What structure(s) reabsorb water and salt, making urine more concentrated?
Proximal convoluted tubule and Henle's loop of nephron and collecting ducts of kidney are all able to reabsorb either water or salt or both, making urine more concentrated.
Along the proximal convoluted tubule (PCT) water from filtrate passively enters blood by osmosis, because solutes are reabsorbed.
Water is also reabsorbed along descending limb of Henle, but not at all along ascending limb. On the other hand salt is reabsorbed along ascending limb of Henle, but not at all along descending limb.
NaCl is also reabsorbed by walls of PCT, and distal convoluted tubule (DCT) of nephron.
Reabsorption of water and salt continues in collecting ducts.