Chapter 26: The Urinary System

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Chapter 26 The Urinary System
Primary sources for figures and content Marieb,
E. N. Human Anatomy Physiology. 6th ed. San
Francisco Pearson Benjamin Cummings,
2004. Martini, F. H. Fundamentals of Anatomy
Physiology. 6th ed. San Francisco Pearson
Benjamin Cummings, 2004.
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Urinary System

• Components
• Kidneys
• Urinary Tract
• Ureters
• Urinary Bladder
• Urethra

Figure 261
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Functions of the Urinary System

• Excretion by the Kidneys
• removal of organic wastes from body fluids
• Elimination by the Urinary Tract
• discharge of waste products
• Homeostatic regulation of plasma volume and
  solute concentrations by the kidneys
• Blood volume, BP
• Concentration of ions
• Stabilize blood pH
• Conserve nutrients
• Assist liver deamination, detoxification

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Functions of the Urinary System

• 4. Other kidney functions
• Gluconeogenesis during starvation
• Produce renin to regulate BP
• Produce erythropoietin for RBC production
• Convert Vitamin D to calcitriol for calcium
  absorption in the GI tract

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The location and structures of the kidneys.
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Gross Anatomy of the Urinary System
Figure 263
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Kidneys

• 1 body weight
• Retroperitoneal, posterior abdominal wall
• Adrenal gland anchored superior
• 3 layers CT anchor kidneys
• Renal capsule
• - collagen fibers covering organ
• Adipose capsule
• - adipose cushion around the renal capsule
• Renal fascia
• - collagen fibers fused to renal capsule and deep
  fascia of body wall and peritoneum

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Kidneys

• Renal ptosis floating kidney
• Cause ? Starvation or injury
• Result ? Kidney becomes loose from body wall
• Kidney could twist blood vessels or ureters

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The Structure of the Kidney
Figure 264
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Kidney

• Hilum (Hilus)
• Point of entry for renal artery and renal nerves
• Ureters enter and exit
• Hilus opens to renal sinus
• Renal sinus lined with renal capsule that is
  contiguous with outside

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Kidney

• Kidney has two layers
• 1. Cortex superficial
• Contact renal capsule
• Houses filtration structures nephrons
• 2. Medulla 6-18 renal pyramids
• Parallel bundles of collection tubules
• Apex papilla, points toward renal sinus
• Kidney divided into sections renal lobes
• Renal lobe
• renal pyramid surrounding cortex called renal
  columns
• Lobe is site for urine production

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Urine Production

• Nephron (cortex) ? collecting ducts (medulla) ?
  papilla ? minor calyx? major calyx ? renal pelvis
• Renal pelvis
• Fills majority of real sinus
• Funnels urine into ureters
• Pyelonephritis
• Inflammation of kidney
• Infection usually enters from ureter and spreads
  up through ducts to nephron

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Blood Supply to the Kidneys
Figure 265
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Blood Supply and Innervation to Kidney

• Receives 20-25 cardiac output
• Highly vascularized, many capillaries involved in
  filtration (nephrons)
• Innervation from Renal Plexus controlled by ANS
• Most is sympathetic to
• Adjust rate of urine formation
• - Change BP and flow at nephron
• Stimulate release of renin
• - Restricts water and sodium loss at nephron

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Blood Supply and Innervation to Kidney

• Two important capillary beds associated with each
  nephron
• 1. Glomerulus filtration
• 2. Peritubular capillaries
• - reclaim filtrate, concentrate urine
• Both connected to arterioles only (not for oxygen
  exchange)
• Afferent arteriole ? capillary ? efferent
  arteriole

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Glomerulus

• Consists of 50 intertwining capillaries
• Blood delivered via afferent arteriole
• Blood leaves in efferent arteriole
• flows into peritubular capillaries
• which drain into small venules
• and return blood to venous system

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The Nephron and Collecting System
Figure 266
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Nephron

