How is the amount of urine produced regulated?

Antidiuretic hormone (ADH) is produced by the hypothalamus and stored in the posterior lobe of the pituitary gland. Its function is to make membrane in collecting ducts permeable to water. it is released in response to a decrease in H2O, decrease in blood volume, and decrease in BP. A decrease in ADH secretion results in an increase in H2O output. A increase in ADH secretion results in a decrease in H2O output.

Na+ absorption also affects urine output. If reabsoprtion of Na+ is inhibited (by caffeine for example), it causes an increase in H2O output.

Gravitational force i believe