Urinary System

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Chapter 26

• Urinary System

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Urinary System Functions

• Filtering of blood
• Regulation of
• blood volume
• concentration of blood solutes
• pH of extracellular fluid
• blood cell synthesis
• Synthesis of Vitamin D

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Urinary System Anatomy
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Location and External Anatomyof Kidneys

• External Anatomy
• Renal capsule
• Surrounds each kidney
• Perirenal fat
• Engulfs renal capsule and acts as cushioning
• Renal fascia
• Anchors kidneys to abdominal wall
• Hilum
• Renal artery and nerves enter and renal vein and
  ureter exit kidneys

• Location
• Lie behind peritoneum on posterior abdominal wall
  on either side of vertebral column
• Lumbar vertebrae and rib cage partially protect
• Right kidney slightly lower than left

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Internal Anatomy of Kidneys

• Cortex Outer area
• Renal columns
• Medulla Inner area
• Renal pyramids
• Calyces
• Major Converge to form pelvis
• Minor Papillae extend
• Nephron Functional unit of kidney
• Juxtamedullary
• Cortical

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The Nephron
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Histology of the Nephron
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Internal Anatomy of Kidneys

• Renal corpuscle
• Bowmans capsule
• Parietal layer
• Visceral layer
• Glomerulus
• Network of capillaries
• Arterioles
• Afferent
• Blood to glomerulus
• Efferent
• Drains

• Tubules
• Proximal (convoluted) tubule
• Loops of Henle
• Descending limb
• Ascending limb
• Distal (convoluted) tubules
• Collecting ducts

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Renal Corpuscle
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Kidney Blood Flow
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Ureters and Urinary Bladder

• Ureters
• Tubes through which urine flows from kidneys to
  urinary bladder
• Urinary bladder
• Stores urine

• Urethra
• Transports urine from bladder to outside of body
• Difference in length between males and females
• Sphincters
• Internal urinary
• External urinary

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Ureters and Urinary Bladder
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Urine Formation
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Filtration

• Filtration
• Renal filtrate
• Plasma minus blood cells and blood proteins
• Most (99) reabsorbed
• Filtration membrane
• Fenestrated endothelium, basement membrane and
  pores formed by podocytes

• Filtration pressure
• Responsible for filtrate formation
• Glomerular capillary pressure (GCP) minus capsule
  pressure (CP) minus colloid osmotic pressure
  (COP)
• Changes caused by glomerular capillary pressure

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Filtration Pressure
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Tubular Reabsorption

• Reabsorption
• Passive transport
• Active transport
• Cotransport
• Specialization of tubule segments

• Substances transported
• Active transport moves Na across nephron wall
• Other ions and molecules moved by cotransport
• Passive transport moves water, urea,
  lipid-soluble, nonpolar compounds

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Tubular Secretion

• Substances enter proximal or distal tubules and
  collecting ducts
• H, K and some substances not produced in body
  are secreted by countertransport mechanisms

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Urine Production

• In ascending limb of loop of Henle
• Na, Cl-, K transported out of filtrate
• Water remains
• In distal 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 tubules
• Na and other substances removed
• Water follows passively
• Filtrate volume reduced
• In descending limb of loop of Henle
• Water exits passively, Filtrate volume reduced 15

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Reabsorption in Proximal Nephron
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Reabsorption in Loop of Henle
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Reabsorption in Loop of Henle
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Medullary Concentration and Urea Cycling
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Urine Concentration Mechanism

• When large volume of water consumed
• Eliminate excess without losing large amounts of
  electrolytes
• Response is kidneys produce large volume of
  dilute urine

• When drinking water not available
• Kidneys produce small volume of concentrated
  urine
• Removes waste and prevents rapid dehydration

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Urine Concentrating Mechanism I
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Urine Concentrating Mechanism II
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Hormonal Mechanisms

• ADH
• Secreted by posterior pituitary
• Increases water permeability in distal tubules
  and collecting ducts
• Aldosterone
• Produced in adrenal cortex
• Affects Na and Cl- transport in nephron and
  collecting ducts

• Renin
• Produced by kidneys, causes production of
  angiotensin II
• Atrial natriuretic hormone
• Produced by heart when blood pressure increases
• Inhibits ADH production
• Reduces ability of kidney to concentrate urine

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Effect of ADH on Nephron
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Aldosterone Effect on Distal Tubule
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Internal Anatomy of Kidneys

• Cortex Outer area
• Renal columns
• Medulla Inner area
• Renal pyramids
• Calyces
• Major Converge to form pelvis
• Minor Papillae extend
• Nephron Functional unit of kidney
• Juxtamedullary
• Cortical

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Clearance and Tubular Load

• Plasma clearance
• Volume of plasma cleared of a specific substance
  each minute
• Used to estimate GFR
• Used to calculate renal plasma flow
• Used to determine which drugs or other substances
  excreted by kidney

• Tubular load
• Total amount of substance that passes through
  filtration membrane into nephrons each minute
• Normally glucose is almost completed reabsorbed

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Tubular Maximum

• Tubular maximum
• Maximum rate at which a substance can be actively
  absorbed
• Each substance has its own tubular maximum

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Urine Flow and Micturition Reflex

• Urine flow
• Hydrostatic pressure forces urine through nephron
• Peristalsis moves urine through ureters

• Micturition reflex
• Stretch of urinary bladder stimulates reflex
  causing bladder to contract, inhibiting urinary
  sphincters
• Higher brain centers can stimulate or inhibit
  reflex

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Tubular flow and Micturition Reflex
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Effects of Aging on Kidneys

• Gradual decrease in size of kidney
• Decrease in kidney size leads to decrease in
  renal blood flow
• Decrease in number of functional nephrons
• Decrease in renin secretion and vitamin D
  synthesis
• Decline in ability of nephron to secrete and
  absorb

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Kidney Dialysis