Fluid & Electrolyte Homeostasis

• Kidneys regulate water loss by altering the concentration of the urine.

  Kidneys’ ability to retain water are the loop of Henle & the collecting duct
• Regulation of Urine Osmolarity

  1. Fluid in PCT is isosmotic (300 mOsM)

  2. Water, but not salts, are reabsorbed in thin segment

  3. Fluid in bottom of loop is hyperosmotic

  4. Salts, but not water, are reabsorbed in thick segment

  5. Fluid in DCT is hypo-osmotic

  6. Urea reabsorption is critical

  7. Interstitial fluid of medullary pyramids becomes more hyperosmotic the deeper you go

  8. Distal DCT and collecting duct can alter their permeability to water
• Aquaporins


  Special channels that allow water to diffuse across cell membranes, regulated by ADH
• How ADH regulates water permeability

  1. ADH binds to receptors in renal collecting duct

  2. Causes vesicles containing aquaporins to insert into membrane facing lumen

  3. Increases absorption of water from filtrate

  4. Results in more concentrated urine and reduced water loss

  5. Also binds to receptors in blood vessels, causing vasoconstriction and increased blood pressure
• Increased ECF osmolarity


  • Detected by osmoreceptors in hypothalamus
  • Increases firing of signals to hypothalamus
  • Increases activity of hypothalamic neurons
  • Increases thirst
  • Releases ADH/vasopressin
• Decreased blood pressure/volume


  • Detected by baroreceptors/atrial stretch receptors
  • Increases firing of signals to hypothalamus
  • Increases activity of hypothalamic neurons
  • Releases ADH/vasopressin
• Regulation of Sodium Balance and ECF Volume

  1. Increased ECF osmolarity and plasma [K+] sensed by adrenal cells

  2. Increases release of aldosterone

  3. Aldosterone increases Na absorption by inserting more Na+ channels and Na/K ATPase in DCT membrane
• Renin-Angiotensin System (RAS)


  1. JG cells in afferent arteriole secrete renin

  2. Renin converts angiotensinogen to angiotensin I

  3. Angiotensin converting enzyme (ACE) converts angiotensin I to angiotensin II

  4. Angiotensin II has many effects: vasoconstriction, aldosterone release, ADH release, thirst, sympathetic stimulation, sodium absorption in nephron
• Decreased blood pressure (afferent arteriole)

  • Decreases flow of Na past macula densa
  • Macula densa signals JG cells
  • JG cells produce renin