Role of urea in concentrating urine

Question 1

Urea excretion

Answer 1

-20-50% of urea is excreted

Question 2

Urea in proximal tubules

Answer 2

-proximal tubules reabsorb 40-50% of filtered urea

Question 3

Urea in descending loop of Henle

Answer 3

-concentration of urea increases because of water reabsorption

Question 4

Urea in other areas of nephron

Answer 4

-thick segment of ascending limb of loop of Henle, distal tubule and collecting tubule are not permeable to urea

Question 5

Ureas role in concentrating urine

Answer 5

-Na, Cl, urea helps significantly with creating hyperosmotic medullary interstitium

Question 6

Urea in collecting tubules

Answer 6

-water reabsorption in collecting tubules resulting in urea concentration increasing creating a concentration gradient allowing urea to diffuse into medullary interstitium

Question 7

How is urea transported into medullary interstitium?

Answer 7

**concentration gradient
-facilitated diffusion by urea transporter-1 and urea transporter-3 (UT-1 and UT-3)

Question 8

Obligatory urine volume

Answer 8

-the least volume of urine required to excrete excess solutes/metabolic waste
**depends on the maximum urine concentrating ability of the kidneys

Question 9

If our kidneys concentrate urine to ~1200 mOsm/L and we need to excrete 600 milliosmoles of solutes per day, what is the obligatory urine volume?

Answer 9

600/1200= 0.5 L/day

Question 10

Drinking sea water

Answer 10

-sea water osmolarity= 1200 mOsm/L
- if you drink 1L of sea water, body needs to excrete 1200 mOsm of additional salt
-means that body needs to excrete 1L of urine because this is the maximum capacity of our kidneys to concentrate urine. BUT since the body needs to excrete waste products too, it would also require another 0.5L of urine

Question 11

Urine specific gravity

Answer 11

-the measure of urine concentration (g/L)

Question 12

Changes in urine specific gravity

Answer 12

-correlates with changes in urine osmolarity
BUT not a complete overlap!!

Question 13

Difference between urine specific gravity and osmolarity

Answer 13

-urine specific gravity considers the number and molecular weight of solutes
-osmolarity only related to the number of molecules

Question 14

Normal urine specific gravity of humans

Answer 14

1.002-1.028 g/L

Question 15

Effects of larger molecules on urine specific gravity

Answer 15

-when larger molecules (such as antibiotics) are excreted in urine, this measurement can be mistakenly interpreted as increased concentration of urine

Question 16

Urine concentration capacity in different species

Answer 16

-varies in different species
-high protein diet (increased urea) helps with concentrating urine (preserving water when needed) however more urea also needs to be excreted too
**when too concentrated, can result in stones

Question 17

Urine concentration capacity

Answer 17

-correlates with renal relative medullary area
>the more relative medullary area, the more concentrated urine that can be produced

Question 18

What 2 characteristics of renal blood flow in the medulla help maintain high solute concentration?

Answer 18

1.slow blood flow in vasa recta capillaries
2.Countercurrent exchanger mechanism

Question 19

Countercurrent exchanger mechanism

Answer 19

-Vasa recta is permeable to water and solutes
-when blood goes down the vertical gradient of medullary interstitium, water is pulled out of vasa recta because of concentration gradient
-when blood goes up the vertical gradient, solutes move out of the vasa recta because of concentration gradient and water moves back into vasa recta because of osmolarity
-blood enters and leaves the medulla via the vasa recta (U-shaped capillary)
-as blood moves deeper in the medulla, its osmolarity increases to match the increasing osmolarity of medulla
-Bottom of u shaped capillary= osmolarity is equal to interstitium (1200 mOsm/L)
-as blood goes up toward cortex, its osmolarity decreases to match that of surrounding interstitium (water moves back in and solutes diffuse into the interstitium)