Loop of Henle Flashcards
What is the major site for reabsorption?
Proximal tubule
What percentage of Na reabsorption occurs at the proximal tubule?
65-75%
Does any protein get through filtering into the renal tubule?
- Some protein does get through
- Particular albumin, about 30g of protein/day which is 0.5% of the total amount presented at the glomerulus
- All completely reabsorbed by a Tm carrier mechanism in proximal tubule
Many drugs are nonpolar and so highly lipid soluble so removal of water in proximal tubule creates concentration gradients for their reabsorption (meaning would never be able to get rid of them). How is this fixed so they can be excreted?
- But liver metabolises them to polar compounds to reduce their permeability and facilitate their excretion
What is the osmocity of the fluid that leaves the proximal tubule?
Is isosmotic with plasma, ie, 300mOsmoles/L
What are the two kinds of nephrons?
Cortical nephron
Juxtamedullary nephrons
What is a juxtamedullary nephron?
Nephron whose renal corpuscle is near the medulla, and whose proximal convoluted tubule and its associated loop of Henle occur deeper in the medulla than the other type of nephron, the cortical nephron
What is a cortical nephron?
Cortical nephrons (the majority of nephrons) start high in the cortex and have a short loop of Henle which does not penetrate deeply into the medulla
What is a special system essentially for water balance?
Loops of Henle of juxtamedullary nephrons
What allows the kidneys to produce concentrated urine in times of water deficit?
A special system essential for water balance is attributable to the loops of Henle of juxtamedullary nephrons
What is the maximum concentration of urine that can be produced?
1200-1400mOsmoles/L
(4x more conentrated than plasma)
How much more concentrated is the maximum concentration of urine than plasma?
4x
What is the concentration of all the waste product that must be excreted each day (Urea, sulphate, phosphate and other waste products and non-waste ions (sodium and potassium))?
About 600mOsmoles/L, which therefore requires a minimum obligatory water loss of 500ml
What is the minimum urine concentration that can be produced?
30-50mOsmoles/L
(10x more diluted than plasma)
How much more diluted is the minimum concentration of urine than plasma?
10x
How are the kidneys able to produce urine of varying concentrations?
Loops of Henle of juxtamedullary nephrons act as counter-current multipliers:
- Counter current meaning fluid flows down the descending limb and up the ascending limb
- Critical characteristics of loops which make them counter-current multipliers are
- The ascending limb of loop of Henle actively co-transports Na and Cl ions out of the tubule lumen into the interstitium, the ascending limb is impermeable to water
- Descending limb is freely permeable to water but relatively impermeable to NaCl
What are critical characteristics of Loops of Henle that makes them counter-current multipliers?
- The ascending limb of loop of Henle actively co-transports Na and Cl ions out of the tubule lumen into the interstitium, the ascending limb is impermeable to water
- Descending limb is freely permeable to water but relatively impermeable to NaCl
What is the permability of the descending and ascendings limbs of the Loops of Henle to water and NaCl?
Descending - permeable to water, impermeable to NaCl
Ascending - impermeable to water, permeable to NaCl
Explain the process of the loop of Henle achieving various concentration gradients?
1) Loop of Henle filled with stationary isomotic fluid (about 300mOsmoles/L)
2) Active removal of NaCl from ascending limb, decreasing osmolarity in tubule and increasing it in interstitium
3) Descending limb now exposed to greater osmolarity in interstium, so water moves out to equate this osmolarity
4) This fluid does not stay in interstitium but is reabsorbed by high oncotic pressure and tissue pressure into vasa recta
5) Fluid is actually moving, entering at proximal and leaving at distal tubule, this concentrates fluid in descending limb rounds the bends and delivers a high concentration to the ascending limb
6) This causes NaCl removal in ascending limb and further concentrates the interstitium
7) Greater concentration of descending limb by removal of water means greater concentration of interstitium by addition of salt from ascending limb
8) Fluid in tubule is progressively concentrated as it moves down descending limb, and progressively diluted as it moves up ascending limb
9) As more and more concentrated fluid is delivered to ascending limb the interstitium becomes more and more concentrated - at any horizontal level there is only a 200mOsmol gradient between ascending limb and interstitium
10) Counter current flow multiplies osmolarity numbers, vertical gradient in interstitium goes from 300-1200mOsmol
In Loop of Henle, at any horizontal level, what is the difference in osmolarity between the ascending limb and the interstitial fluid?
