text: Blood entering the interstitium in the vasa recta is\n300 mosmol/l. As with capillaries elsewhere, these capillaries are\npermeable to salt and water.\n\nAs blood flows, in the vasa recta, from the cortex to\nthe medulla, it equilibrates at each level with the\nsalt and water which is drawn in from the\ninterstitium by the net inward pressure (15mmHg).\n\nAs the blood returns to the cortex, the same\nprocess of equilibration occurs. As the blood leaves to enter the venules, its osmolarity is about 310 mosmol/l., just slightly\nhyper-osmotic with respect to plasma.\n\nThe vasa recta continuously take away a\nsmall amount of salt (osmolarity of 310 vs 300) and\nwater from the interstitium but do not dissipate\nthe gradient.