THE COUNTERCURRENT.pptx

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1. THE COUNTERCURRENT MECHANISM Dr. Pandian M. Assistant Professor Dept . of physiology

2. Introduction It is depends upon the maintenance of a gradient of increasing osmolality along the medullary pyramids . This gradient is produced by the operation of the loops of Henle as countercurrent multipliers and maintained by the operation of the vasa recta as countercurrent exchangers. A countercurrent system is a system in which the inflow runs parallel to, counter to, and in close proximity to the outflow for some distance. This occurs for both the loops of Henle and the vasa recta in the renal medulla

3. Renal circulation . Interlobar arteries divide into arcuate arteries, which give off interlobular arteries in the cortex. The inter-lobular arteries provide an afferent arteriole to each glomerulus. The efferent arteriole from each glomerulus breaks up into capillaries that supply blood to the renal tubules. Venous blood enters interlobular veins, which in turn flow via arcuate veins to the interlobar veins. ( Modifi ed from Boron WF, Boulpaep EL: Medical Physiology. Saunders , 2009.)

4. What is the counter current multiplier mechanism? What is its purpose? Counter current multiplier mechanism is in the loop of Henle (mainly of juxtamedullary nephrons ). Its purpose is to create a very high osmotic pressure ( hyperosmolarity ) of the medullary interstitial fluid .

5. Explain the counter current multiplier mechanism. The loops of Henle of juxtamedullary nephrons dip deep into the medulla. The mechanism depends on this typical anatomical arrangements of loop of Henle. Normal osmolarity of fluid entering the loop is 300 mOsm /L . In medullary interstitial fluid , osmolarity increases to 1200 mOsm /L (or up to 1400 mOsm /L under the effect of ADH) in pelvic tip of medulla.

6. Three different mechanisms are responsible for this:- (a) Active transport of sodium ions out of thick portion of ascending limb of loop of Henle. 1Na + , 2Cl – and 1K + ions are co-transported actively from tubular lumen to medullary interstitial fluid . This movement of salt is not accompanied by water reabsorption (this part of the limb is impermeable to water). So the concentration of salt in the ascending limb decreases, but its concentration in the medullary interstitial fluid rises .

7. In renal medulla all other tubular structures (except ascending limb) are in osmotic equilibrium. The descending limb therefore acquires the increased osmolality of the surrounding fluid. The effect is multiplied as new iso-osmolar filtrate arrives at the descending limb and forces the concentrated tubular contents towards the tip of loop of Henle (hairpin band).

8. (b) Smaller quantities of ions are also transported from collecting ducts to medullary interstitial fluid mainly by active transport of sodium followed by electrogenic passive absorption of chloride ions.

9. (c) When concentration of antidiuretic hormone (ADH) is high in blood, there is further increase in osmolality of medullary interstitial fluid due to increased urea absorption by the following mechanisms: 1. Increased permeability o f collecting ducts to urea. 2. Greatly increased permeability of collecting ducts for water. 3. Increased water reabsorption from the collecting ducts greatly increases urea concentration inside the ducts thereby increasing its diffusion from collecting ducts to medullary interstitial fluid.

10. What is counter current exchange mechanism? What is its purpose? How does it work? Counter current exchange mechanism is in vasa recta. This mechanism is responsible for maintaining hyperosmolality of medullary interstitial fluid (removal of excess solute by blood is prevented). Vasa recta are loop-shaped vessels paralleling loop of Henle. They also loop down from cortex to medulla and then back into the cortex of the kidney.

11. The mechanism of counter current exchange works as follows: Inner medullary blood flow is very slight (1 to 2% of total flow to kidney ). Because flow is sluggish, removal of solutes is minimized.

12. Define Vasa recta A minute vessel of this network runs parallel to the loop of Henle forming a “U” shaped vasa recta. VR is absent or highly reduced in cortical nephrons.

13. (b) Vasa recta function as counter current exchanger. In ‘U’-shaped vasa recta, two arms are lying in close proximity so that fluid and solutes exchange very rapidly between two arms . Both are highly permeable to solutes. Because of rapid exchange of salts and fluid between ascending and descending arms of vasa recta, there is negligible washing out of solutes (e.g. when blood flows down in descending limb of vasa recta, sodium chloride and urea diffuse into the blood and water diffuses outward.

14. When blood flows back in ascending limb, sodium chloride and urea diffuse back into the medullary interstitium and water diffuses into blood. Therefore, by the time blood finally leaves medulla, its osmolar concentration is only slightly greater than blood that initially enters vasa recta.

15. How much is the concentration of urine? Concentration of urine varies from 65 mOsm /L to 1400 mOsm /L, depending upon the concentration of ADH in blood at a given time.

16. Summary Long loop of Henle establishes a vertical osmotic gradient (Countercurrent multiplier) Their vasa recta preserve this gradient while providing blood to renal medulla (Countercurrent exchanger) Collecting ducts of all nephrons use the gradient in conjugation with the hormone vasopressin, to produces urine of varying concentration (osmotic equilibrating device) Collectively this entire functional organization is known as medullary countercurrent system.

17. References Ganong's Review of Medical Physiology, 26 edition. Text book of Medical Physiology Guyton & Hall Human Physiology Vander Text book of Medical Physiology Indukurana Principles of Anatomy and Physiology Totora Net source