Medullary counter-current mechanism

Question 1

What is the concentration of the interstitial fluid in the cortex of the kidney?

Answer 1

300 mOsm/kg

Question 2

What is the concentration of the interstitial fluid in the deep medulla of the kidney?

Answer 2

1200 mOsm/kg

Question 3

How does the concentration of the interstitial fluid from the cortex to the deep medulla of the kidney change?

Answer 3

It increases

Question 4

What is the name of the concentration gradient in the interstitial fluid from the cortex to the medulla?

Answer 4

The corticopapillary gradient

Question 5

What type of nephron creates the corticopapillary gradient?

Answer 5

Juxtamedullary nephrons

Question 6

What part of the juxtamedullary nephron creates the corticopapillary gradient?

Answer 6

Loop of Henle

Question 7

What is the first step in creating the corticopapillary gradient?

Answer 7

Thick ascending limb of loop of Henle
sodium ions and chloride ions move from tubular lumen into interstitial fluid
creates an osmotic gradient of 200 mOsm/kg between the tubular lumen and the interstitial fluid
interstitial fluid has a higher osmolality

Question 8

What is the second step in creating the corticopapillary gradient? (after thick ascending limb step)

Answer 8

Descending limb of loop of Henle
water molecules move from tubular lumen into interstitial fluid
until osmolality in tubular lumen is same as in interstitial fluid
tubular lumen has a higher osmolality than what it started with

Question 9

What is the third step in creating the corticopapillary gradient? (after descending limb step which is after ascending limb step)

Answer 9

Fluid in tubular lumen moves round from the descending limb into the ascending limb

Question 10

What happens after the third step of creating the corticopapillary gradient?

Answer 10

The steps repeat themselves in order

Question 11

What contributes to the corticopapillary gradient?

Answer 11

Urea

Question 12

How does urea contribute to the corticopapillary gradient?

Answer 12

Urea transporters on apical domain of epithelial cells in medullary collecting duct
urea moves from the tubular lumen into the interstitial fluid
high concentration of urea in interstitial fluid in deep medulla

Question 13

What regulates expression of urea transporters on the apical domain of epithelial cells in the medullary collecting duct? How?

Answer 13

ADH

increases expression

Question 14

What happens to urea in the interstitial fluid of the deep medulla?

Answer 14

A small portion of it diffuses back into the loop of Henle

Question 15

What happens to the urea that has diffused back into the loop of Henle from the interstitial fluid in the deep medulla?

Answer 15

It moves with the tubular fluid up the ascending limb of the loop of Henle
into the distal convoluted tubule
into the collecting duct

Question 16

What is the effect of the recirculation of the urea on the corticopapillary gradient? How?

Answer 16

Increases the concentration of urea in the interstitial fluid in the deep medulla
because only a small portion of it diffuses back into the loop of Henle each time, so most it remains there and accumulates there
contributing to corticopapillary gradient

Question 17

What maintains the corticopapillary gradient?

Answer 17

Vasa recta

Question 18

How does blood flow through the vasa recta?

Answer 18

Down the descending limb of the vasa recta

then up the ascending limb of the vasa recta

Question 19

How does blood flow through the vasa recta compare to the flow of tubular fluid in the loop of Henle?

Answer 19

Blood flow down descending limb of vasa recta is ajdacent to tubular fluid flow up the ascending limb of the loop of Henle

Blood flow up ascending limb of vasa recta is adjacent to tubular flud flow down descending limb of loop of Henle

Question 20

What happens to the blood in the descending limb of the vasa recta?

Answer 20

Sodium chloride from interstitial fluid diffuses into it
also it loses water
becomes more concentrated
has a higher osmolality

Question 21

What happens to the blood in the ascending limb of the vasa recta?

Answer 21

Water moves from descending limb of loop of Henle and descending limb of vasa recta move into it
becomes more dilute
has a lower osmolality

Question 22

How does the concentration gradient of the vasa recta compare to the corticopapillary gradient?

Answer 22

They’re the same

Question 23

How do the vasa recta maintain the corticopapillary gradient?

Answer 23

Water moves from descending limb of loop of Henle and descending limb of vasa recta into ascending limb of vasa recta
doesn’t dilute the corticopapillary gradient

Question 24

What is the relative blood flow through the vasa recta? Why?

Answer 24

Slow

To supply metabolic needs of cells
to give time for water to move into it

Question 25

What is the purpose of the corticopapillary gradient?

Answer 25

Allows for water reabsorption in the collecting duct
through the aquaporin channels
from the tubular lumen into the interstitial fluid