Renal: Clearance, Water Balance

Question 1

what is clearance?

Answer 1

a noninvasive evaluation of how the renal tubule handles a given substrate S.

the kidney’s ability to remove substances from the body.

tells the filtering capacity of the glomerulus: i.e., it tells whether the filtering function is okay.

Question 2

what is the formula for clearance?

Answer 2

clearance of [S] = ([S] in urine / [S] in plasma) x urine flow rate

Cs = (Us / Ps) x V

expressed as volume per time

Question 3

what type of compound is insulin?

Answer 3

polysaccharide starch

Question 4

True or False. Insulin is taken up by tissues

Answer 4

false. it is not

it is freely filtered in glomerulus but neither reabsorbed nor secreted insuline clearance = GFR

Question 5

what is another approach to estimating GFR?

Answer 5

creatinine clearance

creatinine is broken down from creatine phosphate and produced at a steady state

Question 6

Why is creatinine a good estimator of GFR?

Answer 6

it’s filtered but not reabsorbed, thus clearance is a good estimator of GFR

however sight over estimate b/c small protion is secreted in the proximal tubule (PT)

Question 7

what are typical serum creatinine ranges?

Answer 7

men 0.7 - 1.2 mg/dL

women 0.5 - 1.0 mg/dL

Question 8

what is useful to estimate RPF (renal plasma flow)?

Answer 8

PAH (para-aminohippurate)
freely filtered and secreted, larger value than GFR

can detect obstruction of renal blood flow

Question 9

True or False. If a substance is completely secreted then its clearance would be smaller than GFR and equal to RPF.

Answer 9

False. It would be larger than GFR.

note: if substance is mostly reabsorbed it is smaller than GFR

Question 10

What is clearance if the substance is completely reabsorbed?

Answer 10

0

Question 11

What is avg urine output per day? what is minimal volume?

Answer 11

a. 1.5 L / day

b. 0.4 - 0.5 L/day (obligatory H2O loss)

Question 12

what is daily production of osmolytes?

Answer 12

600 millosmoles

Question 13

what is max. possible urine osmolarity?

Answer 13

1200 - 1400 mosmol/L

Question 14

True of False. water reabsorption does not occur in the ascending limb and distal tubule.

Answer 14

true

Question 15

what is the osmolarity in the nephron?

Answer 15

tip of loop of henle — hyperosmotic

distal tubule — hyposomotic

Question 16

why does osmolarity increase in the descending loop of henle?

Answer 16

absorbs H2O only (water channels - AQP)

does not absorb Na

Question 17

why does osmolarity decrease in ascending limb?

Answer 17

poor ability to absorb H2O

but does absorb Na (Na/K/Cl cotransporter NKCC)

Question 18

why happens to osmolarity in cortical collecting duct? medullary collecting duct?

Answer 18

a. both Na and H2O are absorbed

b. H2O reabsorption dominates

Question 19

what is responsible for generating an osmotic gradient in the fluid and medullary interstitium.?

Answer 19

asymmetric distribution of water channels and NKCC

Question 20

How does the asymmetric expression of AQP and NKCC contribute to an osmotic gradient in the medulla?

Answer 20

counter current multiplication

Question 21

what provides a driving force for H2O reabsorption in the collecting duct

Answer 21

the medulllary osmolartiy established by the loop of henle

Question 22

in addition to Na, what osmolyte is important for conentrating urine?

Answer 22

urea (via ADH-regulated urea transporters)

in distal tubule and collecting duct