Concentrating mechanisms

• Describe osmolality through the kidney

  Water reabsorption occurs through the medullary collecting ducts, which makes the interstitial fluid surrounding these ducts hyperosmotic. Osmolality increases down the descending limb.
• Describe movement of water and substances in the LOH

  In the descending limb:

  • passive reabsorption of water via Aquaporin 1
  • passive secretion of urea via UT facilitated diffusion and accumulates in the filtrate before reabsorption in the collecting duct
  • No movement of Na+ or Cl-

  In the ascending limb:

  • impermeable to water
  • active reabsorption of Na+ K+ Cl-
• Describe the counter current multiplier

  This is when there is active transport of NaCl with no passive release of water and passive secretion of urea. This creates a concentration gradient for water to be reabsorbed in the collecting duct and concentrates the urine.
• What is Bartter’s syndrome?

  Bartter syndrome is a renal tubular salt-wasting disorder in which the kidneys cannot reabsorb sodium and chloride in the thick ascending limb of the loop of Henle. This leads to increased distal delivery of salt and excessive salt and water loss from the body.
• Factors that increase ability of the LOH to concentrate urine

  • An increase in the length of the loop of Henle (desert rat)
  • A reduction in the flow rate of filtrate through the loop
  • Alter protein in diet? Increase UT transport?
  • Increase in the number or capacity of the pumps (Na+/K+/2Cl-) ­concentration gradient
• Describe the action of ADH

  • Also known as Arginine Vasopressin (AVP)
  • Released from the posterior pituitary gland
  • Rapid acting with a short t½
  • Released in response to a rise in plasma osmolality
  • Stimulated by Angiotensin II
  • Increases permeability of DCT and CD
• Describe the mechanism of ADH action

  1. ADH binds to receptor
  2. Leads to activation of adenylate cyclase, which produces cAMP
  3. Activates PKA, which causes exocytosis of small vesicles containing aquaporins
• Describe the impact of Diabetes Inspidus on water reabsorption

  Inadequate release of ADH or impaired response to ADH, leading to poor water reabsorption. This results in:

  • polyuria
  • polydipsia
  • dehydration

  Treated with vasopressin (ADH)