceral
          – supporting connective tissues

                                   Figure 26–2
• Is protected and stabilized by 3 concentric
  layers of connective tissue:
   1. renal capsule
    1. A layer of collagen fibers
    2. Covers outer surface of entire organ
   2. adipose capsule
    1. A thick layer of adipose tissue
    2. Surrounds renal capsule
  3. renal fascia
    1. A dense, fibrous outer layer
    2. Anchors kidney to surrounding structures
       Typical Adult Kidney

• Is about 10 cm long, 5.5 cm wide, and
  3 cm thick
• Weighs about 150 g
• Point of entry for
  renal artery and
  renal nerves
• Point of exit for
  renal vein and
                Renal Capsule
• Bound to outer surfaces of structures in renal
• Stabilizes positions of ureter, renal blood
  vessels, and nerves
                Renal Cortex
• Superficial portion of kidney in contact with
  renal capsule
• Reddish brown and granular
                 Renal Pyramids
• 6 to 18 distinct conical or triangular structures
  in renal medulla:
  – base abuts cortex
  – tip (renal papilla) projects into renal sinus
                 Major Calyx
• Formed by 4 or 5 minor calyces
Blood Supply to the Kidneys
               • Kidneys receive 20–
                 25% of total
                 cardiac output
               • 1200 ml of blood
                 flows through
                 kidneys each
               • Kidney receives
                 blood through
                 renal artery

                              Figure 26–5
Functional Anatomy of Nephron & Collecting System

                                          Figure 26–6
• Consists of
  renal tubule
  and renal
• Microscopic,
  structures in
  cortex of each
  renal lobe
• Where urine
Renal Tubule
• Long tubular
• Begins at renal

 Renal Corpuscle
• Spherical structure
  consisting of:
  – Bowman’s capsule
  – cup-shaped chamber
  – capillary network
• Consists of 50
• Blood delivered via
  afferent arteriole
• Blood leaves in
  efferent arteriole
• Occurs in renal
• Blood pressure:
  – forces water and
    dissolved solutes
    out of glomerular
    capillaries into
    capsular space
  – produces protein-
    free solution
    (filtrate) similar to
    blood plasma
      3 Functions of Renal Tubule
1. Reabsorb useful organic nutrients that enter
2. Reabsorb more than 90% of water in filtrate
3. Secrete waste products that failed to enter
   renal corpuscle through filtration at
Renal Tubule
• Located in
  – proximal
    tubule (PCT)
  – distal convoluted
    tubule (DCT)
• Separated by
  loop of Henle:
  – U-shaped tube
  – extends partially
    into medulla
Collecting Ducts
• Receive fluid
  from many
• Each collecting
  – begins in cortex
  – descends into
  – carries fluid to
    papillary duct
    that drains into a
    minor calyx
           The Renal Corpuscle
• Each renal corpuscle:
  – is 150–250 µm in diameter
  – includes Bowman’s capsule and glomerulus
• Blood pressure:
  – forces water and small solutes across membrane
    into capsular space
• Larger solutes, such as plasma proteins, are
      Filtration at Renal Corpuscle
   • Is passive
   • Solutes enter capsular space:
      – metabolic wastes and excess ions
      – glucose, free fatty acids, amino acids, and
The Loop of Henle
• Also called nephron loop
• Renal tubule turns
  toward renal medulla:
  – leads to loop of Henle
• Descending limb:
  – fluid flows toward renal
• Ascending limb:
  – fluid flows toward renal
• Each limb contains:
  – thick segment
  – thin segment
The Distal Convoluted
Tubule (DCT)
• The third segment of
  the renal tubule
• Initial portion passes
  between afferent and
  efferent arterioles
• Has a smaller
  diameter than PCT
• Epithelial cells lack
The Collecting System
• The distal
  convoluted tubule:
  – opens into the
    collecting system
• Individual nephrons:
  – drain into a nearby
    collecting duct
• Several collecting ducts:
  – converge into a larger
    papillary duct
  – which empties into a
    minor calyx
                    Renal Physiology
• The goal of urine production:
  –   is to maintain homeostasis
  –   by regulating volume and composition of blood
  –   including excretion of metabolic waste products
      • Urea
         –   Due to breakdown of aa
      • Creatinine
         –   Due to breakdown of creatinine kinase (important in muscle
      • Uric acid
     Organic Waste Products

• Are dissolved in bloodstream
• Are eliminated only while dissolved in
• Removal is accompanied by water loss
• Concentrated urine:
   Differences between Solute
Concentrations in Urine and Plasma

                              Table 26–2
       Renal Threshold for Glucose
• Is approximately 180 mg/dl
• If plasma glucose is greater than 180 mg/dl:
  – Glomular will increase filtrate of glucose
  – glucose appears in urine


   • Is the appearance of glucose in urine
    Renal Threshold for Amino Acids
• Is lower for glucose (65 mg/dl)
• Amino acids commonly appear in urine:
  – after a protein-rich meal


