The production of concentrated urine mainly depends on the counter-current mechanism operating between the Loop of Henle and the vasa recta. The long Loop of Henle in juxta medullary nephrons establishes a concentration gradient in the medulla through the counter-current multiplier system, while the vasa recta maintains this gradient by acting as a countercurrent exchanger. This medullary gradient allows more water to be reabsorbed from the collecting duct under the influence of ADH, leading to concentrated urine. Therefore, the structures playing the most significant role are Henle's loop and vasa recta.

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