Renal Phys 8

• the main regulator of plasma osmolarity; it alters water reabsorption as needed

  ADH
• the osmoreceptor control of ADH secretion

  removes aquaporins from collecting duct -> large volume of dilute urine

  H2O excretion = 2.5 L/day

  
• ADH secretion in dehydration

  H2O excretion = 0.5 L/day

  small volume of concentrated urine

  
• blood plasma and blood pressure are proportional
• what works together with the RAAS reabsorption of NaCl?

  baroreceptor control of ADH secretion
• this helps to keep plasma osmolarity constant by reabsorbing water to go along with the NaCl

  baroreceptor control of ADH secretion
• osmolarity = # of solutes per volume of water
• baroreceptor control of ADH secretion

  1.) decrease in plasma V = decrease in MAP

  2.) ^ Ang II, ^ Aldo, ^ Na+ Reabsorption

  3.) ^ ADH, ^ H2O Reabsorption

  this increases both solutes and water proportionally

  
• Diabetes mellitus (osmotic diuresis):

  failure to reabsorb glucose -> huge amount of glucose in urine causes H2O to be retained in lumen

  • loss of water via urine
  • fails to move glucose into your cells
  • upsets osmotic gradient
  • type I = lack of insulin production or secretion
  • type II = lack of insulin sensitivity
• Diabetes insipidus (water diuresis):

  failure of posterior pituitary to release ADH or failure of kidney to respond to ADH -> H2O permeability is low, increased water loss

  • pure loss of water
  • no aquaporins inserted into the tubule
  • very high water demand
  • urinates > 2.5 L/day
• sweat contains H2O and NaCl and comes out of ECF; it is hypoosmotic
• the response to sweating

  lose more water than NaCl, thus ^ Osm = (small decrease in solute)/(LARGE decrease in water)

  • less excretion because more reabsorption
  • aldosterone and ADH work together

  
• metering of water intake by GI tract

  feed-forward mechanism to ensure that we don't consume too much water

  this decreases thirst
• dry mouth, throat

  increases thirst
• increase in plasma osmolarity stimulates osmoreceptors, which

  increases thirst
• decrease in plasma volume stimulates baroreceptors, which

  increases angiotensin II and increases thirst
• kidneys excrete excess water and NaCl that you ingest but can only reduce the rate at which you lose them when you don't eat/drink enough; must urinate some volume! (you can't excrete dust!)
• you need regular intake to replace regular losses, and extra intake when excess fluid is lost through processes like sweating, vomiting, diarrhea, hemorrhage, etc.
• caffeine dilates afferent arterioles (lose more water)
• alcohol inhibits ADH secretion (lose more water)