Chapter 26: The Urinary System

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Chapter 26 The Urinary System

• BIO 211 Lecture
• Instructor Dr. Gollwitzer

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• Today in class we will discuss
• The interrelationship between the CVS and urinary
  system
• The major functions of the urinary system
• Excretion
• Elimination
• Homeostatic regulation
• The basic principles of urine formation
• Major functions of each portion of the nephron
  and collecting system
• The 3 basic processes involved in urine formation
• Glomerular filtration
• Filtration pressures
• Tubular reabsorption
• Tubular secretion

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CVS and Urinary System

• CVS delivers nutrients (from digestive tract) and
  O2 (from lungs) to cells in peripheral tissues
• CVS carries CO2 and waste products from
  peripheral tissues to sites of excretion
• CO2 removed at lungs
• Most physiological waste products removed by
  urinary system

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Major Functions of Urinary System

• Excretion
• Elimination
• Homeostatic regulation of
• Blood plasma volume
• Solute concentration

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Major Functions of Urinary System

• Excretion
• Removal of organic wastes (e.g., urea, uric acid,
  creatinine) from body fluids ( urine formation)
• Performed by kidneys which act as filtering units
• Elimination
• Discharge of waste products into environment
  (urination)
• Occurs when urinary bladder contracts and forces
  urine through urethra and out of body

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Major Functions of Urinary System Homeostatic
Regulation

• Regulation of blood volume (water balance) and BP
• Adjusts volume of water lost in urine
• Releases
• Renin
• Involved in production of angiotensin II that
  affects BP, thirst, and other hormones (ADH,
  aldosterone) that affect water retention by
  kidneys
• Erythropoietin
• Stimulates erythropoiesis in bone marrow,
  maintains RBC volume

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Major Functions of Urinary System Homeostatic
Regulation

• Regulation of plasma ion concentrations
  (electrolyte balance)
• Controls amounts lost in urine (e.g., Na, K,
  Cl-)
• Controls Ca2 levels by synthesis of calcitriol
• Reabsorption (conservation) of valuable nutrients
• Recycles valuable nutrients
• e.g., amino acids, glucose
• Prevents excretion in urine

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Major Functions of Urinary System Homeostatic
Regulation

• Stabilization of blood pH (acid-base balance)
• Controls loss of H and HCO3- in urine
• Detoxification
• Of poisons, e.g., drugs
• Deamination
• Removes NH2 (amino group) so amino acids can be
  metabolized

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Basic Principles of Urine Formation

• Urine fluid containing
• Water
• Ions
• Soluble compounds
• Goal of urine production
• To maintain homeostasis
• By regulating volume and composition of blood

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Basic Principles of Urine Formation

• Involves excretion of solutes
• (i.e., metabolic/organic waste products)
• Urea
• Most abundant
• Produced by breakdown of amino acids
• Creatinine
• Generated in skeletal muscle by breakdown of
  creatine phosphate (CP, high energy compound that
  plays a role as energy source in muscle
  contraction)
• Uric acid
• Formed by recycling nitrogenous bases from RNA

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Basic Principles of Urine Formation

• Waste products dissolved in bloodstream can only
  be eliminated when dissolved in urine
• Thus removal accompanied by unavoidable water
  loss
• To avoid dehydration, kidneys concentrate
  filtrate (i.e., reabsorb water) produced by
  glomerular filtration

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Functional Anatomy of Nephron and Collecting
System
Figure 266
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3 Processes Involved in Urine Formation

• Glomerular filtration
• Forces water and solutes out of blood in
  glomerulus into capsular space
• ? filtrate
• Tubular reabsorption
• Recovers useful materials from filtrate
• Tubular secretion
• Ejects waste products, toxins, and other
  undesirable solutes into tubules

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Glomerular Filtration

• Occurs in renal corpuscle
• Hydrostatic pressure forces water and solutes
• Out of blood in glomerulus
• Into capsular space ? filtrate
• Occurs solely on basis of size
• Small solute molecules carried with filtrate

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Glomerular Filtration

• Involves passage across filtration membrane which
  is composed of 3 cellular units
• Glomerular capillary endothelium
• Lamina densa
• Filtration slits

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Glomerular Filtration

• Glomerular capillary endothelium
• Filtered through pores in fenestrated capillaries
• Least selective filter
• Pores too small for RBCs to pass through
• Large enough for plasma proteins

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Renal Corpuscle
Figure 268
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Glomerular Filtration