• Two types of nephrons
• Cortical nephron Majority
• In cortex, short loop of Henle
• Juxtamedullary nephrons 15
• At cortex/medulla interface
• Long loops of Henle
• Important for water conservation and concentrated
  urine

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Cortical and Juxtamedullary Nephrons
Figure 267
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Renal Corpuscle

• Site of filtration
• 2 parts
• 1. Glomerular capsule
• Thin parietal epithelium, forms capsule around
  glomerulus
• 2. Glomerulus
• Fenestrated capillaries covered by podocytes
• Podocytes are on the visceral epithelium
• Wrapped around the capillaries, to create
  filtration slits on surface of capillaries
• Slits smaller than fenestrations in glomerular
  capillaries to restrict filtration of large
  molecules

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The Renal Corpuscle
Figure 268
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Renal Corpuscle

• Golmerulonephritis
• Inflammation of glomeruli
• Prevents filtration
• Can be result of antigen/Ab complexes trapped in
  filtration slits following allergy or blood
  infection

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The Nephron and Collecting System
Figure 266
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Renal Tubule

• Reabsorption to process raw filtrate into urine
• 3 parts
• 1. PCT (proximal convoluted tubules)
• Simple cuboidal epithelium with microvilli
• Reabsorbs organic nutrients, ions, water, small
  plasma proteins from filtrate exiting glomerular
  capsule
• 2. Loop of Henle
• Simple squamous epithelium
• Reabsorbs Na, Cl-, H2O form filtrate
• Important to regulate volume and solute
  concentration of urine
• Descending and ascending limbs

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Renal Tubule

• 3. DCT (distal convoluted tubules)
• Simple cuboidal epithelium
• Flat surface
• Four important functions
• Secretion
• Reabsorb Na and Ca from filtrate
• Optional H2O reabsorption from filtrate under
  hormonal control
• Contribute to formation of Juxtaglomerular
  Apparatus

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Table 261
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Juxtaglomerular Apparatus (JGA)

• Consists of two cell types
• Endocrine cells of DCT macula densa
• Granular cells of apparent arteriole
  Juxtaglomerular cells
• Together cells monitor blood and produce
• Renin Enzyme, restricts Na and H2O at nephron
• Erythropoietin hormone, stimulates RBC production

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Collecting System

• Collecting ducts papillary ducts nephrons ? 1
  collecting duct (renal pyramid) ? Many collecting
  ducts ? 1 papillary duct
• Final osmotic concentration of filtrate adjusted
  by collecting duct, after this urine is complete
  and exits kidney
• Papillary ducts (renal papilla) ? minor calyx ?
  major calyx ? renal pelvis ? ureter
• Polycystic Kidney Disease
• Genetic, cysts form that cause swelling of kidney
  tubules, compression reduces function

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KEY CONCEPT

• Kidneys remove waste products from blood
• Nephrons are primary functional units of kidneys
• Kidneys help regulate
• blood volume and pressure
• ion levels
• blood pH

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Which portion of the nephron is NOT in the renal
cortex?

1. proximal convoluted tubule
2. distal convoluted tubule
3. collecting duct
4. loop of Henle

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Why dont plasma proteins pass into the capsular
space under normal circumstances?

1. glomerular capillary pores are too small
2. glomerular blood pressure is too low
3. glomerular filtration rate is too low
4. glomerular blood flow is too slow

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Damage to which part of the nephron would
interfere with the control of blood pressure?

