Final answer:
The countercurrent multiplier operates in the loops of Henle within the nephron, which consists of the ascending and descending limbs. This system creates a concentration gradient essential for urine concentration.
Step-by-step explanation:
The countercurrent multiplier system is a mechanism that occurs in the loops of Henle of the nephron in the kidney. Specifically, it involves both the ascending loop of Henle and the descending loop of Henle. This system creates an osmotic gradient in the renal medulla that is responsible for the concentration of urine. The descending limb of the loop of Henle descends from the renal cortex into the renal medulla and is permeable to water, but not to solutes, whereas the ascending limb is impermeable to water but actively transports solutes out into the medulla, leading to the dilution of the filtrate.
Fine tuning of water recovery or disposal occurs in other parts of the nephron such as the collecting ducts, while the countercurrent multiplier specifically refers to the process in the loops of Henle. The loops of Henle are located between the proximal and distal convoluted tubules, and filtrate from the loop of Henle empties into the distal convoluted tubule.