• Lamina densa
• Basement membrane of glomerular capillaries
• More selective filter
• Blocks passage of large proteins
• Only small polypeptides, nutrients, and ions can
  cross

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Figure 2610, 7th edition
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Glomerular Filtration

• Filtration slits
• Gaps between pedicels of podocytes (visceral
  epithelium around glomerulus)
• Finest filter
• No polypeptides pass through
• Only nutrients, ions into capsular space
• Thus, glomerular filtrate
• Does not contain plasma proteins or polypeptides
• Does contain small organic molecules (e.g.,
  nutrients) and ions in same concentration as in
  plasma

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Filtration Pressures

• Filtration pressure balance between
• Hydrostatic (fluid) pressures
• Glomerular hydrostatic pressure (GHP) in
  capillaries (50 mmg Hg)
• Capsular hydrostatic pressure (CHP) (15 mm Hg)
• Blood osmotic pressure (BOP) (25 mm Hg)

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Filtration Pressures

• Hydrostatic (fluid) pressures
• Glomerular hydrostatic pressure (GHP) (50 mm Hg)
• BP in glomerular capillaries
• Higher in glomerulus than in peripheral
  capillaries (35 mm Hg)
• Because efferent arteriole smaller in diameter
  than afferent arteriole, need higher BP to force
  blood into it
• Promotes filtration pushes water and solutes
  out of plasma in capillaries into filtrate
• Opposed by

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Filtration Pressures

• Hydrostatic (fluid) pressures
• Capsular hydrostatic pressure (CHP) (15 mm Hg)
• Opposes filtration pushes water and solutes out
  of filtrate into plasma in capillaries
• Results from resistance to flow along nephron and
  conducting system that causes water to collect in
  Bowmans capsule
• More water in capsule ? more pressure

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Filtration Pressures

• Blood osmotic pressure (BOP) (25 mm Hg)
• Results from presence of suspended proteins in
  blood
• Promotes return of water into glomerulus
• Opposes filtration
• Tends to draw water out of filtrate and into
  plasma

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Figure 2610, 7th edition
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Summary of Filtration Pressures

• Hydrostatic pressures
• GHP (pushing out of glomerulus) 50 mm Hg
• CHP (pushing into glomerulus) 15 mm Hg
• Net 35 mm Hg (pushing out of glomerulus)
• Osmotic pressure
• BOP (draws into glomerulus) 25 mm Hg
• Filtration pressure 10 mm Hg
• Difference between net hydrostatic pressure and
  blood osmotic pressure

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Summary of Filtration Pressures

• Problems that affect filtration pressure
• Can seriously disrupt kidney function
• Can cause a variety of clinical symptoms, e.g.,
• Drop in systolic pressure from 120 to lt 110 mm Hg
  would eliminate filtration pressure (10 mm Hg)

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• Today in class we will discuss
• The 3 basic processes involved in urine formation
• Glomerular filtration
• Glomerular Filtration Rate
• Renal Failure
• Tubular reabsorption
• PCT, Loop of Henle Countercurrent Exchange,DCT
• Collecting System
• Tubular secretion
• PCT, DCT and Collecting system
• Urine
• Compare/contrast to plasma
• General characteristics
• Hormone influence of volume and concentration
• Voluntary involuntary regulation of urination
  and the micturition reflex

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Glomerular Filtration Rate (GFR)

• Gomerular filtration
• Vital first step essential to all other kidney
  functions
• Must occur so
• Waste products excreted
• pH controlled
• Blood volume maintained
• GFR amount of filtrate kidneys produce per
  minute
• Avg GFR 125 mL/min or 50 gal/day (out of 480
  gallons of blood flow/day)
• 10 of fluid delivered by renal arteries enters
  capsular spaces
• 99 of this reabsorbed so urinate only 0.5
  gallons/day

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Glomerular Filtration Rate (GFR)

• Measured using creatinine clearance test (CCT)
• Breakdown of CP in muscle ? creatinine
• Creatinine enters filtrate at glomerulus and is
  not reabsorbed so is excreted in urine
• Can compare amount of creatinine in blood vs. in
  urine during 24 hour and estimate GFR
• If glomerulus damaged, GFR will be altered (have
  more or less creatinine in urine than normal)

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Glomerular Filtration Rate (GFR)

• GFR depends on
• Adequate blood flow to glomerulus
• Maintenance of normal filtration pressures
• Affected by anything that reduces renal blood
  flow or BP, e.g.,
• Hypotension, hemorrhage, shock, dehydration
• Decreased renal blood volume and/or BP ?
  decreased filtration pressure ? decreased GFR

