The vasa recta bring blood supply at a very slow rate. It consists of a descending limb and an ascending limb. Vasa recta are also known as the straight arteries of the kidney.
Counter-current mechanism first takes place in the region of the nephron called Henle's loop. The two limbs of Henle's loop are counter-current and these tubes are parallel and adjacent.
Then it occurs in a region of the peritubular capillary bed called the 'vasa recta'. The two limbs of the vasa recta are counter-current and these tubes are parallel and adjacent.
Both are involved in establishing an osmotic gradient throughout this region.