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Question
What is the countercurrent mechanism? Describe the physiological basis of the countercurrent mechanism in the kidney.
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Solution
The Countercurrent mechanism is the special mechanism in our kidney which helps in the production of concentrated urine. It takes place in the following manner:- Step:1 Assume that the loop of Henle is filled with a concentration of 300mOsm/L the same as that leaving the proximal tubules. Step:2 The active ion pump of the thick ascending limb on the loop of Henle reduces the concentration inside the tubule and raises the interstitial concentration. Step:3 The tubular fluid in the descending limb and the interstitial fluid quickly reach osmotic equilibrium because of osmosis of water out of the descending limb. Step:4 The Additional flow of the fluid into the loop of Henle from the proximal tubule, which causes the hyperosmotic fluid previously formed in the descending limb to flow into the ascending limb. Step:5 Additional ions pumped into the interstitium with water remaining in the tubular fluid, until a 200-mOsm/L osmotic gradient is established. Step:6 Again, the fluid in the descending limb reaches equilibrium with the hyperosmotic medullary interstitial fluid and as the hyperosmotic tubular fluid from the descending limb flows into the ascending limb, still more solute is continuously pumped out of the tubules and deposited into the medullary interstitium. Step:7 These steps are repeated over and over, with net effect of adding more and more solute to the medulla in excess of water, with sufficient time, this process gradually traps solutes in the medulla and multiplies the concentration gradient established by the active pumping of ions out of the thick ascending limb, eventually raising the interstitial fluid osmolarity to 1200- 1400 mOsm/L .
The physiological basis for the countercurrent mechanism is a special arrangement of the loop of Henle and vasa recta. The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of vasa recta is also in a countercurrent pattern. The proximity between the Henle’s loop and vasa recta, as well as the countercurrent in them, help in maintaining an increasing osmolarity towards the inner medullary interstitium, i.e., from 300 mOsmolL–1 in the cortex to about 1200 mOsmolL–1 in the inner medulla. This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule
| Step:1 | Assume that the loop of Henle is filled with a concentration of 300mOsm/L the same as that leaving the proximal tubules. |
| Step:2 | The active ion pump of the thick ascending limb on the loop of Henle reduces the concentration inside the tubule and raises the interstitial concentration. |
| Step:3 | The tubular fluid in the descending limb and the interstitial fluid quickly reach osmotic equilibrium because of osmosis of water out of the descending limb. |
| Step:4 | The Additional flow of the fluid into the loop of Henle from the proximal tubule, which causes the hyperosmotic fluid previously formed in the descending limb to flow into the ascending limb. |
| Step:5 | Additional ions pumped into the interstitium with water remaining in the tubular fluid, until a 200-mOsm/L osmotic gradient is established. |
| Step:6 | Again, the fluid in the descending limb reaches equilibrium with the hyperosmotic medullary interstitial fluid and as the hyperosmotic tubular fluid from the descending limb flows into the ascending limb, still more solute is continuously pumped out of the tubules and deposited into the medullary interstitium. |
| Step:7 | These steps are repeated over and over, with net effect of adding more and more solute to the medulla in excess of water, with sufficient time, this process gradually traps solutes in the medulla and multiplies the concentration gradient established by the active pumping of ions out of the thick ascending limb, eventually raising the interstitial fluid osmolarity to 1200- 1400 mOsm/L . |
The physiological basis for the countercurrent mechanism is a special arrangement of the loop of Henle and vasa recta. The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of vasa recta is also in a countercurrent pattern. The proximity between the Henle’s loop and vasa recta, as well as the countercurrent in them, help in maintaining an increasing osmolarity towards the inner medullary interstitium, i.e., from 300 mOsmolL–1 in the cortex to about 1200 mOsmolL–1 in the inner medulla. This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule
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