Renal 6 - Urine Concentration and Dilution Flashcards
normal function requires ECF osmolarity of:
300 mOsm
what are the mechanisms that maintain water balance
- elimination of excess water by excreting a dilute urine
- conservation of water by excreting a concentrated urine
- renal feedback mechansims that control extracellular fluid sodium concentration and osmolarity
- thirst and salt appetite mechanisms that determine the intakes of water and salt
what controls how concentrated/dilute urine is
ADH
what is the maximal and minimal urine concentrations
-max: 1200 mOsm
-min: 50 mOsm
urine formed without ADH is ____
dilute
describe how urine is dilute without ADH
- filtrate is isosmotic in PT
- filtrate becomes hyperosmotic in tDL bc water reabsorption
- filtrate becomes hypoosmotic in TAL and early DT bc solute reabsorption
- filtrate stays hypoosmotic in absence of ADH through DT and CD
what does ADH do
increases water permeability of DT and CD
-causes large volume of water diffusion into interstitium
- enters capillaries via vasa recta and removed
-creates concentrated urine
what is obligatory urine volume
the maximal concentrating ability of the kidney dictates how much urine volume must be excreted each day to rid the body of metabolic waste products and ions that are ingested
what is osmotic load (Definition and number)
-solutes ingested and wastes produced by metabolism
- 600 mosmol/day
how do you calculate OUV
OUV = osmotic load/ max urine concentration
what are the requirements for excreting a concentrated urine
- high levels of ADH
- hyperosmotic medullary ISF
- high interstitial fluid osmolarity
what does hyperosmotic medullary ISF set a gradient for
water reabsorption
what does hyperosmotic medullary ISF require
counter current multiplier system
what is a hyperosmotic medullary ISF a function of
juxtamedullary nephrons
-long loop of henle
-vasar recta
-slow flow rate
what is high interstitial fluid osmolarity a result of
- active transport of Na+ and other ions by ascending limb of LOH
-active transport of ions from CD into interstitium - facilitated diffusion of urea by inner medullary CD
-movement of only small amounts of water into medullary interstitium
summarize filtrate handling along the nephron
-continuous delivery of NaCl from PCT into loop of henle
- concentration of filtrate by water reabsorption by descending limb of henle
- continuous reabsorption of solute into interstitium by TAL
- Na+ K+ pumps in CD pump Na+ into interstitium to increase osmolarity and form interstitial osmolar gradient
what happens in the distal tubule and collecting duct in the absence of ADH
- reabsorb solute
- little to no water reabsorption
what happens in the distal tubule and collecting duct in the presence of ADH
- DT and CD become highly permeable to water and reabsorb much water into cortical interstitium
- medullary CD cells reabsorb water but overall amounts much lower
-reabsorbed water carried away by vasa recta
what is urea and what makes it
- waste product of protein metabolism
- produced continuously by liver
how much urea is excreted
normally excrete 20-50% of filtered load
is urea toxic
no but constitutes an osmotic load that must be excreted
what percentage of interstitial osmolarity does urea contribute
40-50% (500-600 mOsm/L)
what is urea passively reabsorbed by
medullary collecting duct cells
where is urea secreted and what is required for urea secretion
- secreted into descending limb and thin ascending limb of loop of henle
-requires ADH
