Final answer:
The hormone ADH primarily affects the collecting ducts in the nephrons of the kidneys, controlling the reabsorption of water back into the bloodstream.
Step-by-step explanation:
The area of the nephron most affected by the hormone ADH (antidiuretic hormone) that facilitates reabsorption of water is the collecting ducts. ADH is critical in regulating the body's water balance by controlling the amount of water reabsorbed by the kidneys. It works by making the collecting ducts permeable to water, allowing water molecules to be reabsorbed into the blood through osmosis. The influence of ADH on water reabsorption is powerful, even though it only directly affects the last 10 percent of water available for recovery after initial filtration at the glomerulus.
This is because 90 percent of the water is already absorbed before reaching the collecting ducts. Depending on the body's hydration status, collecting ducts can reabsorb nearly all of the water that reaches them during states of dehydration or almost none during states of over-hydration.