Mechanisms to Adjust Urine Concentration Flashcards
How can you determine if a loop diuretic is working properly?
FENa should increase
How can you assess renal function for a person on diuretics?
FE of Urea
It’s a good measure of filtration and concentration, and the diuretics don’t screw with it much
A patient had an MI, and now has prerenal azotemia. What does this mean?
Why does he have it?
BUN/Cr > 20:1
Decreased C.O. = decreased renal perfusion = increased tubular reabsorption = more BUN reabsorbed
Why can problems w/ Na/H2O balance cause neurologic problems?
Can cause shrinking or swelling of the brain
How does luminal charge change through the nephron?
Why?
More negative
Tight junctions get tighter, Cl- left in lumen
Reabsorption of __ and __ is linked to Na+ reabsorption
Cl- and H2O
How is water reabsorbed throughout the nephron?
Passive, transcellular, or paracellular
Function of Cl- reabsorption
Balance (+) charges of reabsorbed cations
A normal nephron can concentrate the urine to 20x its starting osmolarity (example). A patient has broken NaKCl2 transporters in the TAL. What is their concentrating ability?
How do you figure?
10x
W/o that transporter, concentrating ability is cut in 1/2
What is the luminal charge of the DCT?
Why?
Negative
Cl- can’t get through tight junctions
What drives the secretion of K+ and H+ in the late DT and CD?
Negative lumen (leftover Cl-)
In a well hydrated person, what is the permeability of H2O in the CD?
In a dehydrated person?
None - no ADH - no aquaporins
High - more ADH and increased Na+ reabsorption in loop
How does the osmolarity in the TAL compare to the interstitium?
200 less
2 main factors that cause urine concentration in the loop
NaKCl2 transporter
Urea recycling (ADH)
In a state of diuresis (volume expansion), how does the medullary interstitium osmolarity compare to normal?
Why?
1/2
No ADH, so no recycling of urea
Obligatory water loss
Minimum volume that must be excreted to rid body of created waste each day
Normal conditions: what is the minimum obligatory water loss amount?
If the kidney’s couldn’t concentrate urine?
Why?
0.5L per day
2L
Blood osmolarity = 300
Max urine osmol. = 1200
4x increase in osmolarity from blood to urine
Osmolar clearance
How to calculate?
The volume of plasma cleared of osmotically active particles per unit time
Same as clearance of any ONE substance, but using total urine and plasma osmolarities
Cosm = (Uosm x V) / Posm
Free water clearance
How to calculate?
Water excreted in excess of that needed to excrete isosmotic urine
V - Cosm
Urine volume minus osmolar clearance
Pt urine collected for 2 hr, total volume = 600 ml. Urine osmolarity = 150 mOsm/l. Plasma osmolarity = 300 mosm/L. Free water clearance?
+ 2.5 ml/min
A patient has neurogenic diabetes insipidus. What is the effect on free water clearance?
Why?
Increased
Not reabsorbing water through aquaporins
Fractional excretion
Percentage of filtered load that is excreted
How to calculate fractional excretion of a substance?
(Ux • Pcr) / (Px • Ucr) x 100
Another way to think of fractional excretion calculation
Amt excreted / Amt filtered
A patient has labs done, then is given a diuretic, then has labs done again. His FENa went __. Why?
Up
More Na+ excretion = more water excretion = diuretic purpose
A patient has labs done, then is given a diuretic, then has labs done again. His FENa went up, but his urine [Na] remained constant.
Why?
Water is proportionately retained in the lumen as Na+. So the [Na] doesn’t change.
