Physiology 441

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Physiology 441

• The Urinary System, Chp. 14
• Text Human Physiology (Sherwood), 6th Ed.

• Julie Balch Samora, MPA, MPH
• jbsamora_at_hsc.wvu.edu
• 293-3412, Room 3145

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The Urinary System

• The Kidneys (form the urine)
• Renal Pelvis (into which urine is drained)
• Ureters (carry to urinary bladder)
• Bladder (stores urine)
• Urethra (tube which allows elimination of urine
  to external environment)

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www.health.uab.edu/show.asp?durki65499
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The Nephron

• The functional unit of the kidney
• Each nephron has two components
• Vascular component
• Tubular component
• Renal cortex- outer region (granular)
• Renal medulla- inner region, made up of triangles
  (renal pyramids)

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Vascular Part of the Nephron

• The glomerulus- (a ball of capillaries) filters
  H2O and solute from blood
• Afferent arterioles (come from renal artery)- one
  supplies each nephron and delivers blood to
  glomerulus
• Efferent arterioles (come from glomerular
  capillaries)- unfiltered blood leaves glomerulus
• Peritubular capillaries- supply renal tissue with
  blood (exchanges w/ tubules)

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Tubular Part of the Nephron

• Bowmans capsule- collects glomerular filtrate
• Proximal tubule- uncontrolled reabsorption and
  secretion of selected substances
• Loop of Henle establishes an osmotic gradient
  in order to concentrate urine to appropriate amt
• Distal tubule- controlled reabsorption and
  secretion occur here
• collecting duct (tubule)- variable, controlled
  reabsorption of Na and H2O, and secretion of K
  and H (fluid leaving here is urine-enters renal
  pelvis)

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Juxtaglomerular apparatus (JGA)

• The ascending limb of Henle passes through a fork
  formed by the afferent and efferent arterioles of
  the same nephron.
• The vascular and tubular cells at this juncture
  are both specialized to form the JGA.

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Distal tubule
Proximal tubule
Collecting duct
Juxtaglomerular apparatus
Efferent arteriole
Afferent arteriole
Bowmans capsule
Glomerulus
Artery
Vein
Cortex Medulla
Peritubular capillaries
Loop of Henle
To renal pelvis
Overview of Functions of Parts of a Nephron
Fig. 14-3, p. 504
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Location of Nephron Matters!

• All nephrons originate in the cortex, but where
  glomeruli lie and length of loops of henle
  designate type of nephron
• Cortical (80) - glomeruli in outer cortex-
  hairpin loops barely reaches medulla
• Juxtamedullary (20) - glomeruli in inner layer
  of cortex, next to medulla- hairpin loop goes
  through the entire depth of medulla- peritubular
  capillaries form vasa recta

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Distal tubule
Distal tubule
Glomerulus
Proximal tubule
Bowmans capsule
Proximal tubule
Cortex
Medulla
Loop of Henle
Collecting duct
Descending limb of loop of Henle
Other nephrons emptying into the same collecting
duct
Vasa recta
Ascending limb of loop of Henle
To renal pelvis
Fig. 14-5, p. 505
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The Kidneys Multiple Roles

• Play a major role in maintaining homeostasis
• Maintain water balance
• Regulate the quantity and concentration of ECF
  ions
• Regulate the plasma volume
• Regulate pH by controlling elimination of acid
  and base in urine
• Maintain osmolarity
• Regulate the concentration of plasma constituents
  (e.g. electrolytes and water)

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The Kidneys Multiple Roles

• Remove Toxic Metabolic Wastes
• Excreting foreign compounds
• Secreting erythropoietin
• Producing renin (imp. in salt handling)
• Converting Vitamin D into its active form

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Basic Renal Processes

• Glomerular filtration
• Tubular reabsorption
• Tubular secretion
• Urine results from these three processes.

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Glomerular Filtration

• The first step in urine formation
• Blood flows through the glomerulus, allowing
  protein-free plasma to be filtered through the
  glomerular capillaries into Bowmans capsule.
• 20 of plasma entering the glomerulus is
  filtered
• 125 ml/min filtered fluid

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Tubular Reabsorption

• Movement of substances from tubular lumen back
  into the blood
• Reabsorbed substances are therefore not lost in
  the urine, but are carried by the peritubular
  capillaries to the venous system
• Most of the filtered plasma is reabsorbed

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Tubular Secretion

• The selective transfer of substances from the
  peritubular capillary into the tubular lumen.
• Allows for rapid elimination of substances from
  the plasma via extraction of the 80 of
  unfiltered plasma in peritubular capillaries and
  adding it to the substances already in tubule as
  result of filtration

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Urine Excretion

• The elimination of substances from the body in
  the urine
• All plasma constituents filtered or secreted, but
  not reabsorbed remain in the tubules and pass
  into the renal pelvis to be excreted as urine and
  eliminated from the body

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Renal Processes 1
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Glomerular Filtration

• Fluid filtered from the glomerulus into Bowmans
  capsule passes through 3 layers
• the glomerular capillary wall
• the basement membrane
• Collagen
• Glycoproteins- negative charge
• the inner layer of Bowmans capsule
• Podocytes
• Filtration slits

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Layers of Glomerulus Membrane
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What Drives Filtration?

