Loop of Henle

Question 1

What is the major site of reabsorption?

Answer 1

Proximal tubule (65-75% of all NaCl and H₂O; all nutritionally important substances)

Question 2

How much of the albumin presented at the glomerulus gets through?

Answer 2

~30g protein/day

0.5% (however this is ultimately all reabsorbed by a T_m carrier mechanism in the proximal tubule)

Question 3

How does the nonpolar and therefore highly lipid soluble character of drugs/pollutants aid in their reabsorption?

Answer 3

The removal of H₂O in the proximal tubule establishes concentration gradients for their reabsorption

Question 4

Therefore since drugs/pollutants are highly lipid soluble you would think we could never get rid of them - but how does the liver aid in their excretion?

Answer 4

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

Question 5

Review of tubule function

Answer 5

Question 6

review of tubule function 2

Answer 6

Question 7

The fluid that leaves the proximal tubule is…

Answer 7

ISOSMOTIC with plasma i.e. 300mOmoles/L

Question 8

Why is the fluid that leaves the proximal tubule isosmotic with plasma?

Answer 8

ALL solute movements are accompanied by equivalent H₂O movements, so that osmotic equilibrium is maintained

Question 9

Where do all nephrons have their proximal and distal tubules? Do all nephrons have common processes for reabsorption and secretion of solutes of the filtrate?

Answer 9

The Cortex

Yes

Question 10

A special system is attributable to the loops of Henle of juxtamedullary nephorns, what is it essential for?

Answer 10

Water balance

Through this mechanism - the kidney can produce concentrated urine in times of H₂O deficient (a major determinant of our ability to survive without water)

Question 12

What is the maximum concentration of urine that can be produced by the human kidney? How much more concentrated is this than plasma?

Answer 12

1200-1400mOsmoles/l

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

Question 13

Urea, sulfate, phosphate, other waste products and non-waste ions (Na+ and K+), which must be excreted each day amount to how many mOsmoles?

Answer 13

600

Question 14

So the substances excreted each day amount to 600mOsmoles, this therefore means a minimum obligatory H₂O loss of how much?

Answer 14

500mls

(this occurs even if there is no H₂O intake - urinate to death!)

Question 15

What is the minimum [urine] in man? (e.g. in conditions of excess H₂O intake when H₂O is excreted in excess of solute)

Answer 15

30-50mOsmoles/L

(i.e. 10x dilution compared with plasma)

Question 16

What do the loops of Henle of juxtamedullary nephrons act as to allow the kidney to produce urine of varying concentrations?

Answer 16

Counter-current multipliers

Question 17

How does fluid flow in counter-current in the loop of Henle?

Answer 17

Fluid flows down the descending limb and up the ascending limb

Question 18

What are the 2 critical characteristics of the loops which make them counter-current multipliers? (hint: 1 for descending limb; 1 for ascending)

Answer 18

1. Descending limb is freely permeable to H₂O but relatively impermeable to NaCl
2. 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 in impermeable to H₂O

Question 19

How does the loop of Henle start off?

Answer 19

Filled with stationary fluid of [300mOsm/l] i.e. isosmotic with plasma

Question 20

As NaCl is pumped out of the ascending limb (key step), what happens to the fluid?

Answer 20

It’s concentration falls and that of the interstitium rises i.e. the osmolarity in the tubule decreases and it increases in the interstitium - this occurs until a limiting gradient of 200mOsm is established

(reaching this 200mOsm gradient is the ‘aim’ of the ascending limb)

Question 22

So due to the concentration gradient produced by the ascending limb, the descending limb is not exposed to greater osmolarity in the interstitium, and what does H₂O therefore do?

Answer 22

Moves out to equate the osmolarity

(it does not then stay in the interstitium - it is reabsorbed by high πp and tissue P into the vasa recta (Starlings)

Question 23

This is all occuring and fluid is actually still moving - entering at proximal and leaving at distal tubule. Concentrated fluid is therefore continuously being delivered from the descending to the ascending limb - what then occurs at the ascending?

Answer 23

Again, active NaCl removal - further concentrating the interstitium

Question 24

Greater concentration of descending limb (by removal of water) means…

Answer 24

greater concentration of interstitium by addition of salt from ascending limb

Question 25

Fluid in tubule is progressively concentrated as it moves down the descending limb and progressively...

Answer 25

diluted as it moves up the ascending limb

Question 26

What happens to the interstitium as more and more concentrated fluid is delivered to the ascending limb?

Answer 26

It becomes more and more concentrated

Question 27

At any horizontal what is the gradient between the ascending limb and the interstitium?

Answer 27

200mOsmol

Question 28

What is the vertical gradient range in the interstitium?

Answer 28

Goes from 300 -\> 1200mOsmol

Question 29

Summary of the osmolarity in the different parts of the kidney

Answer 29

Note the 200mOsmole gradient at each horizontal level of the ascending limb of the loop of Henle reflects the pumping of the active pumps

Question 30

If the key step of transport of NaCl out of the ascending limb is abolished e.g. by use of diuretic frusemide, what happens?

Answer 30

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

Question 31

How much of the initial filtrate is removed by the loop of Henle as a consequence of re-dilution via removal of NaCl?

Answer 31

15-20% (up to 36L)

Question 32

Is the fluid which enters the distal tubule more or less dilute than plasma?

Answer 32

**More**

Question 33

Overwhelming significance of the counter-current multiplier is that is creates...

Answer 33

an increasingly concentrated gradient in the **interstitium**

Question 34

What is the tonicity of he fluid delivered to the distal tubule?

Answer 34

Hyptonic

Question 35

The loop of Henle is all about...

Answer 35

concentrating the medullary interstitium and delivering hypotonic fluid to the distal tubule

Question 36

What is the vasa recta?

Answer 36

The peritubular capillaries of the juxtamedullary nephrons

Question 37

How do the vasa recta participate in the countercurrent mechanism?

Answer 37

By acting as countercurrent exchangers

Question 38

If medullary capillaries drained straight through they would carry away the NaCl removed from the loop of Henle and abolish the interstitial gradient; why does this not happen?

Answer 38

They are arranged as hairpin loops and therefore do not interefere with the gradient

Question 39

As with all capillaries, the vasa recta are freely permeable to what?

Answer 39

H₂O and solutes And therefore equilibrate with the medullary interstitial gradient

Question 40

Give the 3 main functions of the **vasa recta**

Answer 40

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

Question 41

Why are Starlings forces in favour of reabsorption of water into the vasa recta?

Answer 41

High oncotic pressure (π_p) and high hydrostatic pressure (P_t) in the tubule due to tight renal capsule which drives fluid into capillaries

Question 42

Why is the flow rate through the vasa recta very low?

Answer 42

So that there is plenty of time for equilibrium to occur with the interstitium, further ensuring that the medullary gradient is not disturbed

Question 43

The collection duct is the site of water regulation, what controls its permeability?

Answer 43

ADH (Anti-Diuretic Hormone)