What will happen to the luminal fluid when its is surrounded by highly concentrated (salty) environment? (Hint: Osmotic water diffusion in desc. limb of the medulla)
The correct answer and explanation is:
Correct Answer:
When the luminal fluid is surrounded by a highly concentrated (salty) environment, water will diffuse out of the lumen into the surrounding interstitial space due to osmosis, especially in the descending limb of the loop of Henle in the kidney medulla.
300-Word Explanation:
In the descending limb of the loop of Henle, the nephron is permeable to water but impermeable to solutes like sodium and chloride ions. This structure plays a vital role in the kidney’s ability to concentrate urine.
As the filtrate (luminal fluid) descends deeper into the medulla, it encounters a progressively saltier (hypertonic) interstitial environment. This salty environment is maintained by active transport of ions in the ascending limb of the loop and by the countercurrent multiplier system.
According to osmosis, water moves from an area of lower solute concentration (inside the lumen, where the filtrate is more dilute) to an area of higher solute concentration (the salty medullary interstitium). Therefore, as the luminal fluid travels down the descending limb, water exits the tubule into the surrounding hypertonic interstitial fluid. This movement of water occurs passively through aquaporin channels.
The result is a reduction in water volume in the tubular fluid and a rise in solute concentration (increased osmolarity) of the filtrate as it reaches the loop’s bottom. This process is essential for the kidney to produce concentrated urine and conserve water.
Thus, in a highly salty environment, the luminal fluid becomes more concentrated because it loses water to the surrounding tissue. This mechanism helps the body regulate water balance, particularly under conditions of dehydration or low water intake.
This osmotic movement in the descending limb is a key part of the countercurrent exchange system, which enables the kidney to reabsorb water efficiently and maintain homeostasis of body fluids.