A 26-year-old woman takes part in a marathon for

the first time on a hot and humid Sunday afternoon. Halfway through the race,

she collapses. She hasn't drunk enough fluids since the morning. Physical

examination reveals tachycardia and severe dehydration. As a result of this

condition, higher amount of antidiuretic hormone is released from the pars

nervosa of the pituitary gland in response to increased plasma osmolarity

and/or decreased plasma volume. Which part of nephron is directly affected by

this hormone?