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Control of GFR

• GFR increased by
• EPO (relatively minor)
• Renin-angiotensin system
• Natriuretic peptides (ANP and BNP)

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Control of GFR

• Decreased BP and/or blood volume ?
• Decreased O2 ? JGA ? EPO ?
• Increased RBCs ?
• Increased O2 delivery
• Increased blood volume ? increased BP ?
• Increased filtration pressure
• Increased GFR
• Decreased renal blood flow ? JGA ?
  renin-angiotensin system ?
• Increased blood volume ? increased BP ?
• Increased filtration pressure
• Increased GFR

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EPO and Renin
Figure 1819b
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Renin-Angiotensin System

• Renin (enzyme) ? (prohormone) angiotensinogen ?
  (hormone) angiotensin I (in liver)
• Angiotensin I ? angiotensin II (in lung
  capillaries)
• Angiotensin II ? increased blood volume and BP ?
  increased GFR

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Primary Effects of Angiotensin II

• Stimulates constriction of efferent arterioles ?
  increased glomerular pressure
• Directly stimulates reabsorption of Na and H2O
  in DCT ? increased blood volume and BP
• Stimulates adrenal cortex ? aldosterone ?
  reabsorption of Na (and H2O) ? increased blood
  volume and BP
• Stimulates posterior pituitary ? ADH ?
  reabsorption of H2O ? increased blood volume and
  BP
• Stimulates thirst ? increased blood volume and BP
• Stimulates vasoconstriction of arterioles

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Renin-Angiotensin System Response to Reduction
in GFR
Figure 2611-0
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Control of GFR

• Increased blood volume or BP ? stretched cardiac
  muscle cells ? natriuretic peptides
• ANP atrial NP
• BNP brain NP (produced by ventricles)
• Natriuretic peptides
• Increase GFR
• Decrease blood volume and BP
• Via 2 mechanisms

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Natriuretic Peptides Increase GFR

• Act opposite to angiotensin II
• Increase Na and H2O loss
• Inhibit renin release
• Inhibit secretion of aldosterone and ADH
• Suppress thirst
• Prevent increased BP by angiotensin II and NE
• Increase glomerular pressures
• Dilate afferent arterioles
• Constrict efferent arterioles
• Also increase tubular reabsorption of Na
• Decreases blood volume and BP

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Renal Failure

• When filtration (GFR) slows, urine production
  decreases
• Symptoms appear because water, ions, and
  metabolic wastes retained rather than excreted
• Almost all systems affected fluid balance, pH,
  muscular contraction, neural function, digestive
  function, metabolism
• Leads to
• Hypertension (due to blood backing up)
• Anemia due to lack of erythropoietin production
• CNS problems (sleepiness, seizures, delirium,
  coma, death)

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Renal Failure

• Acute renal failure
• From exposure to toxic drugs, renal ischemia,
  urinary obstruction, trauma
• Develops quickly, but usually temporary
• With supportive treatment can survive
• Chronic renal failure
• Condition deteriorates gradually
• Cannot be reversed
• Dialysis or kidney transplant may prolong life

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Reabsorption and Secretion

• Occur in all segments of renal tubules
• Relative importance changes from segment to
  segment

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Tubular Reabsorption

• Molecules move from filtrate ? across tubular
  epithelium into peritubular interstitial fluid
  and blood
• Water, valuable solutes (e.g., nutrients,
  proteins, amino acids, glucose)
• Occurs through diffusion, osmosis (H2O), active
  transport by carrier proteins
• Occurs primarily along PCT (also along renal
  tubule and collecting system)

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Tubular Secretion

• Molecules move from peritubular fluid into
  tubular fluid
• Lowers plasma concentration of undesirable
  materials
• Necessary because filtration does not force all
  solutes out of plasma
• Primary method of excretion for many drugs
• Occurs primarily at PCT and DCT

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Reabsorption and Secretion PCT

• Primarily reabsorption
• 60-70 of filtrate
• Includes
• Organic nutrients (99-100), e.g., glucose, amino
  acids, proteins, lipids, vitamins
• Water (60-70)
• Ions (60-70), e.g., Na, Cl- also K, Ca2,
  HCO3-
• Reabsorbed materials enter peritubular fluid and
  capillaries
• Secretion
• H, NH4, creatinine, drugs, toxins

