Loop of Henle Flashcards
What is the major site of reabsorption?
Proximal tubule (65-75% of all NaCl and H2O; all nutritionally important substances)
How much of the albumin presented at the glomerulus gets through?
~30g protein/day
0.5% (however this is ultimately all reabsorbed by a Tm carrier mechanism in the proximal tubule)
How does the nonpolar and therefore highly lipid soluble character of drugs/pollutants aid in their reabsorption?
The removal of H2O in the proximal tubule establishes concentration gradients for their reabsorption
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?
The liver metabolises them to polar compounds thus reducing their permeability and facilitating their excretion
Review of tubule function
review of tubule function 2
The fluid that leaves the proximal tubule is…
ISOSMOTIC with plasma i.e. 300mOmoles/L
Why is the fluid that leaves the proximal tubule isosmotic with plasma?
ALL solute movements are accompanied by equivalent H2O movements, so that osmotic equilibrium is maintained
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?
The Cortex
Yes
A special system is attributable to the loops of Henle of juxtamedullary nephorns, what is it essential for?
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)
What is the maximum concentration of urine that can be produced by the human kidney? How much more concentrated is this than plasma?
1200-1400mOsmoles/l
i.e. 4x more concentrated than plasma = excess of solute over water
Urea, sulfate, phosphate, other waste products and non-waste ions (Na+ and K+), which must be excreted each day amount to how many mOsmoles?
600
So the substances excreted each day amount to 600mOsmoles, this therefore means a minimum obligatory H2O loss of how much?
500mls
(this occurs even if there is no H2O intake - urinate to death!)
What is the minimum [urine] in man? (e.g. in conditions of excess H2O intake when H2O is excreted in excess of solute)
30-50mOsmoles/L
(i.e. 10x dilution compared with plasma)
What do the loops of Henle of juxtamedullary nephrons act as to allow the kidney to produce urine of varying concentrations?
Counter-current multipliers
How does fluid flow in counter-current in the loop of Henle?
Fluid flows down the descending limb and up the ascending limb
What are the 2 critical characteristics of the loops which make them counter-current multipliers? (hint: 1 for descending limb; 1 for ascending)
- Descending limb is freely permeable to H2O but relatively impermeable to NaCl
- 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
How does the loop of Henle start off?
Filled with stationary fluid of [300mOsm/l] i.e. isosmotic with plasma
As NaCl is pumped out of the ascending limb (key step), what happens to the fluid?
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)
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?
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)
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?
Again, active NaCl removal - further concentrating the interstitium
Greater concentration of descending limb (by removal of water) means…
greater concentration of interstitium by addition of salt from ascending limb
Fluid in tubule is progressively concentrated as it moves down the descending limb and progressively…
diluted as it moves up the ascending limb
What happens to the interstitium as more and more concentrated fluid is delivered to the ascending limb?
It becomes more and more concentrated
At any horizontal what is the gradient between the ascending limb and the interstitium?
200mOsmol
What is the vertical gradient range in the interstitium?
Goes from 300 -> 1200mOsmol
Summary of the osmolarity in the different parts of the kidney
Note the 200mOsmole gradient at each horizontal level of the ascending limb of the loop of Henle reflects the pumping of the active pumps
If the key step of transport of NaCl out of the ascending limb is abolished e.g. by use of diuretic frusemide, what happens?
All concentration differences are lost and the kidney can only produce isotonic urine
How much of the initial filtrate is removed by the loop of Henle as a consequence of re-dilution via removal of NaCl?
15-20% (up to 36L)
Is the fluid which enters the distal tubule more or less dilute than plasma?
More
Overwhelming significance of the counter-current multiplier is that is creates…
an increasingly concentrated gradient in the interstitium
What is the tonicity of he fluid delivered to the distal tubule?
Hyptonic
The loop of Henle is all about…
concentrating the medullary interstitium and delivering hypotonic fluid to the distal tubule
What is the vasa recta?
The peritubular capillaries of the juxtamedullary nephrons
How do the vasa recta participate in the countercurrent mechanism?
By acting as countercurrent exchangers
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?
They are arranged as hairpin loops and therefore do not interefere with the gradient
As with all capillaries, the vasa recta are freely permeable to what?
H2O and solutes
And therefore equilibrate with the medullary interstitial gradient
Give the 3 main 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
Why are Starlings forces in favour of reabsorption of water into the vasa recta?
High oncotic pressure (πp) and high hydrostatic pressure (Pt) in the tubule due to tight renal capsule which drives fluid into capillaries
Why is the flow rate through the vasa recta very low?
So that there is plenty of time for equilibrium to occur with the interstitium, further ensuring that the medullary gradient is not disturbed
The collection duct is the site of water regulation, what controls its permeability?
ADH (Anti-Diuretic Hormone)