1. juxtaglomerular apparatus
2. Bowmans capsule
3. PCT
4. loop of Henle

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Renal Physiology
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Renal Physiology

• Urinary system functions to regulate blood volume
  and concentration
• Removes wastes and produce urine
• Filtrate
• Everything in blood plasma except large proteins
  and cells
• Urine metabolic waste, 1 filtrate

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Common Wastes

• 1. Urea from catabolism of amino acids
• 2. Creatinine from catabolism or damage of
  skeletal muscle tissue
• Creatine phosphate is energy storage of muscle
• 3. Uric acid from recycling of RNA
• 4. Urobilin from breakdown of hemoglobin (yellow
  color)
• All wastes excreted as solution in water
• Loss of filtering ? toxic waste buildup
• death in a few days
• Dialysis ? blood filtering machine used for
  patients with kidney failure

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Urine Formation

• 1. Glomerular Filtrations
• Blood hydrostatic pressure forces water and
  solutes through glomerular wall
• 2. Tubular Reabsorption
• Selective uptake of water and solutes from
  filtrate
• 3. Tubular Secretion
• Transport of wastes from capillaries to tubules

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An Overview of Urine Formation
Figure 269 (Navigator)
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1. Glomerular Filtration

• Occurs through filtration membrane
• Fenestrated endothelium of glomerular capillaries
  
• - Restricts cells
• Podocytes (visceral epithelium of capsule)
• - Filtration slits restrict solutes protein sized
  and larger
• Fused basal lamina for both

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1. Glomerular Filtration

• Filtrations depends on
• Large surface area
• High glomerular blood pressure
• Good permeability
• Glomerular Filtration Rate (GFR)
• Amount of filtrate kidneys produce/minute
• 125 ml/min ? 180L/day
• 99 reabsorbed, 1 lost as urine
• Drop in blood pressure decrease GFR
• Decrease 15 BP 0 GFR

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Glomerular Filtration

• Filtration is passive but all small solutes
  escape e.g. glucose, amino acids, etc.

Figure 2610
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Regulation of Filtrations3 Levels of GFR Control

1. Autoregulation (local level)
2. Hormonal regulation (initiated by kidneys)
3. Autonomic regulation (by sympathetic division of
   ANS)

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1. Autoregulation of GFR

• Functions to maintain constant GFR with normal
  blood pressure fluctuations in systemic arteriole
  pressure
• A. Reduced blood flow/BP triggers
• Dilation of afferent arteriole and glomerular
  capillaries
• Constriction of efferent arteriole
• All functions to INCREASE PRESSURE at the
  glomerulus to INCREASE GFR
• B. High blood flow/BP triggers
• Constriction of afferent arteriole and glomerular
  capillaries
• Dilation of efferent arteriole
• All functions to DECREASE PRESSURE at the
  glomerulus to DECREASE GFR

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2. Hormonal Regulation

• Extrinsic regulation aimed at maintaining
  systemic blood pressure
• A. Renin Enzyme released by juxtaglomerular
  apparatus in response to
• 1. Decline in BP in kidney
• 2. Decline in osmotic concentration of filtrate
• 3. Direct sympathetic stimulation

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2. Hormonal Regulation

• A. Renin
• Renin activates angiotensin in blood to form
  Angiotensin II which triggers
• 1. Arteriole constriction to elevate BP
• 2. Secretion of aldosterone from adrenal glands
• Aldosterone promotes sodium reabsorption in
  kidney tubules
• 3. Thirst
• 4. Release of ADH from pituitary (ADH promotes
  water uptake in tubules)
• Effect
• Increase blood volume
• Decrease urine production

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2. Hormonal Regulation

• B. Natriuretic Peptide
• Hormone released in response to stretching in
  heart or aorta (increase blood volume)
• Triggers
• 1. Dilation of afferent arteriole
• 2. Constriction of efferent arteriole
• Effect
• Increase GFR
• Increase Urine Production
• Decrease blood volume

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3. Autonomic Nervous System Regulation

• Sympathetic causes vasoconstriction
• Decrease GFR
• Decrease Urine Production
• Prolonged sympathetic stimulation can cause
  hypoxia of kidneys and waste accumulation in blood

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Response to Reduction in GFR
Figure 2611
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KEY CONCEPT

• Glomeruli produce about 180 L of filtrate per day
  (70 times plasma volume)
• Almost all fluid volume must be reabsorbed to
  avoid fatal dehydration

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What nephron structures are involved in
filtration?

