Tubular Fluid Remains Isosmotic in the Proximal Tubule As

fluid flows through the proximal tubule, solutes and water are reabsorbed in equal proportions, so that little change in osmolarity occurs; that is, the proximal tubule fluid remains isosmotic to the plasma, with an osmolarity of about 300 mOsm/L.As fluid passes down the descending loop of Henle, water is reabsorbed by osmosis and the tubular fluid reaches equilibrium with the surrounding interstitial fluid of the renal medulla, which is very hypertonic—about two to four times the osmolarity of the original glomerular filtrate. Therefore, the tubular fluid becomes more concentrated as it flows into the inner medulla.

Urine Osmolarity

Plasma osmolarity

Drink 1.0 L H2O

Urine Osmolarity

Plasma osmolarity

-10 60 120 180 Time (minutes)

-10 60 120 180 Time (minutes)

Tubular Fluid Becomes Dilute in the Ascending Loop of Henle. In the ascending limb of the loop of Henle, especially in the thick segment, sodium, potassium, and chloride are avidly reabsorbed. However, this portion of the tubular segment is impermeable to water, even in imav_/i i 12^-'

NaCl

NaCl

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