Loop of Henle Flashcards
what does the proximal tubule do?
Major site of reabsorption, 65-75% of all NaCl and H2O all nutritionally important substances
Is any protein presented and reabsorbed?
Some protein does get through, particularly albumin. around 30g protein/day aorund 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 reabsorbed and excreted?
Many are nonpolar and therefore 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
summary diagram: review of tubular funciton
What are the properties of the fluid that leaves the proximal tubule?
The fluid that leaves the proximal tubule is isosmotic with plasma ie 300 mOmoles/l
The fluid that leaves the proximal tubule is isosmotic with plasma ie 300 mOmoles/l
why is this?
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 _____ and all nephrons have common processes for the reabsorption and secretion of solutes of the filtrate
However a very special system, essential for _____ ______ is attributable to the loops of _____ of juxtamedullary nephrons
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
However a very special system, essential for water balance is attributable to the loops of Henle of juxtamedullary nephrons
the special system, essential for water balance is attributable to the loops of Henle of juxtamedullary nephrons
through thid mechanism, what is the kidney able to produce?
the kidney is able to produce concentrated urine in times of H2O deficit, a major determinant of our ability to survive without water. (Not very good!)
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
(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 how much? and what happens in there is not water intake? and what happens when there is excess urine intake?
around 600 mOsmoles, this therefore 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. N.B. Importance of ingestion for water balance
In conditions of excess H2O intake, 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 the kidneys are able to produce urine of varying concentration?
because the loops of Henle of juxtamedullary nephrons act as counter-current multipliers
Counter-current is easy, fluid flows ____ the descending limb and __ the ascending limb
down
up
What are the critical characteristics of the 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
Let’s start off with the 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 ____ and that of the interstitium _____. This occurs until a limiting gradient of 200mOsm is established
falls
rises
Describe what is happening here as the first parts of the loop of Henle counter current multiplier
- Loop of henle filled with stationary isosmotic fluid (~300 mOsmol/l)
- Key step: active removal of NaCl from ascending limb
As NaCl is removed from ascending limb and put into the interstitium the osmolarity in the tubule ↓ and it ↑ in the interstitium
As NaCl is removed from ascending limb and put into the interstitium the osmolarity in the tubule _ and it _ in the interstitium
↓
↑
As NaCl is removed from ascending limb and put into the interstitium the osmolarity in the tubule ↓ and it ↑ in the interstitium
describe what is happening next in this diagram
- 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 osmotic pressure and tissue P into the vasa recta (Starling’s forces)
As H2O moves out of the descending limb to equate the osmolarity, dexcirbe what is seen in this diagram?
- Fluid is actually moving, entering at the proximal and leaving at the distal tubule
Therefore concentrated fluid in descending limb rounds the bend and delivers a [high] to the ascending limb
Therefore Active NaCl removal
Therefore further concentrates the interstitium
- Greater concentration of descending limb (by removal of water) means greater concentration of interstitium by what?
addition of salt from ascending 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
As more and more concentrated fluid is delivered to the ascending limb, the interstitium becomes more and more _________
concentrated
at any horizontal level, what is the difference in mOsmol gradient between the ascending limb anf the interstitium?
a 200 mOsmol gradient between ascending limb and interstitium
Vertical gradient in interstitium goes from 300 - 1200 mOsmol
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
how do diuretics work?
- The key step is the active transport of NaCl out of the ascending limb. If this is abolished, eg by use of the diuretic frusemide, all concentration differences are lost and the kidney can only produce isotonic urine
Body not able to make this gradient
Don’t reabsorb this salt and if no salt going into interstitium then no removal or water
What has the countercurrent multiplier achieved?
- 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
The overwhelming significance of the countercurrent multiplier is that it creates an increasingly concentrated gradient in the interstitium
Also delivers hypotonic fluid to the distal tubule
** 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
It is all about concentrating the medullary interstitium and delivering hypotonic fluid to the distal tubule
what is the vasa recta permeable to?
As with all capillaries, the vasa recta are freely permeable to H2O and solutes and therefore equilibrate with the medullary interstitial gradient
Where does water regulation occur and what is it controlled by?
The site of water regulation is the Collecting duct, whose permeability is under the 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
Permeability of the collecting duct can be changed depending on the situation we are in e.g. water deprived, allows us to adjust the osmolarity of our urine