1. glomerular capillaries, lamina densa, and
   filtration slits of the podocytes
2. filtration slits of the podocytes, PCT
3. PCT, lamina densa, descending loop of Henle
4. glomerular capillaries, PCT

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What occurs when the plasma concentration of a
substance exceeds its tubular maximum?

1. glomerular blood pressure increases
2. filtration shuts down
3. excess is excreted in urine
4. glomerular osmotic pressure decreases

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How would a decrease in blood pressure affect the
GFR?

1. increase
2. decrease
3. cause random fluctuations
4. no effect due to glomerular compensation

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2. Reabsorption and 3. Secretion

• Reabsorption
• recovers useful materials from filtrate
• Secretion
• ejects waste products, toxins, and other
  undesirable solutes

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2. Tubular Reabsorption

• Transport proteins in renal tubule cells
• Return substances from filtrate to plasma
• When carrier proteins are saturated by the
  substance they carry ? the renal threshold for
  that substance has been reached. All additional
  amounts of that substance will be lost in urine
• E.g. Glycosuria glucose in urine
• Glucose levels in blood/filtrate exceed renal
  threshold

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2. Tubular Reabsorption

• PCT reabsorption
• PCT reabsorbs 60-70 of filtrate
• Reabsorption of 99 of organic nutrients by
  facilitated diffusion and cotransport
• Passive reabsorption of ions by diffusion
• Selective reabsorption of ions by active
  transport
• - Ion pumps controlled by hormones
• Reabsorption of water by osmosis
• - Water follows ions

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2. Tubular Reabsorption

• Loop of Henle reabsorption
• Functions to concentrate filtrate
• Reabsorbs half remaining water and 2/3 Na and
  Cl- by countercurrent multiplications
• Ascending limb pumps ions from filtrate to
  medulla
• High ion concentration then causes water to move
  by osmosis out of descending limb

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Countercurrent Multiplication

• Is exchange that occurs between 2 parallel
  segments of loop of Henle
• the thin, descending limb
• the thick, ascending limb
• Countercurrent Refers to exchange between
  tubular fluids moving in opposite directions
• fluid in descending limb flows toward renal
  pelvis
• fluid in ascending limb flows toward
  medulla/cortex
• Multiplication Refers to effect of exchange
• increases as movement of fluid continues

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Countercurrent Multiplication and Concentration
of Urine
Figure 2613a (Navigator)
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Countercurrent Multiplication and Concentration
of Urine
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The Thin Descending Limb

• Is permeable to water, impermeable to solutes
• As tubular fluid flows along thin descending
  limb
• osmosis moves water into peritubular fluid
• leaving solutes behind
• osmotic concentration of tubular fluid increases
• Normal Maximum Solute Concentration
• Of peritubular fluid near turn of loop of Henle
  1200 mOsm/L
• The Concentration Gradient of the Medulla
• 2/3 (750 mOsm/L) from Na and Cl pumped out of
  ascending limb
• Remainder from urea

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2 Benefits of Countercurrent Multiplication

• Efficiently reabsorbs solutes and water
• before tubular fluid reaches DCT and collecting
  system
• Establishes concentration gradient
• that permits passive reabsorption of water from
  tubular fluid in collecting system

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2. Tubular Reabsorption

• DCT reabsorption
• Aldosterone promotes Na uptake and K loss via
  sodium potassium pump
• Parathyroid hormone and calcitriol promote Ca
  uptake
• ADH stimulates water uptake

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3. Tubular Secretion

• Selectively removes solutes from blood ? delivers
  them to filtrate
• 1. Dispose of drugs and wastes that were not
  filtered
• 2. Eliminate wastes that were reabsorbed
• 3. Rid body of excess K
• 4. Control blood pH Remove H
• CO2 H2O ?? H2CO3 ?? H HCO3-
• Bicarbonate ions used to buffer blood pH but H
  must be secreted into filtrate
• Secretion carried out mostly by DCT, but some
  also occurs in collecting ducts

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KEY CONCEPT

• Reabsorption involves diffusion, osmosis,
  channel-mediated diffusion, and active transport
• Many processes are independently regulated by
  local or hormonal mechanisms
• The primary mechanism governing water
  reabsorption is water follows salt
• Secretion is a selective, carrier mediated
  process

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What effect would increased amounts of
aldosterone have on the K concentration of urine?

