Final answer:
In amphibians, the collecting ducts reabsorb significant amounts of filtrate into the blood, resulting in hyperosmotic urine, thanks to the action of antidiuretic hormone (ADH).
Step-by-step explanation:
In amphibians, much of the filtrate is reabsorbed from the collecting ducts into the blood, producing hyperosmotic urine. The presence of antidiuretic hormone (ADH) from the hypothalamus makes the collecting ducts permeable to water, allowing water molecules to pass through them into the capillaries by osmosis while preventing the passage of ions or other solutes.
As a result, a significant amount of water may be reabsorbed from the urine in the collecting ducts, making the urine more concentrated. This process exemplifies the fine-tuning of water recovery in amphibian excretory systems, particularly in the presence of ADH.