What pituitary hormone works at the level of the collecting duct to either increase or decrease absorption of water in order to maintain plasma volume and thus blood pressure?

Antidiuretic hormone (vasopressin) works at the level of the collecting duct to either increase absorption of water in order to maintain plasma volume and thus blood pressure. This hormone is synthesized by special nerve cells of the hypothalamus and then transported to the pituitary gland where the hormone is released from vesicles into the bloodstream. Vasopressin is released in response to extracellular fluid hypertonicity. His functions are to reabsorb water back into the circulation, from the filtrate in the kidney tubules of the nephrons and to constricts arterioles (raises arterial blood pressure). 

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passive transport is a movement of biochemicals and other atomic or molecular substances across cell membranes without need of energy input. unlike active transport, it does not require an input of cellular energy because it is instead driven by the tendency of the system to grow in entropy.

~wikipedia

so, according to what that says, i believe it's hypertonic and hypotonic.

good luck!