Urinary System

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Chapter 26

• Urinary System

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I. Functions of the Urinary System

• A. Filtering of blood involves three processes-
  filtration, reabsorption, secretion.
• B. Regulation of
• 1. Blood volume
• 2. Concentration of blood solutes Na, Cl-, K,
  Ca2, HPO4-2
• 3. pH of extracellular fluid secrete H
• 4. Blood cell synthesis
• C. Synthesis of vitamin D

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II. Kidney Anatomy and Histology

• A. Retroperitoneal
• B. Right kidney overshadowed by liver
• C. Renal artery and renal vein enter and exit
  hilum
• D. Protected by layer of fat

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E. Internal Anatomy of Kidneys

• 1. cortex
• 2. pyramids
• 3. renal papilla
• 4. medulla
• 5. renal columns
• 6. calyx
• a. Minor
• b. Major
• 7. renal pelvis
• 8. ureter
• 9. hilum

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F. The Nephron

• Functional and histological unit of the kidney
• Renal corpuscle
• a. Bowmans capsule
• b. Glomerulus
• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule
• Collecting Ducts
• Cortex
• Renal pyramid
• Renal papillae
• Juxtamedullary nephrons (15)
• Cortical nephrons

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G. Renal Corpuscle-General view

• Bowmans capsule
• a. Parietal layer
• b. Visceral layer made up of podocytes
• Glomerulus
• Afferent arteriole
• Efferent arteriole
• Juxtaglomerular cells

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H. Bowmans Capsule more closely

• 1. parietal layer
• 2. visceral layer with podocytes with filtration
  slits
• 3. glomerular capillary covered with visceral
  layer of Bowmans capsule
• 4. juxtaglomerular cells with macula densa
  making up the juxtaglomerular apparatus-site of
  renin production
• 5. notice diameters of afferent and efferent
  arterioles

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I. Filtration Membrane

• 1. Fenestrae
• 2. Filtrations slits

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J. Cells of nephron reflect function

• 1. Proximal tubule simple cuboidal epithelium
  with many microvilli-site of reabsorption
• 2. Loops of Henle-production of osmotic gradient
• Descending limb simple squamous epithelium
• Ascending limb distal part thicker and simple
  cuboidal
• 3. Distal tubule shorter than proximal tubule.
  Simple cuboidal, but smaller cells and very few
  microvilli-site of secretion
• 4. Collecting ducts Larger in diameter, simple
  cuboidal epithelium. Site of urine concentration

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K. Arteries and Veins of the Kidneys

• Afferent arterioles
• Glomerulus
• Efferent arterioles
• Peritubular capillaries form a plexus around the
  proximal and distal tubules
• 5. Vasa recta specialized parts of peritubular
  capillaries that course into medulla along with
  loops of Henle, then back toward cortex

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III. Urine Production A. Three basic steps

• Filtration
• Reabsorption
• Secretion

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B. Amount of Filtration

• Movement of fluid, derived from blood flowing
  through the glomerulus, across filtration
  membrane
• Filtrate water, small molecules, ions that can
  pass through membrane
• Pressure difference forces filtrate across
  filtration membrane
• Renal fraction part of total cardiac output that
  passes through the kidneys. averages 21
• Renal blood flow rate 5600 ml/min X .21 1176
  ml/min
• Renal plasma flow rate renal blood flow rate X
  fraction of blood that is plasma 1176 ml/min X
  .55 650 mL/min
• Filtration fraction part of plasma that is
  filtered into lumen of Bowmans capsules average
  19
• Glomerular filtration rate (GFR) amount of
  filtrate produced each minute. therefore
  650ml/min X .19 124ml/min
• Amount of filtrate formed per day 180
  liters/day
• Average urine production/day 1-2 L. Most of
  filtrate must be reabsorbed

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C. Filtration Pressure GCP-CHP-BCOPChanges in
afferent and efferent arteriole diameter alter
filtration pressureDilation of afferent
arterioles/constriction efferent arterioles
increases glomerular capillary pressure,
increasing filtration pressure and thus
glomerular filtration
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D. Regulation of GFR

