Loop of Henle

Question 1

What does the proximal tubule do?

Answer 1

Major site of reabsorption

65-75% of all NaCl and H2O

100% of all nutritionally important substances
-e.g. glucose, amino acids etc

Question 2

What protein is filtered?

Answer 2

Not much at all but some does get through particularly albumin
-about 0.5% of the total amount presented at the glomerulus

Completely reabsorbed by a Tm carrier mechanism in the proximal tubule

Question 3

How are drugs and pollutants excreted?

Answer 3

Many are non polar and so hard to filter

Liver metabolises them to polar compounds thus reducing their permeability and facilitating their excretion

Question 4

What is the fluid that leaves the proximal tubule, entering the loop of henle like?

Why is it like this?

Answer 4

Isotonic with plasma
-300mOmoles/l

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

Question 5

Where are the proximal and distal tubules located?

Answer 5

ALL of the nephrons have their proximal and distal tubules in the cortex and all nephrons have common processes for the reabsorption and secretion of solutes of the filtrate

However a very special system, essential for water balance is attributable to the loops of Henle of Juxtamedullary nephrons

Question 6

What does the loop of Henle do?

Answer 6

Special system, essential for water balance

Through this mechanism, the kidney is able to produce concentrated urine in times of H2O deficit, a major determinant of our ability to survive without water

Question 7

What is the maximum concentration of urine that can be produced by the human kidney?

Answer 7

1200-1400mOsmoles/l

i.e. 4x more concentrated than plasma = excess of solute over water

(Desert species can produce urine as concentrated as 6000mOsmole/l, all H2O needs can be met by metabolic H2O)

Question 8

Why are humans limited to a minimum urine volume?

Answer 8

The urea, sulphate, phosphate, other waste products and non waste ions (Na+ and K+) which must be excreted each day amount to around 600 mOsmoles

This therefore requires a MINIMUM obligatory H2O loss of 500mls

As long as the kidneys are functioning, this volume will be excreted, even if there is NO H2O intake -> urinate to death

Question 9

What is the minimum concentration of urine humans can produce?

Answer 9

In conditions of excess H2O intake, H2O is excreted in excess of solute, MINIMUM [URINE] in man is 30-50mOsmoles/l

i.e. 10 fold dilution compared with plasma

Question 10

How can the loops of Henle of juxtamedullary nephrons help to produce urine of varying concentration?

Answer 10

Act as counter current multipliers

Question 11

What do we mean by counter-current?

Answer 11

Fluid flows DOWN the descending limb and UP the ascending limbs with both limbs lying next to eachother

Question 12

What are the critical characteristics of the loops of Henle which make them counter-current multipliers?

Answer 12

1. The ascending limb of the loop of Henle actively co-transports Na+ and Cl- ions out of the tubule lumen into the interstitium.

The ascending limb is impermeable to H2O

2. The descending limb is freely permeable to H2O but relatively impermeable to NaCl

Question 13

The loop of Henle starts off filled with stationary fluid of [300mOsm/l]
i.e. isosmotic with plasma.

What happens to the interstitium as NaCl is pumped out of the ascending limb?

Answer 13

Concentration of NaCl falls and that of the interstitium rises.

This occurs until a limiting gradient of 200mOsm is established in the ascending limb.

Question 14

What happens to the descending limb when NaCl from the ascending limb moves into the interstitium?

Answer 14

Descending limb is now exposed to greater osmolarity in the interstitium, H2O will move out to equate the osmolarity.

Question 15

What happens to water that leaves the descending limb and enters the interstitium?

Answer 15

H2O does not stay in the interstitium, but is reabsorbed by the high oncotic pressure and tissue pressure into the vasa recta (Starling’s forces)

Constantly removed

Question 16

How does a high osmolarity entering the ascending limb perpetuate the loop of Henle’s process?

Answer 16

Fluid moves round the loop of Henle entering the proximal and leaving at the distal tubule

Concentrated fluid in descending limb rounds the bend and delivers a [high] to the ascending limb

Active NaCl removal

further concentrates the interstitium

MULTIPLIER EFFECT

Question 17

How does the greater concentration of descending limb perpetuate the multiplier effect?

Answer 17

Greater concentration of descending limb (by removal of water) means greater concentration of interstitium by addition of salt from ascending limb

MULTIPLIER EFFECT

Question 18

What is the end change to the fluid that exists the loop of Henle into the distal tubule?

Answer 18

The fluid has been dilated without adding any water.

Water has been removed but even more NaCl removed so dilute

Question 19

What is the difference in mOsmole gradient at each horizontal level of the loop of Henle?

Answer 19

200mOsmole difference between descending and ascending limb

Question 20

How does frusemide work?

Answer 20

The key step in water retention by the kidney is the active transport of NaCl out of the ascending limb.

Without this the high osmolarity of the interstitium isnt set up and urine can only be isotonic to plasma.

This is what furesimide does

Question 21

What does the counter-curent multiplier achieve?

Answer 21

1. Concentrates fluid on the way down and promptly re-dilutes it on the way back up, NOT by adding H2O, but by removing NaCl.
2. One consequence of this is that 15-20% of the initial filtrate (up to 36l) is removed from the loop of Henle.
3. Fluid which enters the distal tubule is MORE DILUTE than plasma

CONCENTRATES THE MEDULLARY INTERSTITIUM

DELIVERS HYPOTONIC FLUID TO THE DISTAL TUBULE

Question 22

What are the vasa recta?

Answer 22

Branch off the efferent arterioles of juxtamedullary nephrons (those nephrons closest to the medulla), enter medulla, and surround the loop of Henle.

Are peritubular capillaries, specifically those that surround the loop of Henle

Specialised arrangement participate in the countercurrent mechanism by acting as counter-current exchangers.

As with all capillaries the vasa recta are freely permeable to H2O and solutes and therefore equilibriate with the medullary interstitial gradient.

Question 23

Why are the vasa recta needed / why does the loop of henle need to loop away from the proximal and distal tubules?

Answer 23

If medullary capillaries drained straight through they would carry away the NaCl removed from the loop of Henle and abolish the interstitial gradient.

Does not happen because they are arranged as hairpin loops and therefore do not interfere with the interstitial gradient

Question 24

What are the functions of the vasa recta?

Answer 24

1. Provide O2 for medulla
2. In providing O2 must not disturb gradient
3. Removes volume from the interstitium, up to 36l/day

Question 25

Why does flow rate help the vasa recta do their function?

Answer 25

Very low flow rate so that there is plenty of time for equilibration to occur with the interstitium, further ensuring that the medullary gradient is not disturbed

Question 26

Where is the site of water regulation in the kidney?

Answer 26

Collecting duct, whose permeability is under the control of ADH = Anti-Diuretic Hormone (Vasopressin)