What STARTS the countercurrent mechanism?

Answer 1

If you are referring to the countercurrent exchange that occurs in the nephron in terms of solutes and water to maintain the osmotic gradient of the renal pyramids, this process begins at the initiation of the thick descending loop of Henle, as ions and water are exchanged between the loop of Henle and the vasa recta. The vasa recta is the capillary system that surrounds the loop of Henle component of the nephron.