1. increase
2. decrease
3. no effect
4. impossible to predict

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What effect would a decrease in the Na
concentration of filtrate have on the pH of
tubular fluid?

1. higher
2. lower
3. no effect
4. impossible to predict

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How would the lack of juxtamedullary nephrons
affect the volume and osmotic concentration of
urine?

1. increase volume decrease osmotic concentration
2. decrease volume decrease osmotic concentration
3. increase volume increase osmotic concentration
4. decrease volume increase osmotic concentration

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Why does a decrease in the amount of Na in the
distal convoluted tubule lead to an increase in
blood pressure?

1. Because it increases renin production.
2. Because it decreases water content in blood.
3. Because it increases filtration rate.
4. Because it increases water loss through kidneys.

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Control of Water Volume

• Control of Water Volume
• Obligatory water reabsorption occurs by osmosis
  in PCT and descending loop of Henle
• Cannot be prevented
• Facultative water reabsorption can occur in DCT
  and collecting ducts
• Usually impermeable
• ADH causes formation of water channels by
  triggering insertion of aquaporin proteins in
  cell membrane of DCT and collecting ducts
• Aquaporins allow more osmosis to concentrate
  urine and conserve water

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The Effects of ADH on the DCT and Collecting Duct
Figure 2615 (Navigator)
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Control of Water Volume

• Control of Water Volume
• 1. Diuretics Substance that cause water loss
• Osmostic diuretics
• Substances that cannot be reabsorbed and thus
  take water with then
• Hypertension and edema meds
• Prevent Na uptake
• Water follows salt
• Alcohol
• Inhibits ADH preventing facultative water
  reabsorption

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Control of Water Volume

• Control of Water Volume
• 2. Diabetes insipidus not enough ADH
• Produce large quantities of dilute urine
• Up to 24 L/day, normal 1.2 L/day
• 3. Anuria low urinary output
• Less than 150 ml/day
• Usually due to events that block filtration
• Nephritis
• Immune reactions
• Crushing injuries

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Urine Transport, Storage, Elimination

• Urine Transport, Storage and Elimination
• Urine production and modification
• Renal tubules and collecting system
• Once in renal pelvis ? Urine Complete ? excreted
  via ureters, bladder, urethra
• Nephrolithiasis Blockage of urinary passage
• E.g. Calculi (kidney stones)
• Crystallized deposits of calcium, magnesium, or
  uric acid
• Form in renal pelvis, can become lodged in
  ureters
• Large ones may need disruption by a lithotripter

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Ureters

• Ureters
• Connect renal pelvis to urinary bladder
• Wall layers
• Mucosa, with transitional epithelium
• Muscularis, with two layers of smooth muscle
• Adventitia, attaches to posterior body wall
• Contractions occur every 30 sec to force urine
  toward bladder

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Urinary Bladder

• Urinary Bladder
• Wall folded into rugae when empty expands
• Wall layers
• Mucosa with transitional epithelium
• Muscularis with 3 layers of smooth muscle
  detrusor muscle
• 1. Contraction causes expulsion of urine from
  bladder
• 2. Detrusor muscle thickened around urethral
  opening to create the internal urethral sphincter
• Provides involuntary control over release of
  urine
• 3. Adventitia Fibrous, Anchors bladder to
  pelvic
• floor

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Urethra

• Urethra
• Single tube, connects bladder to environment
• Lined with pseudostratified columnar epithelium
• Passes through band of skeletal muscle that forms
  external urethral sphincter
• under voluntary control
• relaxation results in micturition

