Flashcards in Lecture 5- Renal Tubular Function Deck (29):
high water intake
excretion of dilute urine (50mOsm/L)
typical water intake
excretion of urine of normal osmolarity (500 mOsm/L)
low water intake
excretion of concentrated urine (120 mOsm/L)
reabsorption of water and solutes in proximal tubule is
In the thin descending limb there is increased water permeability so tubular osmolarity increases/decreases ?
increases- (due to high osmolarity of interstitial fluid in renal medulla drawing water out)
In the thick ascending limb, only solutes are absorbed so fluid is
In the distal tubule and collecting duct, there is continued absorption of solutes, but not is water if ADH absent so
further ↓ in urine osmolarity
-Patients either fail to make ADH or principal cells fail to respond to ADH
-Lack of production/response to ADH > excrete large volumes of urine (up to max excretion of 18L/day)
What is the maximal urine excretion a day?
(10% of filtered water)
key factors in excretion of concentrated urine (2)
1. high osmolarity of medullary interstitium
2. high water permeability of DT and CD (from high ADH)
How much solute must you excrete a day?
600 mOsm (50% urea)
since mas osmolarity of urine is 1200 mOsm, the minimum volume excreted is 0.5L
High ADH - high water permeability of DT and CD will lead to
high water reabsorption
What does extensive water reabsorption in the DT and CD prevent?
excessive dilution of medullary intersitium
Urea is how much percent of max medullary osmolarity?
40% (500 mOsm/L) of the 1200 mOsm/L
What is urea?
a waste product from AA catabolism
How much urea do the kidneys need to clear?
25-35gm uea/day (50% of filtered urea is excreted
urea in PT
↑[urea] due to water reabsorption that's faster than urea
urea in thin loop of henle
↑[urea] due to high permeability & concentration in medullary interstitium
urea in thick loop of henle, DT, cortical CD
impermeable to urea
↑ ADH = ↑ water reabsorption from DT and CCD = ↑ [urea]
permeable to urea > urea diffuses into medullary interstitium
urea cycles from loop of henle back to medullary collecting duct maintains high/low concentration?
delivers blood to medulla without "washout" of solutes
Blood flow is slow (1-2% of total renal BF)
due to high permeability of vasa recta to solutes and water
↑ BF > ↑ "washout" of solutes > ↓ ability to form concentrated urine
purpose of free water clearance
to determine whether water is excreted in excess of solutes or solutes are excreted in excess of water (relative to plasma)
volume of plasma cleared of solutes
C(osm) = U (osm) x V/P(osm)
Free water clearance
difference between urine flow rate and osmolar clearance
(can be positive or negative)
water excretion > solute excretion