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Chapter 25 Urinary System

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					Chapter 25 Urinary System                                   Inhibit the internal and external sphincters
Urinary System Organs (picture in text)               Physical Characteristics of Urine
Ureters                                                Color and transparency
 Slender tubes that convey urine from the                  Clear, pale to deep yellow (due to
  kidneys to the bladder                                      urochrome)
 Ureters enter the base of the bladder through             Concentrated urine has a deeper yellow
  the posterior wall                                          color
      This closes their distal ends as bladder             Drugs, vitamin supplements, and diet can
       pressure increases and prevents backflow               change the color of urine
       of urine into the ureters                            Cloudy urine may indicate infection of the
Urinary Bladder                                               urinary tract
 Smooth, collapsible, muscular sac that stores        Odor
  urine                                                     Fresh urine is slightly aromatic
      Males – prostate gland surrounds the neck            Standing urine develops an ammonia odor
       inferiorly                                           Some drugs and vegetables (asparagus)
      Females – anterior to the vagina and uterus            alter the usual odor
Urinary Bladder                                        pH
 The bladder wall has three layers                         Slightly acidic (pH 6) with a range of 4.5 to
      Transitional epithelial mucosa                         8.0
      A thick muscular layer                               Diet can alter pH
      A fibrous adventitia                            Specific gravity
 The bladder is distensible and collapses when             Ranges from 1.001 to 1.035
  empty                                                     Is dependent on solute concentration
 As urine accumulates, the bladder expands           Chemical Composition of Urine
  without significant rise in internal pressure        Urine is 95% water and 5% solutes
Urethra                                                Nitrogenous wastes: urea, uric acid, and
 Sphincters keep the urethra closed when urine         creatinine
  is not being passed                                  Other normal solutes include:
      Internal urethral sphincter – involuntary            Sodium, potassium, phosphate, and sulfate
       sphincter at the bladder-urethra junction              ions
      External urethral sphincter – voluntary              Calcium, magnesium, and bicarbonate ions
       sphincter surrounding the urethra as it         Abnormally high concentrations of any urinary
       passes through the urogenital diaphragm          constituents may indicate pathology
      Levator ani muscle – voluntary urethral        Kidney Functions
       sphincter                                       Filter 200 liters of blood daily, allowing toxins,
 The female urethra is tightly bound to the            metabolic wastes, and excess ions to leave the
  anterior vaginal wall                                 body in urine
 Its external opening lies anterior to the vaginal    Regulate volume and chemical makeup of the
  opening and posterior to the clitoris                 blood
 The male urethra has three named regions             Maintain the proper balance between water and
      Prostatic urethra – runs within the prostate     salts, and acids and bases
       gland                                           Gluconeogenesis during prolonged fasting
      Membranous urethra – runs through the           Production of rennin to help regulate blood
       urogenital diaphragm                             pressure and erythropoietin to stimulate RBC
      Spongy (penile) urethra – passes through         production
       the penis and opens via the external            Activation of vitamin D
       urethral orifice                               Internal Anatomy
Micturition (Voiding or Urination)                    Cortex – the light colored, granular
 The act of emptying the bladder                     superficial region
 Distension of bladder walls initiates spinal         Medulla – exhibits cone-shaped medullary
  reflexes that:                                        (renal) pyramids separated by columns
      Stimulate contraction of the external                The medullary pyramid and its surrounding
       urethral sphincter                                     capsule constitute a lobe
      Inhibit the detrusor muscle and internal        Renal pelvis – flat funnel shaped tube lateral to
       sphincter (temporarily)                          the hilus within the renal sinus
 Voiding reflexes:
      Stimulate the detrusor muscle to contract
Internal Anatomy                                        Connecting Tubules
 Major calyces – large branches of the renal            The distal portion of the distal convoluted
  pelvis                                                  tubule nearer to the collecting ducts
      Collect urine draining from papillae                        Function in maintaining the acid-base
      Empty urine into the pelvis                                  balance of the body
 Urine flows through the pelvis and ureters to                    Help maintain the body’s water and salt
  the bladder                                                       balance
Blood and Nerve Supply                                  Capillary Beds of the Nephron
 Approximately one-fourth (1200 ml) of systemic         Blood pressure in the glomerulus is high
  cardiac output flows through the kidneys each           because:
  minute                                                      Arterioles are high-resistance vessels
The Nephron                                                   Afferent arterioles have larger diameters
 Nephrons are the structural and functional units              than efferent arterioles
  that form urine, consisting of:                        Fluids and solutes are forced out of the blood
      Glomerulus – a tuft of capillaries associated      throughout the entire length of the glomerulus
        with a renal tubule                              Peritubular beds are low-pressure, porous
      Glomerular (Bowman’s) capsule – blind,             capillaries adapted for absorption that:
        cup-shaped end of a renal tubule that                 Arise from efferent arterioles
        completely surrounds the glomerulus                   Cling to adjacent renal tubules
      Renal corpuscle – the glomerulus and its               Empty into the renal venous system
        Bowman’s capsule                                 Vasa recta – long, straight efferent arterioles of
      Glomerular endothelium – fenestrated               juxtamedullary nephrons
        epithelium that allows solute-rich, virtually   Vascular Resistance in Microcirculation
        protein-free filtrate to pass from the blood     Afferent and efferent arterioles offer high
        into the glomerular capsule                       resistance to blood flow
Capillary Beds of the Nephron                            Blood pressure declines from 95mm Hg in renal
 Every nephron has two capillary beds                    arteries to 8 mm Hg in renal veins
      Glomerulus                                        Resistance in afferent arterioles:
      Peritubular capillaries                                Protects glomeruli from fluctuations in
