Functions of Loop of Henle

Loop of Henle with its complex anatomy and even more complicated physiology has long remained an enigma to researchers all around the world. Here we discuss about the functional anatomy and the transport characteristics of Loop of Henle.

1. Functions of Loop of Henle Dr. Saran A K

2. Loop of Henle A. Functions of LoH Role in Reabsorption Thin Descending Limb Thin Ascending Limb Thick Ascending Limb Role in Concentration of Urine Role in Acid Base Balance B. Applied Aspects References

3. Continuation of Pars Recta of Proximal Tubule Fredrich Gustav Jacob Henle U shaped segment Functionally distinct segments Thin Descending Segment Thin Ascending Segment Thick Ascending Segment Highly specialized nephron site extreme heterogeneity anatomic configuration Organization of Human Nephron Fig 33.2 Berne & Levy Physiology 7th Edition

4. Cortical Nephrons (85%) JG Nephrons (15%) Glomeruli lies in the outer cortex Glomeruli juxtamedullary cortex / inner part of cortex LoH has 2 parts ( tDL , TAL ) 3 parts ( tDL , tAL , TAL ) Thin segment is very short barely penetrating the inner medulla Long thin limb extending into the medullary pyramids Drains into the peritubular capillaries Drains into peritubular capillaries and vasa recta  Excretion of waste products Concentration of urine by creating a hyper osmotic medulla

5. Thin Descending Limb Thick Ascending Limb 2-14mm ,30 µ in diameter 12mm , 60 µ diameter Flat squamous cell Leaky Tight Junctions Cuboidal cell Tight Tight Junctions Few mitochondria Large number of mitochondria Poorly developed luminal and basolateral surface Extensive invagination of basal membrane 5 Cellular Ultrastructure and Primary Transport in LoH Fig 27-8 Guyton and Hall Medical Physiology

6. Functions of Loop of Henle Role in Reabsorption Thin Descending Limb ( tDL ) Absorption of H2O 15% of the filtered water is reabsorbed  Thin descending limb is permeable to water, AQP 1   Owing to the high osmolality of the medullary interstitium (because of the activity of TAL) Changes in the % of filtered substances along nephron Fig 38-1 Ganong’s Review of Medical Physiology 26 th Edition

7. 3/4th of H2O entering tDL of Long Looped Nephrons is rreabsorbed here 1/2th of H2O entering tDL of Short Looped Nephrons is reabsorbed here 4. Tubular Fluid Osmolality Osmolality increases due to reabsorption of water 300 to 1200 in Long looped nephrons x4 times 300 to 600 in Short looped nephrons x2 times Osmolality changes as filtrate flows through nephron Fig 20-4 Silverthorne Human Physiology Absorption of H2O : Continued

8. Long Looped Tubular Fluid Peritubular Fluid Electrolyte 1120 600 Urea 80 600 1200 1200 Short Looped Tubular Fluid Peritubular Fluid Electrolyte 560 400 Urea 40 200 600 600 Both in long and short looped nephrons, the predominant solutes in the tubular fluid are Na + and Cl - , but substantial fraction of the peritubular fluid is Urea .  5. Composition of Tubular and Peritubular Fluid (at the tip of LoH )   Absorption of H2O : Continued

9. The medullary interstitial gradient – Na + Cl - and Urea contribution Fig 35.8 Berne & Levy Physiology 7 th Edition Changes in Osmolality of Tubular Fluid in different segments Fig 28-8 Guyton and Hall Medical Physiology 12 th edition

10. Thin Ascending Limb ( tAL ) In long looped nephrons, tAL is seen after the hairpin. Though structurally similar to the tDL , the tAL has completely different transport and permeability characteristics. Permeability of TAL  virtually impermeable to water highly permeable to Na + and Cl - moderately permeable to urea Passive diffusion of Na + , Cl - (absorption) and Urea (secretion) takes place along their concentration gradients.

