2.4. The Loop of Henle - Counter-Current Multiplier Mechanisms of Action

Question 1

What is the osmolarity of the solution in the loop of henle?

Answer 1

Isosmotic with plasma = 300mOsmoles/L

Question 2

What happens in the ascending limb of the loop of henle?

Answer 2

Na and Cl are pumped out forming a limiting gradient

Question 3

What is the value of the limiting gradient?

Answer 3

200mOsmoles

Question 4

What establishes the 200mOsmole limiting gradient?

Answer 4

1. The concentration inside the tubule falling

2. The concentration inside the interstitium rising

Question 5

What is the descending limb exposed to, due to the removal of Na and Cl in the ascending limb?

Answer 5

A greater osmolarity in the interstitum

Question 6

What is the result of the descending limb being exposed to a greater interstitial osmolarity

Answer 6

H2O will be removed to equate the osmolarity

Question 7

What happens to the H2O, once it has left the descending limb of the loop of henle, to equate the osmolarity?

Answer 7

It is reabsorbed by the high oncotic and tissue pressure into the Vasa Recta

Question 8

What happens to the tubule fluid as you go down the descending limb?

Answer 8

It becomes more concentrated

Question 9

Where is the tubule fluid most concentrated? What roughly is that concentration?

Answer 9

it becomes most concentrated as it rounds the bend and moves towards the ascending limb = about 1200mOsmoles

Question 10

What happens to the tubule fluid as you go up the ascending limb?

Answer 10

It becomes more diluted

Question 11

How does the tubule fluid become more diluted as it ascends up the ascending limb?

Answer 11

This is due to the Na and Cl being pumped out - not the reabsorption of water into the tubule

Question 12

What is the gradient between the ascending limb and the interstitium at all times?

Answer 12

200mOsmoles

Question 13

What does the decreasing concentration of the descending limb result in?

Answer 13

This results in a greater concentration of the interstitium by the addition of salt from the ascending limb

Question 14

What is the key step in the counter-current multiplier?

Answer 14

The step where the Na and Cl are pumped out of the ascending limb, creating the interstitial gradient

Question 15

1. What happens if there is no Na and Cl being pumped out of the ascending limb?
2. What drug acts by this mechanism?

Answer 15

1. All of the concentration differences are lost and an isotonic urine is produced
2. Frusemide (Diuretic)

Question 16

What happens to the concentrated gradient in the interstium?

Answer 16

It is also increased, alongside the tubule

Question 17

What does the increasingly concentrated interstitium allow for?

Answer 17

1. Concentrate the tubule fluid on the way down / re-dilute it on the way up by removing salt
2. 15-20% of the initial filtrate is removed
3. The fluid which enters into the distal tubule is more diluted than plasma