200mOsmole
What does the vertical gradient of the interstitium around the loop of Henle progress from and to?
300-1200mOsmole
What does the 200mOsmole gradient at each horizontal level of loop of Henle reflect?
Pumping of the active pumps
What is the key step in achieving the gradients of the loop of Henle?
Active transport of NaCl out of ascending limb
If this is abolished, such as by use of diuretic frusemide, all concentration differences are lost and the kidney can only produce isotonic urine
What is an example of a drug that abolishes the active transport of NaCl from the ascending limb, losing the concentration gradients?
Diuretic frusemide
What does the counter current multiplier of the Loop of Henle achieve?
- Concentrates fluid on the way down and promptly re-dilates it on the way back up, not by adding water but by removing NaCl
- One consequence of this is that 15-20% of the initial filtrate (up to 36L) is removed from loop of Henle
- Fluid which enters the distal tubule is more dilate than plasma
- Delivers hypotonic fluid to the distal tubule
- Fluid enters at 300mOsm and leaves at 100, what is left behind concentrate the intersitium
Only 200mOsm gradient exists at any horizontal level
How does the counter current multiplier dilate fluid up the ascending limb?
Not by adding water but by removing NaCl
What is a consequence of the loop of Henle redilating fluid up the ascending limb by removing NaCl?
15-20% of initial filtrate (up to 36L) is removed from loop of Henle
What tonicity of fluid is delivered to the distal tubule in the loop of Henle?
- Delivers hypotonic fluid to the distal tubule
- Fluid enters at 300mOsm and leaves at 100, what is left behind concentrate the intersitium
Only 200mOsm gradient exists at any horizontal level
What is the most important thing that the countercurrent miltiplier of the loop of Henle achieves?
Increasingly concentrated gradient in the interstitium
What is the main function of the loop of Henle?
Concentrating the medullary interstitium and delivering hypotonic fluid to the distal tubule
What are vasa recta?
Straight arteries coming off from arcades in the mesentery of the jejunum and ileum, and heading toward the intestines
How does the specialised arrangement of peritubular capillaries (vasa recta) of the juxtamedullary nephons also participate in the countercurrent mechanism?
Acting as countercurrent exchangers:
- If medullary capillaries drained straight though they would carry away NaCl removed from loop of Henle and abolish the interstitial gradient
- Does not happen because they are arranged as hairpin loops and therefore do not interfere with the gradient
- As with all capillaries, vasa recta are freely permeable to water and solutes and therefore equilibrate with the medullary interstitial gradient
What allows the vasa recta to not interfere with the concentration gradients in the interstitum around the loops of Henle?
They are arranged as hairpin loops
What are some functions of vasa recta?
- Provide oxygen for medulla
- In providing oxygen must not disturb gradient
- Removes volume from the interstitial, up to 36L/day
How much volume can be removed from the interstitium around the loops of Henle due to vasa recta?
Up to 36L/day
Are Starling’s forces in the vasa racta in favour of reabsorption or filtration?
Reabsorption because of high oncotic pressure and high Pt (tissue pressure) due to tight renal capsule which drives fluid into capillaries
Is the flow rate through the vasa recta fast or slow?
Flow rate through the vasa recta is very low so that there is plenty of time for equilibrium to occur with the interstitium, further ensuring that medullary gradient is not disturbed
What is the site of water regulation?
Collecting duct
What is the permeability of the collecting duct to water under the control of?
ADH (anti-diuretic hormone)
What does ADH stand for?
Anti-diuretic hormone
What is anti-diuretic hormone (ADH) also known as?
Vasopressin
What does whether or not the dilate urine delivered to the distal tubule is concentrated and to what extent depends on what?
Presence or absence of vasopressin (ADH)