11. The number of moles of Na + plus Cl - leaving the lumen exceeds the number of moles of urea entering due to greater permeability of tAL to Na + and Cl - . But this urea re-cycling helps to prevent urea washout from renal medulla, thus contributing to hyperosmotic renal medulla. Osmolality of Tubular and Peritubular Fluid The osmolality of tubular fluid slightly falls below that of the surrounding peritubular fluid.

12. Thick Ascending Limb (TAL) Permeability of Thick Ascending Limb The permeability to water is negligible The urea permeability is quite low Actively transports ions including Na + , K + and Cl - from lumen to peritubular space Osmolality of Tubular and Peritubular Fluid The tubular fluid leaving the LoH is hypotonic Urea concentration is considerably greater than PT Fluid The TF/P remains constant as the volume of the tubular fluid does not change

13. The Na + -K + -2Cl - Symport The loop of Henle is responsible for the reabsorption of ~ 40% of filtered Na + , mostly in the TAL. Sodium is then actively extruded across the basal lateral surfaces by the Na + -K + ATPase. The entry of Na ions in the thick ascending limb is coupled to the entry of K + ion and two Cl - ions.  NKCC2 identified by Gamba Mechanism of sodium, chloride and potassium transport TAL Fig 27-9 Guyton and Hall Medical Physiology

14. Load Dependent Na + Reabsorption  An important characteristic of the Na + -K + -2Cl - symport is that an increased amount of Na + will be reabsorbed by the thick ascending limb if an increased load of Na + is delivered to it. Also seen in Proximal Convoluted Tubule The active reabsorption of Na + and Cl - in TAL is inhibited by certain prostaglandins and loop diuretics such as Frusemide.  

15. Kidney Chloride Channels and Barttin Chlorides leave the cells at the basolateral membrane through ClC family of kidney chloride channels also known as ClC -K ClC -Ka and ClC-Kb are expressed in the TAL. They contain a beta subunit called as barttin and is an essential portion for the functioning of these channels. A number of genes and their products have been identified and implicated in various salt losing tubulopathies. Mechanism of sodium, chloride and potassium transport TAL Fig 27-9 Guyton and Hall Medical Physiology

16. ROMK Channels Some of the K that enters the cells through the Na + -K + -2Cl - symport leaks back across the apical membrane into the tubular lumen via ROMK channel (renal outer medullary K + channel) These channels ensure K +  recycling to the lumen , essential for salt reabsorption set a positive trans-epithelial voltage , that drives paracellular reabsorption of cations Additional Na+ reabsorption Other cations namely Ca 2+ and Mg 2+ Mechanism of sodium, chloride and potassium transport TAL Fig 27-9 Guyton and Hall Medical Physiology

17. Calcium Reabsorption Bulk of Ca 2+  reabsorption paracellular pathway NKCC2 and in particular ROMK generate the “driving force” Magnesium Reabsorption 60% of the filtered Mg 2+  is reabsorbed in the TAL Passive paracellular transit is the main route, and it is driven by the lumen-positive voltage. Mechanism of sodium, chloride and potassium transport TAL Fig 27-9 Guyton and Hall Medical Physiology

18. The synergic activity of the main transporters and channels involved in salt absorption (NKCC2, ROMK, the chloride channel ClC with the  Barttin subunit) and the integrity of Tight Junctions are the prerequisite to prevent electrolytes imbalance. 

19. The osmolality of the renal medulla goes on increasing progressively from about 300 mOsm /kg H2O at corticomedullary junction to about 1200 mOsm /kg H2O at papilla. Hyperosmotic interstitial fluid of the medulla is critically important in concentrating the urine. An increasing interstitial osmotic gradient is guaranteed by Counter Current System Countercurrent Multiplier System Countercurrent Exchanger System  2. Role in Concentration of Urine : The Medullary Gradient Vertical Osmotic Gradient in Renal Medulla Fig 14-24 Sherwood Human Physiology 7 th Ed