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Micturition Reflex

• When bladder contains 200ml urine
• 1. Stretch receptors triggered
• 2. Signal conscious awareness of pressure
• 3. Stimulates contraction of detrusor muscle
• Voluntary maintenance of contracted external
  urethral sphincter prevents urination
• 1. Detrusor will relax
• 2. Opening will open internal urethral sphincter
• 3. Urination will occur
• Continued increase in urinary volume will
  repeatedly trigger reflex

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Micturition Reflex

• If volume exceeds 500 ml
• Forced relaxation of internal and external
  urethral sphincters will result in non-voluntary
  urination/micturition
• Incontinence
• Inability to voluntarily control urine excretion
• Due to
• loss of muscle tone
• Damage to sphincters
• Damage to nerves or control centers in brain

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The Micturition Reflex
Figure 2620 (Navigator)
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Age Related Changes

• 1. Decline in functional nephrons
• 2. Reduction in GFR
• Damage or decrease blood flow
• 3. Reduced sensitivity to ADH dilute urine
• 4. Problems with micturition
• Incontinence
• Urinary retention, enlarged prostate

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What effect would a high-protein diet have on the
composition of urine?

1. increased urea
2. increased potassium
3. increased fluid volume
4. A and C are correct

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An obstruction of a ureter by a kidney stone
would interfere with the flow of urine between
which two points?

1. ureter and urethra
2. renal medulla and renal pelvis
3. renal medulla and urethra
4. renal pelvis and urinary bladder

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The ability to control the micturition reflex
depends on your ability to control which muscle?

1. urogenital diaphragm
2. internal urinary sphincter
3. external urinary sphincter
4. coccygeus

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Urinalysis

• Is the analysis of a urine sample
• an important diagnostic tool
• Includes color and appearance of urine

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General Characteristics of Normal Urine
Table 265
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Typical Values Obtained from Standard Urinalysis
Table 266
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A Summary of Renal Function
Figure 2616a
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A Summary of Renal Function
Figure 2616b
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Step 1 Glomerulus

• Filtrate produced at renal corpuscle has the same
  composition as blood plasma
• without plasma proteins

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Step 2 Proximal Convoluted Tubule (PCT)

• Active removal of ions and organic substrates
• produces osmotic water flow out of tubular fluid
• reduces volume of filtrate
• keeps solutions inside and outside tubule isotonic

92
Step 3 PCT and Descending Limb

• Water moves into peritubular fluids, leaving
  highly concentrated tubular fluid
• Reduction in volume occurs by obligatory water
  reabsorption

93
Step 4 Thick Ascending Limb

• Tubular cells actively transport Na and Cl out
  of tubule
• Urea becomes higher proportion of total osmotic
  concentration

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Step 5 DCT and Collecting Ducts

• Final adjustments in composition of tubular fluid
  
• Osmotic concentration is adjusted through active
  transport (reabsorption or secretion)

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Step 6 DCT and Collecting Ducts

• Final adjustments in volume and osmotic
  concentration of tubular fluid
• Exposure to ADH determines final urine
  concentration

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Organs for the Conduction and Storage of Urine
Figure 2618a
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Organs for the Conduction and Storage of Urine
Figure 2618b
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Organs for the Conduction and Storage of Urine
Figure 2618c
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Organs that Collect and Transport Urine
Figure 2619
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SUMMARY

• Urinary system functions
• excretion
• Elimination
• Kidneys
• Urine
• Urination (micturition)
• Nephron
• Renal corpuscle
• Renal tubule
• Filtrate
• Tubular fluid
• Collecting system
• Loop of Henle

101
SUMMARY

• Urine formation
• Filtration
• Reabsorption
• Secretion
• Glomerulus filtration
• Countercurrent multiplication
• Ureters
• Urinary bladder
• Urethra
• Micturition reflex