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tubule osmolality along the corse of the nephron
the proximal tubule reabsorbs filtered fluid isosmotically; the thin descending limb of the loop of henle is permeable to water and the fluid osmolality increases (by water reabsorption) in parallel with interstitial osmolality in the medulla; as the descending loop makes the hairpin turn and becomes the thin ascending loop it becomes impermeable to water but permeable to NaCl which is reabsorbed passively ([NaCl] is higher in the lumen) and the tubular fluid osmolality decreases; in the thick ascending limb which is also impermeable to water there is active NaCl reabsorption which renders the tubular fluid hypotonic; this is the segment where free water is generated; as tubular fluid enters the cortical and medullary collecting ducts if there is antidiuretic hormone present as in water restriction these segments will become permeable to water and tubular fluid osmolality will equilibrate with the osmolality of the surrounding tissue; in the cortex the osmolality is the same as plasma and that is the osmolality that the tubular fluid will reach; still in antidiuresis as the fluid enters the medullary collecting duct the luminal osmolality rises sharply as the tubular fluid equilibrates with the hyperosmotic medulla; in the condition of excess water intake ADH is not present and the collecting duct remain impermeable to water; the fluid entering these segments is hyposmotic and as salt is reabsorbed it becomes increasingly hyposmotic
generating hyperosmolar medulla and urine
the renal medullar is hyperosmotic during water diuresis and antidiuresis; single effect and countercurrent multiplier; distinct pattern of water and salt permeabilities along the loop of Henle; the ascending loop of Henle plays a key role in both the dilution and concentration of urine; the main function of the loop is to remove NaCl from the lumen and deposit it in the interstitium of the renal medulla; by separating tubule NaCl from tubule water it participates directly in the formation of dilute urine; because it deposits the NaCl in the medulla it contributes to its hyperosmolality and therefore indirectly to the generation of concentrated urine; the hyperosmolar medulla is the result of the dingle effect and counter current multiplier and the distinct water and salt permeabilities along the loop of Henle
how does the coutercurrent multiplication in the loop of henle work? stage 1
the stating condition is with isosmotic fluid (300 mOsm) throughout the ascending and descending limbs and in the interstitium
stage 2
the 'single effect' (the one thing that really causes the whole thing to be set up); Na Cl transport from the lumen of the ascending limb to the interstitium increasing osmolality (it is impermeable to water even in the presence of AVP)
stage 3
increasing osmolality rapidly equilibrates with the lumen of the descending limb (it is permeable to water)
stage 4
an 'axial shift' of tubule fluid along the loop of henle with a rapid equilibration between the lumen of the descending limb and the interstitium leading to osmolarities of 300, 700, 1000, and 1200 in the interstitium from top to bottom
the movement of NaCl in the TAL
the TAL moves NaCl from lumen out to interstitium using a combination of transcellular and paracellular pathways
the movement of NaCl in the tAL
the tAL transports NaCl passively driven by the concentration of NaCl in the lumen which exceeds that of the interstitium of the inner medulla
what about the tDL?
the work of concentrating the NaCl in the lumen is performed here; the tDL has a high water permeability owed to a high expression of aquaporin 1 (AQP1); the tDL has a very low permeability to NaCl and a finite urea permeability resulting from the presence of the UT-A2 urea transporter; the interstitium of the inner medulla has very high [NaCl] and [urea] which provide the osmotic energy for passive water reabsorption which secondarily concentrates NaCl in the lumen of the tDL
concentrations of NaCl and urea in the kidney (cortex and medulla)
in the interstitium [NaCl] and [urea] all rise along the axis from the cortex to the papillary tip of the renal medulla; in the outer medulla a steep rise in interstitial [NaCl] occurs owing to the pumping of NaCl out of the TAL that is largely responsible for producing the hypertonicity; the [urea] rises steeply from the middle of the outer medulla to the papilla; at the tip of the papilla urea and NaCl each contribute approx 1/2 of the interstitial osmolality
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