what is counter current in human kidney??
Answers
Answer:
Countercurrent multiplication in the kidneys is the process of using energy to generate an osmotic gradient that enables you to reabsorb water from the tubular fluid and produce concentrated urine. This mechanism prevents you from producing litres and litres of dilute urine every day, and is the reason why you don’t need to be continually drinking in order to stay hydrated.
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Mechanism of Concentration of the Filtrate/ Counter Current Mechanism:
Loop of Henle and vasa recta play a significant role in concentrating the filtrate.
The flow of filtrate in two limbs of the loop and flow of blood in vasa recta are in opposite directions; forming a counter current system.
It helps maintain concentration gradient in interstitial fluid of kidney from cortex (300 mOsmol/ litre) to inner medulla (1200 mOsmol/ litre). This gradient is mainly caused by Na+ and Cl-.
Recall that descending loop of Henle is permeable to water whereas ascending loop of Henle is permeable to ions.
As the filtrate flows down the descending limb of loop, more and more water is reabsorbed which results in progressive increase in osmolarity of the filtrate inside the loop, i.e., the filtrate becomes progressively hypertonic with respect to the interstitial fluid.
Now, as the filtrate enters ascending limb of loop which is permeable to solutes (Na+ and Cl-) but impermeable to water, more and more Na+ and Cl- get reabsorbed. The reabsorption of solutes is maximum in the deep medulla due to highest concentration gradient between the filtrate and the interstitial fluid (Recall that the filtrate becomes progressively hypertonic as it flows down the descending limb of the loop). As the filtrate ascends the loop, it becomes progressively hypotonic. Thus the reabsorption of ions also gets decreased. This differential reabsorption of water and solutes in different limbs of the loop generate concentration gradient in the kidney which progressively increases from cortex to medulla.
Now we will see how this concentration gradient helps efficient reabsorption of water and thus concentrating the urine. Recall that vasa recta enters the medullary region as descending limb which runs adjacent to the ascending limb of the loop of Henle. The descending limb of vasa recta starts taking up Na+ and Cl- transported by the loop. As a result, blood attains concentration of 1200 mOsmol/ litre in deep medulla.
The concentrated blood now flows into the ascending portion of the vasa recta which runs through decreasing concentration gradient in the interstitial fluid.
Thus, blood reabsorbs more and more water from the interstitial fluid and attains concentration of 300 mOsmol/ litre as it reaches cortex. In this way, blood leaves vasa recta with its normal osmolarity and at the same time reabsorbs more and more water from the filtrate.
The filtrate runs thrice through the medulla in its journey- in the descending limb of loop, the ascending limb of loop and in the collecting tubule.
As urine flows downwards in the collecting tubule (into deeper medullary region), it encounters higher and higher concentrations of solutes in the interstitial fluid. Hence it too progressively loses water due to osmosis. This is how urine is concentrated.
Note: Human kidneys can produce urine nearly four times concentrated (1200 mOsmol per litre) than the initial filtrate formed.
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