Excretion

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Functions of the urinary system

• Excretion
• The removal of organic waste products from body
  fluids
• Elimination
• The discharge of waste products into the
  environment
• Homeostatic regulation of blood plasma
• Regulating blood volume and pressure
• Regulating plasma ion concentrations
• Stabilizing blood pH
• Conserving nutrients

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Figure 26.1 An introduction to the Urinary System
Figure 26.1
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Figure 26.2 The Position of the Kidneys
Figure 26.2a, b
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Figure 26.3 The Urinary System in Gross
Dissection
Figure 26.3
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Sectional anatomy of the kidneys

• Superficial outer cortex and inner medulla
• The medulla consists of 6-18 renal pyramids
• The cortex is composed of roughly 1.25 million
  nephrons
• Major and minor calyces along with the pelvis
  drain urine to the ureters

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Figure 26.4 The Structure of the Kidney
Figure 26.4a, b
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Figure 26.5 The Blood Supply to the Kidneys
Figure 26.5c, d
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Figure 26.5 The Blood Supply to the Kidneys
Figure 26.5a, b
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Figure 26.6 A Representative Nephron
Figure 26.6
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Nephron functions include

• Production of filtrate
• Reabsorption of organic nutrients
• Reabsorption of water and ions
• Secretion of waste products into tubular fluid

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Two types of nephron

• Cortical nephrons
• 85 of all nephrons
• Located in the cortex
• Juxtamedullary nephrons
• Closer to renal medulla
• Loops of Henle extend deep into renal pyramids

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Figure 26.7 Cortical and Juxtamedullary Nephrons
Figure 26.7a
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Figure 26.7 Cortical and Juxtamedullary Nephrons
Figure 26.7b, c
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Figure 26.8 The Renal Corpuscle
Figure 26.8a, b
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Figure 26.8 The Renal Corpuscle
Figure 26.8c, d
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Functional anatomy of the nephron

• Proximal convoluted tubule (PCT)
• Actively reabsorbs nutrients, plasma proteins and
  ions from filtrate
• Released into peritubular fluid
• Loop of Henle
• Descending limb
• Ascending limb
• Each limb has a thick and thin section

Animation Urinary System Anatomy
PLAY
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Functional anatomy of the nephron

• Distal convoluted tubule (DCT)
• Actively secretes ions, toxins, drugs
• Reabsorbs sodium ions from tubular fluid

PLAY
Animation Urinary System Dissection and
Flythrough
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SECTION 26-3 Principles of Renal Physiology
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Urine production maintains homeostasis

• Regulating blood volume and composition
• Excreting waste products
• Urea
• Creatinine
• Uric acid

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Basic processes of urine formation

• Filtration
• Blood pressure
• Water and solutes across glomerular capillaries
• Reabsorption
• The removal of water and solutes from the
  filtrate
• Secretion
• Transport of solutes from the peritubular fluid
  into the tubular fluid

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Carrier Mediated Transport

• Filtration in the kidneys modified by carrier
  mediated transport
• Facilitated diffusion
• Active transport
• Cotransport
• Countertransport
• Carrier proteins have a transport maximum (Tm)
• Determines renal threshold

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Reabsorption and secretion

• Accomplished via diffusion, osmosis, and
  carrier-mediated transport
• Tm determines renal threshold for reabsorption of
  substances in tubular fluid

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Figure 26.9 An Overview of Urine Formation
Figure 26.9
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Figure 26.10 Glomerular Filtration
Figure 26.10
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Figure 26.10 Glomerular Filtration
Figure 26.10a, b
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Glomerular filtration rate (GFR)

• Amount of filtrate produced in the kidneys each
  minute
• Factors that alter filtration pressure change GFR

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Factors controlling the GFR

• A drop in filtration pressure stimulates
  Juxtaglomerular apparatus (JGA)
• Releases renin and erythropoietin

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Figure 26.11 The Response to a Reduction in the
GFR
Figure 26.11a
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Figure 26.11 The Response to a Reduction in the
GFR
Figure 26.11b
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Sympathetic activation

• Produces powerful vasoconstriction of afferent
  arterioles
• Decreases GFR and slows production of filtrate
• Changes the regional pattern of blood flow
• Alters GFR
• Stimulates release of renin by JGA

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Reabsorption and secretion at the PCT

• Glomerular filtration produces fluid similar to
  plasma without proteins
• The PCT reabsorbs 60-70 of the filtrate produced
  
• Reabsorption of most organic nutrients
• Active and passive reabsorption of sodium and
  other ions
• Reabsorption of water
• Secretion also occurs in the PCT

Animation Early Filtrate Processing
PLAY
Animation Glomerular filtration
PLAY
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Figure 26.12 Transport Activities at the PCT
Animation Proximal Convoluted Tubule
PLAY
Figure 26.12
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The loop of Henle and countercurrent
multiplication

• Countercurrent multiplication
• Between ascending and descending limbs of loop
• Creates osmotic gradient in medulla
• Facilitates reabsorption of water and solutes
  before the DCT
• Permits passive reabsorption of water from
  tubular fluid

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Figure 26.13 Countercurrent Multiplication and
Concentration of Urine
Figure 26.13a
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Figure 26.13 Countercurrent Multiplication and
Concentration of Urine
Figure 26.13b
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Figure 26.13 Countercurrent Multiplication and
Concentration of Urine
Figure 26.13c
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Reabsorption and secretion at the DCT

• DCT performs final adjustment of urine
• Active secretion or absorption
• Absorption
• Tubular cells actively resorb Na and Cl-
• In exchange for potassium or hydrogen ions
  (secreted)

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Figure 26.14 Tubular Secretion and Solute
Reabsorption at the DCT
Animation Distal Convoluted Tubule
PLAY
Figure 26.14
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Figure 26.14 Tubular Secretion and Solute
Reabsorption at the DCT
Figure 26.14c
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Reabsorption and secretion along the collecting
system

• Water and solute loss is regulated by aldosterone
  and ADH
• Reabsorption
• Sodium ion, bicarbonate, and urea are resorbed
• Secretion
• pH is controlled by secretion of hydrogen or
  bicarbonate ions

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Control of urine volume and osmotic concentration

• Urine volume and osmotic concentration are
  regulated by controlling water reabsorption
• Precise control allowed via facultative water
  reabsorption

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Figure 26.15 The Effects of ADH on the DCT and
Collecting Ducts
Figure 26.15
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Figure 26.15 The Effects of ADH on the DCT and
Collecting Ducts
Figure 26.15a, b
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Function of the vasa recta

• Removes solutes and water
• Balances solute reabsorption and osmosis in the
  medulla

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Figure 26.16 A Summary of Renal Function
Figure 26.16a