Glomerular%20Filtration

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Glomerular Filtration
Normally, 3 Starling forces are at work in
glomerular filtration
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Glomerular Filtration

• Regulation of the GFR is critical to maintaining
  homeostasis and is regulated by an assortment of
  local and systemic mechanisms
• Renal autoregulation occurs when the kidneys
  themselves regulate GFR.
• Neural regulation occurs when the ANS regulates
  renal blood flow and GFR.
• Hormonal regulation involves angiotensin II and
  atrial natriuretic peptide (ANP).

3
Glomerular Filtration

• Renal autoregulation of GFR occurs by two means
• Stretching in the glomerular capillaries triggers
  myogenic contraction of smooth muscle
• cells in afferent arterioles (reduces GFR).
• Pressure and flow monitored in the
• macula densa provides tubuloglomerular
• feedback to the glomerulus, causing the
• afferent arterioles to constrict (decreasing
• blood flow and GFR) or dilate (increasing
• blood flow and GFR) appropriately.

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Glomerular Filtration

• Neural regulation of GFR is possible because the
  renal blood vessels are supplied by sympathetic
  ANS fibers that release norepinephrine causing
  vasoconstriction.
• Sympathetic input to
• the kidneys is most
• important with extreme
• drops of B.P. (as occurs
• with hemorrhage).

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Glomerular Filtration

• Two hormones contribute to regulation of GFR
• Angiotensin II is a potent vasoconstrictor of
  both afferent and efferent arterioles (reduces
  GFR).
• A sudden large increase in BP stretches the
  cardiac atria and releases atrial
• natriuretic peptide (ANP).
• ANP causes the
• glomerulus to relax,
• increasing the surface
• area for filtration.

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The Filtration Membrane
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Glomerular Filtration(Interactions
Animation)Renal Filtration
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Pressures That Drive Glomerular Filtration
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Tubular Reabsorption

• Tubular reabsorption is the process of returning
  important substances (good stuff) from the
  filtrate back into the renal interstitium, then
  into the renal blood vessels... and ultimately
  back into the body.

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Tubular Reabsorption

• The good stuff is glucose, electrolytes,
  vitamins, water, amino acids, and any small
  proteins that might have inadvertently escaped
  from the blood into the filtrate.
• Ninety nine percent of the glomerular filtrate is
  reabsorbed (most of it before the end of the
  PCT)!
• To appreciate the magnitude of tubular
  reabsorption, look once again at the table in the
  next slide and compare the amounts of substances
  that are filtered, reabsorbed, and excreted in
  urine.

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Tubular Reabsorption
Total Amount in Plasma Amount in 180 L of filtrate (/day) Amount returned to blood/d (Reabsorbed) Amount in Urine (/day)
Water (passive) 3 L 180 L 178-179 L 1-2 L
Protein (active) 200 g 2 g 1.9 g 0.1 g
Glucose (active) 3 g 162 g 162 g 0 g
Urea (passive) 1 g 54 g 24 g (about 1/2) 30 g (about 1/2)
Creatinine 0.03 g 1.6 g 0 g (all filtered) 1.6 g (none reabsorbed)
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Tubular Reabsorption

• Reabsorption into the interstitium has two
  routes
• Paracellular reabsorption is a passive process
  that occurs between adjacent tubule
• cells (tight junctions do
• not completely seal off
• interstitial fluid from
• tubule fluid.)
• Transcellular reabsorption
• is movement through an
• individual cell.

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Tubular Reabsorption

• It is a tremendous feat to reabsorb all of the
  nutrients and fluid we must to survive, while
  still filtering out, concentrating and excreting
  toxic substance.
• To accomplish this, the kidney establishes a
  countercurrent flow between the filtrate in the
  limbs of the Loops of Henle and the blood in the
  peritubular capillaries and Vasa Recta.
• Two types of countercurrent mechanisms exist in
  the kidneys countercurrent multiplication and
  countercurrent exchange.

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Tubular Reabsorption

• Countercurrent multiplication is the process by
  which a progressively increasing osmotic gradient
  is formed in the interstitial fluid of the renal
  medulla as a result of countercurrent flow.
• Countercurrent exchange is the process by which
  solutes and water are passively exchanged between
  the blood of the vasa recta and interstitial
  fluid of the renal medulla as a result of
  countercurrent flow.
• This provides oxygen and nutrients to the renal
  medulla without washing out or diminishing the
  gradient.

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Tubular Reabsorption

• Both mechanisms contribute to reabsorption of
  fluid and electrolytes and the formation of
  concentrated urine.

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Tubular Reabsorption

• Reabsorption of fluids, ions, and other
  substances occurs
• by active and passive means.
• A variety of symporters and antiporters actively
  transport Na , Cl , Ca2, H, HCO3 , glucose,
  HPO42 , SO42 , NH4, urea, all amino acids, and
  lactic acid.
• Reabsorption of water can be obligatory or
  facultative, but it always moves by osmosis down
  its concentration gradient depending on the
  permeability of the tubule cells (which varies
  between the PCT, the different portions of the
  loop of Henle, DCT, and collecting ducts).

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Tubular Reabsorption

• Obligatory reabsorption of water occurs when it
  is obliged to follow the solutes as they are
  reabsorbed (to maintain the osmotic gradient).
• Facultative reabsorption describes variable
  water reabsorption, adapted to specific needs.
• It is regulated by the effects
• of ADH and aldosterone on
• the principal cells of the renal
• tubules and collecting ducts.

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Tubular Reabsorption

• This graphic depicts the formation of a dilute
  urine, mostly through obligatory
• reabsorption of water.
• Compare this process to
• the one depicted on the
• next slide where urine is
• concentrated by the action
• of ADH on the DCT and
• collecting ducts of
• juxtamedullary nephrons.