10 – Urine Concentration

Question 1

How much of filtered urea is excreted?

Answer 1

-20-50%

Question 2

Proximal tubule reabsorption of filtered urea:

Answer 2

-40-50%
>due to large amounts of water reabsorption (solvent drag)

Question 3

Descending loop of Henle and the collecting duct and the concentration of urea:

Answer 3

-increases because of water reabsorption
>permeable to urea: goes into interstitum due to concentration gradient

Question 4

What segments of the nephron are NOT permeable to urea?

Answer 4

-thick segment of ascending limb of loop of Henle
-distal tubule
-collecting tubule
*urea concentration increases

Question 5

Urea helps significantly with creating hyperosmotic medullary interstitium:

Answer 5

-diffuses into medullary interstitium because of concentration gradient
>facilitated by urea transporter-1 and 3
-part of the reabsorbed urea diffuses back into loop of Henle and recirculates before excretion

Question 6

Urine transporter-3:

Answer 6

-ADH can increase it
>enhances interstitium gradient

Question 7

Obligatory urine volume:

Answer 7

-least volume of urine required to excrete excess solutes/metabolic waste
*depends on maximum urine concentrating ability of the kidneys
Ex. 1200mOsm/L for humans

Question 8

What happens if we drink sea water?

Answer 8

-osmolarity of 1200mOsm/L=need to excrete that much additional salt
>would need to excrete 1L of urine to get ride of 1200mOsm of salt
*also needs to excrete waste products (ex. urea=600mOsm/day)=need an additional 0.5L of urine

Question 9

Urine specific gravity:

Answer 9

-measure of urine concentration (g/L)
-often used in clinics
-changes correlates with changes in urine osmolarity (linear), but not a complete overlap

Question 10

Urine specific gravity value considers:

Answer 10

-number AND molecular weight of solutes (size)
>vs. osmolarity is ONLY related to number of molecules

Question 11

What can cause the urine specific gravity to be mistakenly interpreted as increased concentration of urine?

Answer 11

-when larger molecules are excreted in urine

Question 12

What happens if a rabbit has not had water for 2 days?

Answer 12

-increase in maximum urine osmolarity
>able to concentrate it more to try and ‘conserve’ water

Question 13

Urine concentration capacity varies in different species and diets:

Answer 13

-high protein helps with concentrating urine, but then more urea needs to be excreted
*need to keep rehydrated to limit risk for kidney stones (from urine being so concentrated!)

Question 14

Urine concentration capacity correlates with:

Answer 14

-renal relative medullary area (RATIO)
>more medulla compared to cortex=allows for generation of more concentrated urine
>can establish the medullary interstitium concentration gradient better

Question 15

Camels medulla to cortex ratio:

Answer 15

-very high (10.6)
>able to concentrate urine more

Question 16

What are 2 characteristics of renal blood flow in the medulla that help with maintaining its high solute concentration?

Answer 16

1. Slow blood flow in vasa recta capillaries (in juxtamedullary nephrons)
2. Countercurrent exchanger mechanism

Question 17

How does blood enter and leave the medulla?

Answer 17

-via vasa recta (U-shaped capillary)

Question 18

Vasa recta is permeable to:

Answer 18

-water
-solutes

Question 19

What happens when blood goes down the vertical gradient of medullary interstitium? (water and solute movements)

Answer 19

-water is pulled OUT of vasa recta b/c of high osmolarity of interstitium
-solutes move INTO vasa recta b/c of concentration gradient

Question 20

Blood osmolarity as it moves deeper into medulla:

Answer 20

-increases to match increasing osmolarity of medulla

Question 21

What is blood osmolarity at the bottom of the U-shaped vasa recta?

Answer 21

-osmolarity is equal to interstitium

Question 22

What happens when blood goes up the vertical gradient of medullary interstitium? (water and solute movements)

Answer 22

-water moves back INTO vasa recta b/c of osmolarity
-solutes move OUT of vasa recta b/c of concentration gradient

Question 23

Blood osmolarity as it goes up toward cortex?

Answer 23

-decreases to match surround interstitium