4th hour

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4th hour
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9.3.2
Urine formation
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• Objectives
• Students should be able to explain the formation
  and concentration of urine involving
  ultrafiltration, reabsorption and secretion

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Ultrafiltration

• Filters out molecules which are much smaller than
  RBC and plasma proteins

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• Substance forced through the capillary wall and
  between the podocytes by blood pressure enter the
  lumen of Bowmans capsule

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• Contains salt, glucose vitamin, also
  nitrogenous waste urea

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Factors contribute to ultrafiltration process

• a) The hydrostatic blood pressure in the
  glomerulus is higher than in other
  capillaries.
• This high pressure is mainly due to the high
  resistance to outflow presented by the efferent
  arteriole, which is smaller in diameter than the
  afferent arteriole

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• b) Large amount of glomerulus filtrate
• large surface area for filtration provided
  by the highly coiled glomerulus capillaries

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• c) High permeability of the glomerulus
• The wall of Bowmans capsule in contact
  with capillaries consists of specialized
  epithelial cell called podocytes.
• These cells have numerous cytoplasmic
  extension called foot processes that cover most
  of the capillaries.
• Foot processes of adjacent podocytes are
  separated by narrow gaps called filtration
  slits.

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• The perforated walls of the capillaries and the
  podocytes form a filtration membrane that permits
  fluid and small solutes dissolved in the plasma,
  such as glucose, amino acids, sodium,
  potassium,chloride, bicarbonate, other salts, and
  urea to pass through and become part of the
  filtrate.
• This filtrate membrane holds back blood cells,
  platelets and most of the plasma protein.

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Reabsorption

• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule
• Collecting duct

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• Proximal tubule
• Help maintain a constant pH in body fluid
• By controlled secretion of hydrogen ions and
• By reabsorbing about 90 of HCO3- from filtrate
• Reabsorbing of NaCl (salt) and water
• Salt moves from the filtrate to the interstitial
  fluid
• Water follows passively by osmosis

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• ii) Descending limb of the loop of Henle
• Permeable to water but not to salt
• Water move out of the tubule by osmosis
• Filtrate moving downward from the cortex to inner
  medulla
• Filtrate continues to lose water
• NaCl concentration of Henle increase

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• iii) Ascending limb of the loop of Henle
• Consist of thin segment and thick segment
• Both have epithelia that are virtually
  impermeable to water
• Permeable to NaCl
• Salt that was concentrated now diffuse out of the
  ascending limb (thin segment)

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• Contributing to a high interstitial osmolarity in
  the medulla of the kidney
• The thick segment continues the transfer of salt
  from the filtrate to interstitial fluid but now
  the active transport
• Filtrate becomes progressively more dilute as it
  moves up to the cortex again in the ascending
  limb of the loop

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• iv) Distal convoluted tubule
• Plays a key role in regulating the K and NaCl
  concentration of body fluids
• By varying the amount of the K that is secreted
  into the filtrate and the amount of NaCl that is
  reabsorbed from the filtrate
• Contributes to pH regulation by controlled
  secretion of H by the reabsorption of
  bicarbonate HCO3-

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• v) Collecting duct
• Carries the filtrate back in the direction of the
  medulla and renal pelvis
• The special tissue epithelium is permeable to
  water but not to salt
• The duct carries the filtrate toward the renal
  medulla for a second time

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• And the filtrate become more and more
  concentrated as water is lost to the interstitial
  fluid
• The bottom portion is permeable to urea and
  leakage of this solute into the interstitial
  fluid contributes to the high osmolarity of the
  medulla

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Secretion

• As filtrate travels through the nephron tubule,
  it is joined by substance that are transported
  across the tubule epithelium from the surrounding
  interstitial fluid
• Because small molecules pass freely from plasma
  within capillaries into interstitial fluid, the
  net effect of renal secretion is the addition of
  plasma solute to the filtrate within the tubule

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• The proximal and distal tubules are the most
  common site of secretion
• Unlike filtration, which is nonselective,
  secretion is a very selective process involving
  both passive and passive transport

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Summary of the processes in a nephron
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