• How does fluid move from the plasma across the
  glomerular membrane into Bowmans capsule?
• No active transport mechanisms
• No local energy expenditure
• Simple passive physical forces accomplish
  filtration
• - Filtration occurs throughout the length of the
  capillaries

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Forces involved in Filtration

• Glomerular capillary blood pressure (favors
  filtration)
• Plasma-colloid osmotic pressure (opposes
  filtration)
• Bowmans capsule hydrostatic pressure (opposes
  filtration)

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Glomerular Capillary Blood Pressure

• Fluid pressure exerted by the blood within the
  glomerular capillaries
• Glomerular capillary pressure is significantly
  higher than other capillary blood pressures
• This is due to the larger diameter of the
  afferent arteriole compared with the efferent
  arteriole
• Blood pressure does not fall along the length of
  this capillary, which pushes fluid out of the
  glomerulus into Bowmans capsule
• (pressure build-up in glom. Cap. 55mmHg)

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Pressure opposing filtration

• Plasma-colloid oncotic pressure- caused by the
  unequal distribution of plasma proteins across
  the glomerular membrane
• (30mmHg)
• Bowmans capsule hydrostatic pressure- the
  pressure exerted by the fluid in this initial
  part of the tubule- tends to push fluid out of
  Bowmans capsule
• (15mmHg)

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Net Filtration Pressure

• Force favoring filtration (glomerular capillary
  blood pressure of 55 mmHg) minus forces opposing
  filtration (plasma colloid osmotic pressure of 30
  mmHg Bowmans capsule pressure of 15 mmHg)
• 55 (30 15) 10 mmHg

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Glomerular Filtration Rate

• Depends on
• The net filtration pressure
• How much glomerular surface area is available for
  penetration
• How permeable the glomerular membrane is
• GFR Kf x net filtration pressure
• Where (Kf) filtration coefficient (a product of
  the above two glomerular properties)
• - Roughly 125 ml/min in males

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GFR

• Changes in GFR primarily result from changes in
  the glomerular capillary blood pressure
• As glomerular capillary blood pressure ?, the net
  filtration pressure ?, as does GFR
• If afferent arteriolar resistance ?, GFR ?

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Mechanisms to Regulate GFR

• Autoregulation (prevent spontaneous changes in
  GFR)
• Involves myogenic and tubuloglomerular feedback
  mechanisms
• Extrinsic sympathetic control (long-term
  regulation of arterial BP)
• Mediated by the sympathetic nervous system
• Can override autoregulatory mechanisms

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Autoregulation Myogenic mechanism

• Response to changes in pressure within the
  nephrons vascular component
• Arterioles contract inherently in response to the
  stretch accompanying ? pressure. Vessel
  automatically constricts, which helps limit blood
  flow into glomerulus despite increased systemic
  pressure
• Opposite reaction occurs when smooth muscles
  sense a drop in pressure

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Autoregulation Tubuloglomerular feedback

• Juxtaglomerular apparatus
• the combination of tubular and vascular cells
  where the tubule passes through the angle formed
  by the afferent and efferent arterioles as they
  join the glomerulus
• Smooth muscle cells within the afferent arteriole
  form granular cells
• Specialized tubular cells in this region known as
  macula densa- sense changes in salt level of
  tubular fluid

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Importance of Autoregulation of GFR

• The myogenic and tubuloglomerular feedback
  mechanisms work in tandem to autoregulate GFR
  within a MAP range of 80-180 mmHg
• Autoregulation greatly blunts the direct effect
  that changes in arterial pressure might otherwise
  have on GFR and preserves water and solute
  homeostasis and allows waste excretion to carry
  on as usual

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Extrinsic Sympathetic Control

• GFR can be changed purposefully, even when MAP is
  within the autoregulatory range
• GFR is reduced by the baroreceptor reflex
  response to a fall in blood pressure (the SNS
  causes vasoconstriction in most arterioles as a
  compensatory mechanism to ? TPR)
• Afferent arterioles innervated with sympathetic
  vasoconstrictor fibers much more than are the
  efferent aa.
• ? GFR causes ? urine output, conserving some
  water and salt, helping to restore plasma volume
  to normal

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Baroreceptor Reflex Influence on the GFR in
Long-term Regulation of Arterial Blood Pressure
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Filtration Fraction

• The Percentage of Renal Plasma Flow that is
  Filtered
• FF GFR/RPF
• Roughly 20