• 1. Autoregulation
• Involves changes in degree of constriction in
  afferent arterioles
• Myogenic mechanism As systemic BP increases,
  afferent arterioles constrict and prevent
  increase in renal blood flow
• Tubuloglomerular feedback Increased rate of
  blood flow of filtrate past cells of macula
  densa signal sent to juxtaglomerular apparatus,
  afferent arteriole constricts

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2. Sympathetic regulationnorepinephrine

• a. What do you think will happen?
• b. Constricts small arteries and afferent
  arterioles
• c. Decreases renal blood flow and thus filtrate
  formation
• d. During shock or intense exercise intense
  sympathetic stimulation, rate of filtrate
  formation drops to a few mm
• e. Not a problem over short duration-normal
  response as blood is shunted to skeletal muscle
• f. Prolonged shock can damage kidneys

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E. Tubular Reabsorption in general

• 1. occurs as filtrate flows through the lumens
  of proximal tubule (majority), loop of Henle,
  distal tubule, and collecting ducts
• Processes involve diffusion, facilitated
  diffusion, active transpot, and osmosis
• Substances transported to interstitial fluid and
  reabsorbed into peritubular capillaries
• 99 of filtrate volume is reabsorbed.
• These substances return to general circulation
  through venous system

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F. Reabsorption in PCT

• 1. Substances pass through
• Apical surface
• Basal surface
• Lateral surfaces
• 2. Active transport of Na across the basal
  membrane from cytoplasm to interstitial fluid
  linked to reabsorption of most solutes
• 3. Number of carrier molecules limits rate of
  transport ie. diabetes mellitus
• 4. Filtrate volume reduced by 65 due to osmosis
  of water
• 5. Osmotic pressure of filtrate remains 300 mOsm
  due to permeability char of membrane

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G. Reabsorption in Loop of Henle

• 1. Descending loop of Henle passes into an ever
  increasing concentrated environment. Simple
  squamous epithelium.
• 2. Descending limb is highly permeable to water
  and moderately permeable to urea, sodium, most
  other ions
• 3. In descending limb, water moves out of
  nephron, solutes in. Volume of filtrate reduced
  by another 15.

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Reabsorption in Loop of Henle

• The wall of the ascending limb of the loop of
  Henle is not permeable to water.
• Ascending limb moves Na across the wall of the
  basal membrane by active transport.
• At the end of the loop of Henle, inside of
  nephron is 100 mOsm/kg.
• Interstitial fluid in the cortex is 300mOsm/kg.
• Filtrate within DCT is much more dilute than the
  interstitial fluid which surrounds it.
• Loop of Henle like the trap below the sink in the
  kitchen
• Establishes concentration gradient as you pass
  into the medulla

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H. Reabsorption in DCT and Collecting Duct

• Filtrate which reaches DCT is dilute with respect
  to interstitial fluid
• DCT and collecting duct are the sites where
  hormonal control of urine volume occurs-finishing
  touches
• Two ducts are affected by ADH
• In presence of ADH these two become very
  permeable to water
• In absence of ADH, relatively impermeable
• DCT can also reaborb more sodium ion, but this is
  also under hormonal control of aldosterone

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I. Tubular Secretion

• 1. Moves metabolic by-products, drugs, molecules
  not normally produced by the body into tubule of
  nephron
• 2. Active or passive
• 3. Ammonia produced by epithelial cells of
  nephron from deamination of amino acids. Diffuses
  into lumen
• H, K, penicillin, and substances such as
  para-aminohippuric acid (PAH) actively secreted
  into nephron
• Hydrogen ions are moved into tubule as Na ions
  are reabsorbed-major mechanism of body in
  regulating tissue fluid pH

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Urine Production Summary

• In ascending limb of loop of Henle
• Na, Cl-, K transported out of filtrate
• Water remains
• In distal convoluted tubules and collecting ducts
• Water movement out regulated by ADH
• If absent, water not reabsorbed and dilute urine
  produced
• If ADH present, water moves out, concentrated
  urine produced

