Final answer:
The statement is true and describes the countercurrent multiplier mechanism in the nephron that enables the creation of a concentrated interstitial fluid, which in turn allows the collecting ducts to concentrate urine.
Step-by-step explanation:
The statement that the countercurrent mechanism in the nephron creates a highly concentrated interstitial fluid so that urine can be concentrated by the collecting ducts when they are permeable to water is True. The nephron uses a system known as the countercurrent multiplier, which involves different segments of the loop of Henle operating with opposite functions. The descending limb is permeable to water but not to solutes, allowing water to flow out of the filtrate and into the interstitial fluid, increasing the osmolality of the filtrate as it moves deeper into the renal medulla. Conversely, the ascending limb is permeable to solutes but not to water, which means Na+ and Cl- ions are reabsorbed into the interstitial fluid. Additionally, collecting ducts can actively pump urea into the interstitial spaces, further contributing to the osmolarity of the medulla. This concentration gradient enables the collecting ducts to concentrate the urine, especially when they are permeable to water, as influenced by hormones like antidiuretic hormone (ADH). Therefore, the osmotic gradient created by the loop of Henle and the urine concentrating ability of the collecting ducts are integral to the body's water balance and urine concentration.