Loop of Henle

Question 1

What are the two limbs that form the loop of henle?

Answer 1

Descending (cortex to medulla) - H2O permeable
Thick Ascending (medulla to cortex) - H2O impermeable

Question 2

What happens to the osmolality as the filtrate passes through the loop?

Answer 2

Enters the loop at 285mOsm but leaves hypotonic at 90mOsm as solute concentration falls.

Question 3

What is the function of the ascending limb?

Answer 3

To maintain a hypertonic environment in the medulla to drive H2O absorption in the descending limb and collecting ducts. This concentrates urine.

Question 4

How is water reabsorbed in the descending limb?

Answer 4

Moves paracellularly due to the osmotic force in the medulla, created by a hypertonic environment.

Question 5

How much osmotic gradient is maintained between the descending and ascending loops?

Answer 5

200mOsm - maintained by ascending limb.

Question 6

How does the ascending limb create the hyperosmotic medulla environment?

Answer 6

Uses NKCC2 to move ions into the highly osmotic interstitial space of the medulla. It is a Na-K-2Cl cotransporter moving them from the filtrate into the cell.
The cotransported requires K to be recyclyed as extracellular concentration is low. Transported back into cell via Na/K ATPase and then across the apical membrane into the filtrate for NKCC2 function. Na/K ATPase, K channels and Cl channels move the ions into the interstitium across the BL membrane.

Question 7

What is furosemide and how does it work?

Answer 7

A potent diuretic that blocks NKCC2 for 20% Na excretion with water following = natriuresis and diuresis.
Used in cardiac failure and renal failure to drive excretion in the remaining nephrons and to reduce fluid overload.
S.E include hypovolaemia, hypokalaemia, arrhythmia, metabolic acidiosis and loss of Mg and Ca.

Question 8

How much sodium is normally excreted?

Answer 8

0.7% of filtered Na

Question 9

How much water is reabsorbed in the descending limb?

Answer 9

15%

Question 10

How much NaCl is reabsorbed in the ascending limb?

Answer 10

15%

Question 11

What is the main function of the loop of Henle?

Answer 11

To create high osmotic concentrations in the medulla so that the filtrate can concentrated in the collecting ducts.

Question 12

What is the countercurrent multiplier?

Answer 12

The mechanism which allows maximum NaCl reabsorption by using opposite flow of filtrate in the two limbs. It creates a longitudinal gradient of so that the fluid leaving the medulla to the distal tubule is hypotonic.

Question 13

What is the osmolality of the interstitium equal to and why?

Answer 13

Interstitium is equal to the descending limb as the fluid forms an equilibrium as it is water permeable.

Question 14

How does the ascending limb increase the omsolality of the interstitium?

Answer 14

Actively transports Na into interstitium to maintain an osmotic gradient between the ascending limb and interstitium of 200mOsm.

Question 15

What happens to the water that is reabsorbed into the interstitium and why is this important?

Answer 15

H2O is absorbed by the vasa recta and this removal ensures a gradient for water to move down by preventing equilibrium forming.

Question 16

How does Na move in the ascending limb?

Answer 16

It can move passively in the thin ascending limb as the walls are permeable.
In the thick ascending limb it requires active transport using Na/K ATPase on the basolateral membrane, once it has entered the cell via NKCC2.

Question 17

Briefly describe the countercurrent multiplier.

Answer 17

As more water is reabsorbed into the hyperosmotic interstitium, the filtrate of the descending limb becomes hyperosmotic as the limb is not permeable to Na, and so solute concentration rises. As the loop changes into the ascending limb, it becomes impermeable to water but allows solute movement from the hypertonic filtrate into the hypertonic medulla via active transport. As more solute leaves the limb, the filtrate becomes hypotonic, ready to enter the distal tubule for concentration.

Question 18

Where is the highest osmolality achieved in the medulla?

Answer 18

At the tip of the loop it reaches 1400mOsm. This creates a longitudinal gradient from the tip to the cortex of 1400 to 90mOsm

Question 19

Why is the countercurrent better than a linear gradient?

Answer 19

The countercurrent produces a higher starting osmolality and therefore greater gradient for Na and Cl to move down for maximum reabsorption. Linear gradient would begin at 285mOsm and would quickly reach maximum ionic gradient.