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Reabsorption Loop of Henle

• Reabsorption
• Na, Cl-
• Water
• Accomplished by countercurrent exchange
• Refers to exchange by tubular fluids moving in
  opposite directions
• Fluid in descending limb flows toward renal
  pelvis
• Fluid in ascending limb flows toward cortex

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Countercurrent Exchange

• Occurs because of different permeabilities of
  segments of LOH
• Descending limb (thin)
• Permeable to water
• Relatively impermeable to solutes
• Ascending limb (thick)
• Relatively impermeable to water and solutes
• Has active transport mechanisms
• Pump Na and Cl- from tubular fluid into
  peritubular fluid

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Countercurrent Exchange

• Na and Cl- pumped out of thick ascending limb
  into peritubular fluid
• Increases osmotic concentration in peritubular
  fluid around thin descending limb
• Results in osmotic flow of H2O out of thin
  descending limb into peritubular fluid ?
  increased solute concentration in thin descending
  limb
• Arrival of concentrated solution in thick
  ascending limb increases transport of Na and Cl-
  into peritubular fluid

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Overview of Urine Formation
Figure 2616
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Reabsorption and Secretion DCT

• Reabsorption (by vasa recta)
• Na (under influence of aldosterone), Cl-
• Ca2(under influence of PTH and calcitriol)
• H2O (under influence of ADH)
• Secretion
• K (in exchange for Na), H
• NH4 (from deamination produces lactic acid,
  ketone bodies ? acidosis)
• Creatinine, drugs, toxins

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Reabsorption and Secretion Collecting System

• Makes final adjustments to ion concentration and
  urine volume
• Reabsorption
• Na (under influence of aldosterone)
• H2O (under influence of ADH)
• HCO3-
• Urea (distal portion)
• Secretion
• K, H

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Figure 2615
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Summary Urine Formation

• Involves all parts of nephron and collecting
  system
• Processes occur primarily in certain areas
• Glomerular filtration at the renal corpuscle
• Nutrient reabsorption in the PCT
• Water and salt conservation in loop of Henle
• Tubular secretion in the DCT
• Regulation of final volume and solute
  concentration occurs in loops of Henle and
  collecting system

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Normal Kidney Function

• Continues as long as filtration, reabsorption,
  and secretion function within narrow limits
• Disruption of kidney function has immediate
  effects on composition of circulating blood
• If both kidneys affected, death occurs within few
  days

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Normal Kidney Function

• Glomeruli produce approx 48 gallons (180 L) of
  filtrate/day
• 70X plasma volume!
• Almost all fluid volume must be reabsorbed to
  avoid fatal dehydration

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Urine

• Clear, sterile solution
• Yellow (straw) color due to pigment (urobilin)
• Urinalysis analysis of urine sample
• Results from filtration, absorption, secretion
  activities of nephron

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Table 265
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Urine vs. Plasma

• Little to no metabolites and nutrients (glucose,
  lipids, amino acids, proteins)
• Slightly increased Na, greatly increased K and
  Cl-, and greatly decreased HCO3-
• Very high levels of nitrogenous wastes
  (creatinine, urea, ammonia, uric acid)
• Lower pH (6.0 vs. 7.4)
• Much greater water content (95 vs. 50)

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Table 262
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Diuresis

• Elimination of urine
• Usually used to indicate production large volumes
  of urine
• Diuretics
• Drugs that promote water loss in urine
• Reduce
• Blood volume
• Blood pressure
• Extracellular fluid volume

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Micturition Reflex

• Coordinates the process of urination
• Begins when stretch receptors in bladder
  stimulate parasympathetic neurons
• Results in contraction of detrusor muscle
  contraction
• Voluntary relaxation of external urethral
  sphincter causes relaxation of internal urethral
  sphincter

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Micturition Reflex
Figure 2620
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Voluntary Control

• Infants
• Lack voluntary control over urination
• Corticospinal connections are not established
• Incontinence
• Inability to voluntarily control urination
• May be caused by trauma to internal or external
  urethral sphincter

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Age-Related Changes in Urinary System

• Decline in number of functional nephrons
• Reduction in GFR
• Reduced sensitivity to ADH

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Age-Related Changes in Urinary System

• Problems with micturition reflex
• Sphincter muscles lose tone ? incontinence
• Lose control due to
• Stroke
• Alzheimers disease
• CNS problems
• In males, enlarged prostate compresses urethra,
  restricts urine flow ?urinary retention

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The Excretory System

• Includes all systems with excretory functions
  that affect body fluids composition
• Urinary system
• Integumentary system
• Respiratory system
• Digestive system