Loop of Hen Flashcards
function of proximal tubule?
Major site of reabsorption, 65-75% of all NaCl and H2O all nutritionally important substances.
what proteins are reabsorbed by proximal tubule?
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.
how are drugs and pollutants absorbed by the proximal tubule?
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.
review the function of the proximal tubule?
how would you describe the fluid that leaves the proximal tubule?
isosmotic with plasma ie 300 mOmoles/l
why is the fluid that leaves the proximal tubule described as isosmotic?
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.
where do all of the nephrons have their proximal and distal tubules?
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.
what is a very special system, essential for water balance?
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.
what is the maximum concentration of water that can be produced by the human kidney?
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.
how concentrated is the urine of desert species in comparison to humans?
(Desert species can produce urine as concentrated as 6000mOsmole/l, all H2O needs can be met by metabolic H2O).
The urea, sulphate, phosphate, other waste products and non-waste ions (Na+ and K+ ) which must be excreted each day amount to….
≈ 600 mOsmoles
what mininum h20 loss is associated with 600 mOsmoles of waste product?
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
In conditions of excess H2O intake how is H20 excreted?
H2O is excreted in excess of solute, minimum [urine] in man is 30-50 mOsmoles/l ie 10 fold dilution compared with plasma.
how are kidneys able to produce urine of varying concentrations?
because the loops of Henle of juxtamedullary nephrons act as counter-current multipliers
what is counter current?
fluid flows down the descending limb and up the ascending limb.
what are critical characteristics of loops which make them counter current multipliers?
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.
how does the loop of henle begin and what happens to NaCl?
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.
what happens once the descending limb is now exposed to greater osmolarity in the interstitium?
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).
is fluid moving in the loop of henle?
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.
how does fluid concentration differ between the ascending and descending limb?
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.
what does greater concentration of the descending limb mean for concentration of the interstitium?
Greater concentration of descending limb (by removal of water) means greater concentration of interstitium by addition of salt from ascending limb.
what happens as more and more concentrated fluid is delivered to the ascending limb?
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
summarise the mechanism of the juxtaglomerular nephron?
what is the key step of loop of henle?
The key step is the active transport of NaCl out of the ascending limb
what happens if active transport of NaCl out of the ascending limb is abolished?
If this is abolished, eg by use of the diuretic frusemide, all concentration differences are lost and the kidney can only produce isotonic urine.
what three things have the countercurrent multiplier acheived?
- Concentrates fluid on the way down and promptly re-dilutes it on the way back up, NOT by adding H2O, but by removing NaCl.
- One consequence of this is that 15-20% of the initial filtrate (up to 36 l) is removed from the loop of Henle.
- Fluid which enters the distal tubule is more dilute than plasma.
Also delivers hypotonic fluid to the distal tubule
The overwhelming significance of the countercurrent multiplier is?
that it creates an increasingly concentrated gradient in the interstitium
how does fluid differ throughout concurrent multiplier?
** 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
what is the vasa recta?
The specialized arrangement of the peritubular capillaries of the juxtamedullary nephrons also participate in the countercurrent mechanism by acting as countercurrent exchangers.
what is the purpose of the vasa recta?
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.
what are functions of the vasa recta?
- Provide O2 for medulla.
- In providing O2 must not disturb gradient.
- Removes volume from the interstitium, up to 36l/day.
describe the flow rate through the vasa recta?
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.
what is the site of water regulation?
the Collecting duct, whose permeability is under the control of ADH = Anti-Diuretic Hormone (Vasopressin).
what is the permeablility of the collecting duct under control of?
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.