 Each glomerulus is:                                           systemic blood pressure
      Fed by an afferent arteriole                      Resistance in efferent arterioles:
      Drained by an efferent arteriole                       Reinforces high glomerular pressure
Anatomy of the Glomerular Capsule                             Reduces hydrostatic pressure in peritubular
 The visceral layer consists of modified,                      capillaries
  branching epithelial podocytes                        Juxtaglomerular Apparatus (JGA)
 Extensions of the octopus-like podocytes               Where the distal tubule lies against the afferent
  terminate in foot processes                             (sometimes efferent) arteriole
 Filtration slits – openings between the foot           Arteriole walls have juxtaglomerular (JG) cells
  processes that allow filtrate to pass into the              Enlarged, smooth muscle cells
  capsular space                                              Have secretory granules containing renin
Renal Tubule                                                  Act as mechanoreceptors
 Proximal convoluted tubule (PCT) – composed            Macula densa
  of cuboidal cells with numerous microvilli and              Tall, closely packed distal tubule cells
  mitochondria                                                Lie adjacent to JG cells
      Reabsorbs water and solutes from filtrate              Function as chemoreceptors or
        and secretes substances into it                         osmoreceptors
 Loop of Henle – a hairpin-shaped loop of the          Juxtaglomerular Apparatus (JGA)
  renal tubule                                          Filtration Membrane
      Proximal part is similar to the proximal          Filter that lies between the blood and the
        convoluted tubule                                 interior of the glomerular capsule
      Proximal part is followed by the thin             It is composed of three layers
        segment (simple squamous cells) and the               Fenestrated endothelium of the glomerular
        thick segment (cuboidal to columnar cells)              capillaries
 Distal convoluted tubule (DCT) – cuboidal cells             Visceral membrane of the glomerular
  without microvilli that function more in secretion            capsule (podocytes)
  than reabsorption                                           Basement membrane composed of fused
                                                                basal laminae of the other layers
Mechanisms of Urine Formation                               Epinephrine is released by the adrenal
 The kidneys filter the body’s entire plasma                medulla
  volume 60 times each day                                  Afferent arterioles constrict and filtration is
 The filtrate:                                              inhibited
      Contains all plasma components except            The sympathetic nervous system also stimulates
       protein                                           the renin-angiotensin mechanism
      Loses water, nutrients, and essential ions      Renin-Angiotensin Mechanism
       to become urine                                  Is triggered when the JG cells release renin
 The urine contains metabolic wastes and               Renin acts on angiotensinogen to release
  unneeded substances                                    angiotensin I
 Urine formation and adjustment of blood               Angiotensin I is converted to angiotensin II
  composition involves three major processes            Angiotensin II:
      Glomerular filtration                                Causes mean arterial pressure to rise
      Tubular reabsorption                                 Stimulates the adrenal cortex to release
      Secretion                                               aldosterone
Glomerular Filtration                                   As a result, both systemic and glomerular
 Principles of fluid dynamics that account for          hydrostatic pressure rise
  tissue fluid in all capillary beds apply to the      Renin Release
  glomerulus as well                                    Renin release is triggered by:
 The glomerulus is more efficient than other               Reduced stretch of the granular JG cells
  capillary beds because:                                   Stimulation of the JG cells by activated
      Its filtration membrane is more permeable               macula densa cells
      Glomerular blood pressure is higher                  Direct stimulation of the JG cells via  1-
      It has a higher net filtration pressure                 adrenergic receptors by renal nerves
 Plasma proteins are not filtered and are used to          Angiotensin II
  maintain oncotic pressure of the blood               Tubular Reabsorption
Glomerular Filtration Rate (GFR)                        A transepithelial process whereby most tubule
 The total amount of filtrate formed per minute         contents are returned to the blood
                                                                    2+   2+    +             +
  by the kidneys                                        Only Ca , Mg , K , and some Na are
 Factors governing filtration rate at the capillary     reabsorbed via paracellular pathways
  bed are:                                             Tubular Reabsorption
      Total surface area available for filtration      All organic nutrients are reabsorbed
      Filtration membrane permeability                 Water and ion reabsorption is hormonally
      Net filtration pressure                           controlled
Regulation of Glomerular Filtration                     Reabsorption may be an active (requiring ATP)
 If the GFR is too high:                                or passive process
      Needed substances cannot be reabsorbed          Tubular Secretion
       quickly enough and are lost in the urine         Essentially reabsorption in reverse, where
 If the GFR is too low:                                 substances move from peritubular capillaries or
      Everything is reabsorbed, including wastes        tubule cells into filtrate
       that are normally disposed of                    Tubular secretion is important for:
 Three mechanisms control the GFR                          Disposing of substances not already in the
      Renal autoregulation (intrinsic system)                 filtrate
      Neural controls                                      Eliminating undesirable substances such as
      Hormonal mechanism (the renin-                          urea and uric acid
       angiotensin system)                                  Ridding the body of excess potassium ions
Intrinsic Controls                                          Controlling blood pH
 Under normal conditions, renal autoregulation        Developmental Aspects
  maintains a nearly constant glomerular filtration     Infants have small bladders and the kidneys
  rate                                                   cannot concentrate urine, resulting in frequent
Extrinsic Controls                                       micturition
 When the sympathetic nervous system is at             Control of the voluntary urethral sphincter
  rest:                                                  develops with the nervous system
      Renal blood vessels are maximally dilated        E. coli bacteria account for 80% of all urinary
 Under stress:                                          tract infections
      Norepinephrine is released by the                Sexually transmitted diseases can also inflame
       sympathetic nervous system                        the urinary tract

				
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