Down, up and down again.

• Mammals can produce concentrated urine, and Henle's loop and the vasa recta are central to this.
• Filtrate flows in opposite directions in the two limbs of Henle's loop, forming a counter current. Blood in the two limbs of the vasa recta also flows in a counter current pattern.
• Because the loop and vasa recta lie close together and both carry counter currents, the osmolarity of the interstitium rises steadily towards the inner medulla: from 300 mOsmol L⁻¹ in the cortex to about 1200 mOsmol L⁻¹ in the inner medulla.
• The gradient is made mainly by NaCl and urea.
• NaCl is transported out by the ascending limb of Henle's loop and exchanged with the descending limb of the vasa recta; the ascending part of the vasa recta returns it to the interstitium.
• A little urea enters the thin part of the ascending limb of Henle's loop; the collecting tubule later hands urea back to the interstitium.
• This transport, made possible by the special arrangement of Henle's loop and the vasa recta, is the counter current mechanism. It keeps the concentration gradient in the medullary interstitium.
• The gradient lets water pass easily out of the collecting tubule, concentrating the filtrate. Human kidneys can make urine nearly four times as concentrated as the initial filtrate (300 to 1200 mOsmol L⁻¹).

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