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
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 the main function of the loop of Henle?
Concentrating the medullary interstitium and delivering hypotonic fluid to the distal tubule
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 is the permeability of the collecting duct to water under the control of?
ADH (anti-diuretic hormone)