• In Proximal convoluted tubules
• Na and other substances removed
• Water follows passively
• Filtrate volume reduced by 65
• In descending limb of loop of Henle
• Water exits passively, solute enters
• Filtrate volume reduced an additional 15-only
  20 of original filtrate remains

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Loop of Henle
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Urine Concentrating Mechanisms
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IV. Regulation of Urine Concentration and
Volume A. Renin/Angiotensin/Aldosterone
Mechanism

• Mainly a mechanism to regulate blood pressure
• Cascade of events occurs to result in Na ion
  reabsorption and therefore water
• Sensory mechanism for the system is located in
  the juxtaglomerular complex
• Two stimuli result in the release of renin from
  this area
• a. Decreased stretch of the afferent
  arteriole of the glomerulus
• b. Decreased concentration of Na ions in
  the distal convoluted tubule
• Released renin converts a liver protein
  angiotensinogen into angiotensin I
• AngiotensinI -gt angiotensinII by a proteolytic
  enzyme found in capillaries of lung
• Enzyme is called angiotensin-converting enzyme
  (ACE)
• Angiotensin II is a very powerful vasoconstrictor
• Angiotensin II also increases aldosterone
  secretion-increases reabsorption from distal
  convoluted tubule

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The juxtaglomerular complex is the site of renin
release
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B. ADH Mechanism

• Mainly regulating osmolarity of the blood and
  tissue fluids
• Secondarily affecting blood pressure
• Osmoreceptors are found in hypothalamus of the
  brain
• Reacts secondarily to pressure receptors in
  atria, carotid sinuses, and aortic arch
• ADH effects the permeability of the distal
  convoluted tubule and collecting ducts
• Water reabsorption before this portion of the
  nephron is obligatory
• Filtrate entering the distal convoluted tubule
  possesses an osmolarity of 100 mOsm
• Depending upon the amount of ADH released,
  urines final osmolarity can vary from 100 to
  1200 mOsm
• From as little as less than 1 liter to 20 liters
  in the total absense of ADH (diabetes insipidus)

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ADH Mechanism
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C. ANH-atrial natriuretic hormone

• Produced by right atrium of heart when blood
  volume increases stretching cells
• Inhibits Na reabsorption
• Inhibits ADH production
• Increases volume of urine produced
• Venous return is lowered, volume in right atrium
  decreases

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Some Youtube sites

• http//www.youtube.com/watch?vuo-NOr-P49I

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V. Urine Movement

• A. Hydrostatic pressure forces urine through
  nephron
• B. Peristalsis moves urine through ureters from
  region of renal pelvis to urinary bladder. C.
  Occur from once every few seconds to once every
  2-3 minutes
• 1. Parasympathetic stimulation increase
  frequency
• 2. Sympathetic stimulation decrease frequency
• D. Ureters enter bladder obliquely through
  trigone. Pressure in bladder compresses ureter
  and prevents backflow

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VI. Anatomy and Histology of Ureters and Bladder

• A. Ureters bring urine from renal pelvis to
  urinary bladder. Lined by transitional epithelium
• B. Urinary bladder hollow muscular container.
  In pelvic cavity posterior to symphysis pubis.
  Lined with transitional epithelium muscle part
  of wall is detrusor

C. Trigone interior of urinary bladder.
Triangular area between the entry of the two
ureters and the exit of the urethra. Area expands
less than rest of bladder during filling
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Anatomy and Histology of Urethra

• Male extends from the inferior part of the
  urinary bladder through the penis
• Female shorter opens into vestibule anterior to
  vaginal opening
• Internal urinary sphincter in males, elastic
  connective tissue and smooth muscle keep semen
  from entering urinary bladder during ejaculation
• External urinary sphincter skeletal muscle
  surrounds urethra as it extends through pelvic
  floor. Acts as a valve