The Loop of Henle

Question 1

How much of the albumin gets through the glomerulus?

Answer 1

30 grams a day (0.5% of the total amount presented at the glomerulus)

However it is completely reabsorbed by a Tm carrier mechanism in the proximal tubule

Question 2

How does the liver aid excretion of drugs?

Answer 2

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

When drugs are lipid soluble the removal of H2O in the proximal tubule establishes concentration gradients for their reabsorption - lipid solubility means we would never ged rid of them

Question 3

Describe the osmolarity of the fluid that leaves the proximal tubule

Answer 3

It is isoosmotic with the plasma (300 mosmoles/l)

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

Question 4

Where are the proximal and distal tubules?

Answer 4

In the cortex

Question 5

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

Answer 5

1200-1400 mOsmoles/l (4 times more concentrated than the plasma)

Question 6

What is the minimum amount of waste products and non-waste ions that must be excreted each day?

Answer 6

600 mOsmoles - requires a minimum obligatory H2O loss of 500 mls

This occurs even if there is no O2 intake

Question 7

What is the minimum concentration of urine?

Answer 7

30-50 mOsmoles/l

Question 8

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

Answer 8

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

Question 9

How does the ascending limb affect the concentration of the interstitium?

Answer 9

NaCl is pumped out of the ascending limb, its concentration falls and that of the interstitium rises. This occurs until a limiting gradient of 200mOsm is established. (this is the maximum gradient that can be produced)

Question 10

What is the effect of the high osmolarity of the interstitium on the descending loop?

Answer 10

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

Question 11

Where does the H2O travel after it passes into the interstitium?

Answer 11

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

Question 12

Descrbie the change in concentration that happens during the ascent and descent of the loop of henle?

Answer 12

The fluid in the tubule is progressively concentrated as it moves down the descending limb and progressively diluted as it moves up the ascending limb.

Question 13

What is the reult of greater concentration of the descending limb?

Answer 13

Means the interstitium will become more concentrated as a result of addition of salt from the ascending limb

Question 14

What is the maximum values of the horizontal gradient and the vertical gradient?

Answer 14

Horizontal - there is only ever a maximum of 200 mOsmol gradient between the ascending limb and the interstitium

Vertical gradient in the interstitium goes from 300 - 1200 mOsmol

Question 15

How does the drug furosemide work?

Answer 15

Furosemide stops the transport of sodium and chloride movement – so there is no osmotic gradient set up in the interstitium – water cannot move into the interstitium

Question 16

What is one of the consequences of the counter current multiplier?

Answer 16

15 - 20% of the initial filtrate (up to 36 l) is removed from the loop of henle

Question 17

Describe the osmolarity of the fluid that enters the distal tubule

Answer 17

This is more dilute than plasma

Question 18

What is the significance of the counter current regulatory system?

Answer 18

The overwhelming significance of the countercurrent multiplier is that it creates an increasingly concentrated gradient in the interstitium.

Question 19

What is the name given to the peritubular capillaries of the juxtamedullary nephrons?

Answer 19

Vasa recta

Question 20

What is the role of the vasa recta?

Answer 20

They act as counter current exchangers

Do not interfere with the gradient

Freely permeable to H2O and solutes and therefore equilibrate with the medullary interstitial gradient

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

Question 21

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

Answer 21

High oncotic pressure and high hydrostatic pressure in the tubule due to tight renal capsule wihch drives fluid into the capillaties

Question 22

Why is the flow rate low through the vasa recta?

Answer 22

So that there is plenty of time for the equilibration to occur with the interstitium - ensuring that medullary gradient is not disturbed

Question 23

What controls permeability of the colecting duct?

Answer 23

ADH - produced by the posterior pituitary