20. Countercurrent System:    A system where inflow runs parallel to, counter to, and in close proximity to the outflow for some distance. In the kidney, the structures which form the countercurrent system are  loop of Henle and the vasa recta Countercurrent Multiplier System  formed by the  operation of U shaped loop of Henle and is responsible for the production of hyperosmolality and a gradient in renal medulla Countercurrent Exchanger System   is a similar arrangement of the surrounding vasa recta prevents osmotic gradient dissipation Origin of single effect (main driving force) Multiplication of the single effect

21. Counter Current Multiplier System Origin of a Single Effect (Horizontal Gradient) Multiplication of the single effect (Vertical Gradient) Main Driving Force . Produces a gradient of 200 mOsm /kg H20 The mechanism of origin of single effect in outer medulla is different from that of the inner medulla. The hyper-osmolality and medullary gradient is generated by the  multiplication of the single effect by the countercurrent multiplier Outer Medulla Active addition of Na Cl through water impermeable TAL Inner Medulla Passive transport of Sodium Ions from tAL Active Transport of Sodium and Urea from the CD into the medullary interstitium . tDL : High permeability of a water but not to solutes tAL : Impermeability to water but high permeability to NaCl TAL : Impermeability to water and ability to actively absorb

22. Reabsorbs a significant fraction ( 15% ) of the filtered bicarbonate Bicarbonate concentration increases significantly resulting from due to water reabsorption along the descending limb In the TAL , bicarbonate is reabsorbed via the transcellular pathway resembles bicarbonate reabsorption in the PT through Na+ /H+  exchanger (NHE3) activity Bicarbonate exit from the cells is mediated by the Cl − /HCO 3−  exchanger 2 (AE2) Transport mechanisms in TAL LoH Fig 24.7 Berne & Levy Physiology 7th Edition 3. Role in Acid Base Balance

23. Urine ammonia excretion derives mainly from renal ammonia genesis , rather than glomerular filtration.   It is produced from glutamine in the Proximal Tubule as ammonium ion (NH 4 + ) and is released to the luminal fluid. TAL has a crucial role in ammonia reabsorption via NKCC2 , at the K +   binding site substituting for potassium. Basolateral exit is mediated by the Na + /NH 4 +  exchanged via NHE4 Na + - HCO 3 - Co- transporter Cl − -dependent pathway

24. Applied Aspects Loop Diuretics   Furosemide ,  Torsemide, and Bumetanide bind NKCC2 in a  reversible fashion. reduction in Na+ , K+ , and Cl−  absorption overrules that the cortico-medullar osmotic gradient increases urine output impairs paracellular cations reabsorption.   This property has predictable beneficial effects in several conditions, and loop diuretics are the main therapy in fluid retentive states and hypercalcemic conditions.

25. Autosomal Recessive Disorder caused by inactivating mutations in the gene coding for the Na + -K + -2Cl − symporter (NKCC2) the apical K + channel (ROMK) basolateral Cl − channel – Classical Barrter Decreases NaCl reabsorption and K + reabsorption by the TAL causes hypokalemia and a decrease in ECFV which activates RAAS mechanism. characterized by hypokalemia, metabolic alkalosis, hyperaldosteronism 2. Barrter Syndrome

26. Mutations in the tight junction protein claudin -16 (CLDN16) reduce the permeability of paracellular pathway of Ca2+ and Mg 2+, reducing reabsorption Characterized by enhanced excretion of Ca 2+ and Mg 2+ high levels of Ca++ in urine which leads to nephrolithiasis 3. Familial Hypo- magnesemic Hypercalciuria

27. References Ganong . Review of Medical Physiology 22 nd Edition McGraw Hill Best and Taylor’s Physiological Basis of Medical Practice 12 th Edition Williams & Wilkins Guyton & Hall. Textbook of Medical Physiology 11 th Edition Saunders Elsevier Walter F Boron. Medical Physiology 2nd Edition Saunders Elsevier Berne & Levy Physiology 7 th Edition Elsevier Ganong . Review of Medical Physiology 26 th Edition McGraw Hill