on the late DCT & collecting duct, the osmolarity of the tubular fluid INCREASES (~1200) as it flows through the rest of the nephron and collecting duct. These changes result from the following: 1. Countercurrent flow through the descending and ascending limbs establishes an osmotic gradient in the renal medulla. Since the thick ascending limb is constantly reabsorbing Na+ and Cl-, and thus Na+ and Cl- become increasingly concentrated in the interstitial fluid of the medulla which results in the formation of an osmotic gradient ranging from 300 to 1200. The descending limb is very permeable to water but impermeable to solutes except urea. Because the osmolarity of the interstitial fluid outside the descending limb is HIGHER than the tubular fluid within it, water moves out of the descending limb via osmosis. Osmolarity increases as fluid continues along the descending limb reaching a max of ~ 1200 at the hairpin turn. In the ascending limb, this area is impermeable to water, but symporters reabsorb Na+ and Cl- from the tubular fluid into the interstitial fluid of the renal medulla so osmolarity of the tubular fluid progressively decreases as it flows through the ascending limb. Overall, tubular fluid becomes increasingly more concentrated as it flows along the descending limb and progressively more dilute as it moves along the ascending limb. 2. Cells in the collecting ducts reabsorb more water and urea . As ADH increases the water permeability of the principal cells, water quickly moves via osmosis out of the collecting duct tubular fluid, into the interstitial fluid of the inner medulla and then into the vasa recta, therefore leaving the tubular fluid concentrated.
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