Urine concentrating mechanisms

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Urine concentrating mechanisms

• Dr. Eman El Eter

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• The kidney can excrete urine as dilute as 5
  mOsm/L and as concentrate as high as 100-1400
  mOsm/L.
• The kidney can excrete large volume or small
  volume of urine without affecting the rate of
  solute excretion
• What is the relation of ADH to urine osmolarity,
  i.e. diluted or concentrated urine?

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Changes in the osmolarity of tubular fluid
throughout the nephrone
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Changes in the osmolarity of tubular fluid
throughout the nephrone
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Counter current multiplier mechanism

• Responsible for the high osmolality of the renal
  medulla
• 1200-1400 mOsm.
• The renal medulla must accumulates solutes in
  great excess of water.
• It is maintained by the balanced inflow and
  outflow of solutes and water in medulla.
• A countercurrent mechanisms the inflow is
  parallel to, close to but opposite to outflow.

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Mechanisms responsible for creation of
hyperosmolar medulla

• 1- Active transport of Na and co-transport of
  K, Cl- and other ions out of loop of Henle into
  medullary interstitium.
• 2- Active transport of ions from collecting ducts
  into medullary interstituim.
• 3- Diffusion of large amounts of urea from the
  inner medullary collecting ducts into the
  medullary interstituim.
• 4- Diffusion of only small amounts of water from
  the medullary collecting tubules into
• the interstitium, less than the reabsorption of
  solutes into medullary interstitium.
• 5- No water diffusion into the medulla from the
  ascending loop of Henle.

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Counter current multiplier
ISF interstitial fluid
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Counter current exchanger in the vasa recta

• Function Maintains the hyperosmolarity of the
  renal medulla. Why?
• Vasa Recta has two main features
• 1- Blood flow is slow (1-2 of total renal blood
  flow).
• 2- The vasa recta act as a counter current
  exchanger to minimize washout of solutes from the
  medullary interstitium. This is due to the U
  shape of vasa recta capillaries.

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Disorders of urininary concentrating ability

• Diabetes insipidus
• Cause inability to produce or release ADH
• Urine low fixed specific gravity (diluted urine)
• Polyuria
• Polydypsia.
• Nephrogenic diabetes insipidus
• Cause inability of kidney to respond to ADH
• Diabetes mellitus
• High specific gravity urine