CORTEX 285 MEDULLA
Figure 4.5 Counter-current multiplier.
interstitial fluid solute concentration gradient. The osmolality of interstitial fluid in medulla progressively increases from the corticomedullary junction (isosmo-tic) to the tips of the renal papillae. An osmotic gradient exists between the tubular fluid and the interstitial fluid along the entire medullary collecting duct. Osmotic equilibration of the tubular fluid with the hypertonic medullary interstitium forms hypotonic urine.
If two adjacent currents flow in opposite directions in a hairpin loop, then any exchange of material in a transverse direction is multiplied longitudinally as one progresses towards the apex of the loop. The flow (current) in the ascending limb is in the opposite direction (counter) to flow in the descending limb. (U-shaped
S' o ta y counterflow arrangement.) A small transverse osmotic pressure gradient between the ascending and descending limbs of the loop of Henle is thus multiplied into a larger longitudinal gradient.
The ascending limb is water impermeable and actively transports solute (Cl ions actively, followed by Na ions passively) from tubular fluid into the interstitial fluid.
The descending limb is highly permeable to water and solutes and the increased osmolality of medullary interstitium causes water absorption and tubular fluid concentration. The maximal concentration that the urine can attain is equal to that of the medullary interstitium and papilla.
Isosmotic fluid enters the thin descending limbs and is concentrated as it flows down to the bend of the loop of Henle. As it flows up the ascending limbs it is diluted so that the fluid that emerges is hypotonic to plasma.
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