• Is the appearance of amino acids in urine
• Lactic acidosis:
  – develops after exhaustive muscle activity
     • (bulging muscles can cut off blood supply)
  – due to anaerobic respiration
• Ketoacidosis:
  – Lower blood pH, higher acid, due to presence of ketones
  – develops in starvation or diabetes
  – Body does not have suff. glucose/glycogen to sustain met
     • Muscle loss can occur - dieting
       ADH – antidiuretic hormone
• Hormone causes special water channels to
• Increases rate of osmotic water movement
• Higher levels of ADH increases:
  – number of water channels
  – water permeability of DCT and collecting system
• No ADH, water is not reabsorbed
  – All fluid reaching DCT is lost in urine
    producing large amounts of dilute urine
              The Hypothalamus
• Continuously secretes low levels of ADH:
• At normal ADH levels:
  – collecting system reabsorbs 16,800 ml fluid/ day
    (9.3% of filtrate)
• A healthy adult produces:
  – 1200 ml urine per day (0.6% of filtrate)

• Are drugs that promote water loss in
  urine (diuresis)
• Diuretic therapy reduces:
  – blood volume
  – blood pressure
  – extracellular fluid volume
        The Composition of Urine
• Results from filtration, absorption, and
  secretion activities of nephrons
• Some compounds (such as urea) are neither
  actively excreted nor reabsorbed along
• Organic nutrients are completely reabsorbed:
  – other compounds missed by filtration process
    (e.g., creatine)
• The Concentration of components
  – in a urine sample depends on osmotic
    movement of water

•Normal Urine
  •Is a clear, sterile solution
  •Yellow color (pigment urobilin) generated in kidneys from
  urobilinogens (breakdown of Bilirubin from erythrocytes)
A Summary of Renal Function

                          Figure 26–16a
         Urine Transport,
     Storage, and Elimination
• Takes place in the urinary tract:
  – ureters
  – urinary bladder
  – urethra
Organs for the Conduction
  and Storage of Urine

                            Figure 26–18a
Organs for the Conduction
  and Storage of Urine

                            Figure 26–18b
Organs for the Conduction
  and Storage of Urine

                            Figure 26–18c
                    The Ureters
• Are a pair of muscular tubes
• Extend from kidneys to urinary bladder
• Begin at renal pelvis
• attached to posterior abdominal wall
• Penetrate posterior wall of the urinary bladder
• Pass through bladder wall at oblique angle
• Ureteral openings are slitlike rather than
• Shape helps prevent backflow of urine:
    – when urinary bladder contracts
       3 Layers of the Ureter Wall
• Inner mucosa:
  – transitional epithelium and lamina propria
• Middle muscular layer:
  – longitudinal and circular bands of smooth muscle
• Outer connective-tissue layer:
  – continuous with fibrous renal capsule and
        Peristaltic Contractions

•   Begin at renal pelvis
•   Sweep along ureter
•   Force urine toward urinary bladder
•   Every 30 seconds
             The Urinary Bladder
• Is a hollow, muscular organ
• Functions as temporary reservoir urine storage
• Full bladder can contain 1 liter of urine
                Bladder Position
• Is stabilized by several peritoneal folds
• Posterior, inferior, and anterior surfaces:
  – lie outside peritoneal cavity
• Ligamentous bands:
  – anchor urinary bladder to pelvic and pubic bones
Umbilical Ligaments
• Are vestiges of 2
  umbilical arteries
• Middle umbilical
  ligament extends:
  – from anterior,
    superior border
  – toward umbilicus
• Lateral umbilical
  – pass along sides of
    bladder to umbilicus
                   The Mucosa
• Lining the urinary bladder has folds (rugae):
  – that disappear as bladder fills

  The Trigone of the Urinary Bladder
• Is a triangular area bounded by:
  – openings of ureters
  – entrance to urethra
• Acts as a funnel:
  – channels urine from bladder into urethra
       The Urethral Entrance
• Lies at apex of trigone:
  – at most inferior point in urinary bladder
                The Urethra
• Extends from neck of urinary bladder
• To the exterior of the body

             The Male Urethra
• Extends from neck of urinary bladder
• To tip of penis (18–20 cm)
 How is urination regulated
voluntarily and involuntarily
 and what is the micturition
            The Micturition Reflex
• Coordinates the process of urination
• As the bladder fills with urine:
  – stretch receptors in urinary bladder (>500 ml):
     • stimulate pelvic nerve
  – stimulus travels from pelvic nerves:
     • stimulate ganglionic neurons in wall of bladder
  – postganglionic neuron in intramural ganglion:
     • stimulates detruscor muscle contraction
  – interneuron relays sensation to thalamus and
    deliver sensation to cerebral cortex
  – voluntary relaxation of external thus internal
    urethral sphincter
• Lack voluntary control over urination
• Corticospinal connections are not established

 Incontinence-Is the inability to control
 urination voluntarily
Age-Related Changes in Urinary System
• Decline in number of functional nephrons
• Reduced sensitivity to ADH
• Problems with micturition reflex
      3 Micturition Reflex Problems
1. Sphincter muscles lose tone:
  –   leading to incontinence
2. Control of micturition can be lost due to:
  –   a stroke
  –   Alzheimer’s disease
  –   CNS problems affecting cerebral cortex or
3. In males, urinary retention may develop if
   enlarged prostate gland compresses the
   urethra and restricts urine flow

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