Loop of Henle

Question 1

If drugs are nonpolar and high lipid soluble, when water is absorbed in the renal system leaving high concentrations of these substances to be reabsorbed, what stops drugs and pollutants being reabsorbed?

Answer 1

The liver metabolizes them to polar compounds - meaning they get excreted

Question 2

The fluid that leaves the proximal tubule is…

Answer 2

Isosmotic with plasma (meaning 300 mOmoles/L)

Question 3

How is fluid leaving the proximal tubule isosmotic?

Answer 3

All the solute movements are accompanied by equivalent H2O movements, so that osmotic equilibrium is maintained.

Question 4

Do all of the nephrons have their proximal and distal tubules in the cortex?

Answer 4

Yes

Question 5

Do all nephrons have common or different process for reabsorption and secretion ?

Answer 5

Common

Question 6

Where does the “very special system” essential for water balance occur? What does it do?

Answer 6

Loops of Henle of juxtamedullary nephrons

Allows the kidneys to create concentrated urine in times of water deficit

Question 7

What is the max conc. of urine that can be made by human kidneys?

Answer 7

1200-1400 mOsmoles/L

4x more conc. than plasma

Question 8

What does all the waste products that must be secreted each day amount to in mOsmoles? How much water is lost for this to happen?

Answer 8

600 mOsmoles requiring a minimum obligatory H2O loss of 500 mls

Question 9

Does this minimum of 500mls of water always get excreted?

Answer 9

As long as the kidney’s are functioning yes.

It won’t even stop if there is no water intake meaning you will keep losing water

Question 10

What happens if there is excess water intake?

Answer 10

Water is excreted in excess of solute

Question 11

What is the minimum urine concentration of water in man?

Answer 11

30-50 mOsmoles/L

Question 12

How are kidneys able to produce urine of varying concentrations?

Answer 12

Loops of Henle of juxtamedullary nephrons act as counter-current multipliers

Question 13

What does counter current mean in the context of the loop of henle?

Answer 13

Fluid flows down the descending limb and up the ascending limb

Question 14

What critical characteristics of the loops of henle make them countercurrent MULTIPLIERS?

Answer 14

Ascending limb actively co-transports Na and Cl ions out of the tubule into the interstitium and is impermeable to H2O

The descending limb is freely permeable to H2O but doesn’t pump out Na or Cl

Question 15

Explain how this system works.

Answer 15

As NaCl is pumped out of the ascending limb, the concentration inside the lumen falls and in the interstitium is rises. This occurs until a gradient of 200 mOsm is established

This increased concentration/osmolarity in the interstitium means that when fluid comes down the descending limb, water leaves the lumen and enters the interstitium which concentrates the fluid inside the descending limb. The interstitium concentration is unaffected by the increase in water from descending limb

Question 16

Why does water from the descending limb not affect the conc. of the interstitium?

Answer 16

It doesn’t stay there, is reabsorbed by the high osmotic pressure and tissue pressure into the vasa recta

Question 17

Continue explaining the system.

Answer 17

So since fluid is actually moving down the descending and up ascending limbs, this means that at the bottom of the ascending limb you get very highly concentrated solution that has just left the descending limb.

In order to keep the 200 mOsmol’s conc. gradient with the interstitium, the interstitium near the bottom is much more concentrated than near the top where the ascending limb fluid is less concentrated

Question 18

From top to bottom, what is the vertical gradient of the interstitium?

Answer 18

300-1200 mOsmol

Question 19

If the pumping on NaCl from the ascending limb is abolished, what would happen? how can this happen?

Answer 19

All concentration differences are lost and the kidney can only produce isotonic urine

Diuretic frusemide

Question 20

So in the end, what has the countercurrent multiplier achieved?

Answer 20

1. Concentrates fluid on the way down and promptly re-dilutes it on the way back up by removing NaCl.
2. One consequence of this is that 15-20% of the initial filtrate (up to 36 l) is removed from the loop of Henle
3. Fluid which enters the distal tubule is more dilute than plasma and is hypotonic.

Most important effect is the creation of an increasingly concentrated gradient in the interstitium.

Question 21

If fluid enters the loop of Henle at 300 mOsm, what level does it leave at?

Answer 21

100 mOsm

Question 22

What is the vasa recta? How are they important?

Answer 22

A specialized arrangement of the peritubular capillaries of the juxtamedullary nephrons that supply O2 to the medullary WITHOUT disrupting the interstitial gradient set up by loops of henle

Question 23

How does the vasa recta not destroy the gradient?

Answer 23

The blood flow in the vasa recta is sluggish, allowing time for the plasma in the vasa recta to equilibrate with the surrounding interstitial fluid. The osmolarity of the plasma inside the vasa recta increases as it descends into the medulla, and then decreases again on the ascending side.

This allows blood to flow to the medulla, without eliminating the osmotic gradient.

Question 24

So what is the point of setting up this interstitial gradient?

Answer 24

When water exits the loop of henle, it enters the collecting ducts.

If the body needs to conserve water, ADH is released into the collecting ducts and binds to aquaporins opening water channels into the interstitium

Since the interstitium is now salty due to the gradient, water can flow freely out and get reabsorbed and the urine gets more concentrated the further down the gradient it does

If water does not need to be conserved the aquaporins simply stay shut

Question 25

If any of this remains confusing, youtube is a great source and re-read the lecture again!

Answer 25

Will do