Loop of Hen

Question 1

function of proximal tubule?

Answer 1

Major site of reabsorption, 65-75% of all NaCl and H2O all nutritionally important substances.

Question 2

what proteins are reabsorbed by proximal tubule?

Answer 2

N.B. Some protein does get through, particularly albumin. ≈ 30g protein/day ≡ 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 absorbed by the proximal tubule?

Answer 3

Many are nonpolar and ∴ highly lipid soluble so that the removal of H2O in the proximal tubule establishes concentration gradients for their reabsorption. Because of their lipid solubility would never get rid of them!!

But the liver metabolizes them to polar compounds thus reducing their permeability and facilitating their excretion.

Question 4

review the function of the proximal tubule?

Answer 4

Question 5

how would you describe the fluid that leaves the proximal tubule?

Answer 5

isosmotic with plasma ie 300 mOmoles/l

Question 6

why is the fluid that leaves the proximal tubule described as isosmotic?

Answer 6

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

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.

Question 7

where do all of the nephrons have their proximal and distal tubules?

Answer 7

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.

Question 8

what is a very special system, essential for water balance?

Answer 8

loops of Henle of juxtamedullary nephrons

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 9

what is the maximum concentration of water that can be produced by the human kidney?

Answer 9

Maximum concentration of urine that can be produced by the human kidney = 1200-1400mOsmoles/l ie 4x more concentrated than plasma = excess of solute over water.

Question 10

how concentrated is the urine of desert species in comparison to humans?

Answer 10

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

Question 11

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

Answer 11

≈ 600 mOsmoles

Question 12

what mininum h20 loss is associated with 600 mOsmoles of waste product?

Answer 12

This ∴ 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 13

In conditions of excess H2O intake how is H20 excreted?

Answer 13

H2O is excreted in excess of solute, minimum [urine] in man is 30-50 mOsmoles/l ie 10 fold dilution compared with plasma.

Question 14

how are kidneys able to produce urine of varying concentrations?

Answer 14

because the loops of Henle of juxtamedullary nephrons act as counter-current multipliers

Question 15

what is counter current?

Answer 15

fluid flows down the descending limb and up the ascending limb.

Question 16

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

Answer 16

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.

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

Question 17

how does the loop of henle begin and what happens to NaCl?

Answer 17

loop of Henle filled with a stationery fluid of [300mOsm/l] ie isosomotic with plasma.

As 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.

Question 18

what happens once the descending limb is now exposed to greater osmolarity in the interstitium?

Answer 18

H2O will move out to equate the osmolarity.

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

Question 19

is fluid moving in the loop of henle?

Answer 19

Fluid is actually moving, entering at 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.

Question 20

how does fluid concentration differ between the ascending and descending limb?

Answer 20

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 21

what does greater concentration of the descending limb mean for concentration of the interstitium?

Answer 21

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

Question 22

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

Answer 22

As more and more concentrated fluid is delivered to the ascending limb, the interstitium becomes more and more concentrated.
At any horizontal level the is only a 200 mOsmol gradient between ascending limb and interstitium

Counter-current flow multiplies the gradient – see final values in next slide
Vertical gradient in interstitium goes from
300 🡪 1200 mOsmol

Question 23

summarise the mechanism of the juxtaglomerular nephron?

Answer 23

Question 24

what is the key step of loop of henle?

Answer 24

The key step is the active transport of NaCl out of the ascending limb

Question 25

what happens if active transport of NaCl out of the ascending limb is abolished?

Answer 25

If this is abolished, eg by use of the diuretic frusemide, all concentration differences are lost and the kidney can only produce isotonic urine.

Question 26

what three things have the countercurrent multiplier acheived?

Answer 26

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 36 l) is removed from the loop of Henle.
3. Fluid which enters the distal tubule is more dilute than plasma.

Also delivers hypotonic fluid to the distal tubule

Question 27

The overwhelming significance of the countercurrent multiplier is?

Answer 27

that it creates an increasingly concentrated gradient in the interstitium

Question 28

how does fluid differ throughout concurrent multiplier?

Answer 28

** Fluid enters at 300 mOsm and leaves at 100. What is left behind concentrates the interstitium.

Only a 200 mOsm gradient exists at any horizontal level, but its effect is multiplied by the countercurrent flow

Question 29

what is the vasa recta?

Answer 29

The specialized arrangement of the peritubular capillaries of the juxtamedullary nephrons also participate in the countercurrent mechanism by acting as countercurrent exchangers.

Question 30

what is the purpose of the vasa recta?

Answer 30

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

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

As with all capillaries, the vasa recta are freely permeable to H2O and solutes and ∴equilibrate with the medullary interstitial gradient.

Question 31

what are functions of the vasa recta?

Answer 31

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

Question 32

describe the flow rate through the vasa recta?

Answer 32

The balance of Starling’s forces are very much in favour of reabsorption because of high Πp, and high Pt due to tight renal capsule which drives fluid into capillaries.

The flow rate through the vasa recta is very low so that there is plenty of time for equilibration to occur with the interstitium, further ensuring that the medullary gradient is not disturbed.

Question 33

what is the site of water regulation?

Answer 33

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

Question 34

what is the permeablility of the collecting duct under control of?

Answer 34

ADH = Anti-Diuretic Hormone (Vasopressin)

Whether or not the dilute urine delivered to the distal tubule is concentrated and to what extent depends on the presence or absence of the posterior pituitary hormone, ADH.