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Question
Give a detailed account of osmoregulation by kidneys in mammals. What do you understand by countercurrent mechanism? Explain its significance.
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Solution
Osmoregulation is maintaining constant levels of water in the body. The kidneys are the main osmoregulatory organs in mammalian systems; they function to filter blood and maintain the osmolarity of body fluids at 300 mOsm. The kidneys accomplish this task using countercurrent mechanisms. The countercurrent multiplication system relies on the interaction between the flow of filtrate through the ascending and descending limbs of the long nephron loops of the juxtamedullary nephrons. It depends on actively transporting solutes out of the ascending limb of the loop of Henle. The countercurrent multiplier allows the nephron to maintain the medullary osmotic gradient in order to vary the urine concentration dramatically. 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 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 transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the countercurrent mechanism.
Countercurrent mechanism helps to maintain a concentration gradient in the medullary interstitium. Presence of such interstitial gradient helps in easy passage of water from the collecting tubule thereby concentrating the filtrate (urine). Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed.
| 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 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 transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the countercurrent mechanism.
Countercurrent mechanism helps to maintain a concentration gradient in the medullary interstitium. Presence of such interstitial gradient helps in easy passage of water from the collecting tubule thereby concentrating the filtrate (urine). Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed.
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