V plasma & ^ Urine. Vasopressin/adh increases the amount of water reabsorbed by the kidneys and decreases the amount of water excreted into the urine. With more water in the bloodstream, the concentration of everything else decreases (so osmolality goes down), while the concentration of everything in the urine goes up (as dose the urine osmolality).
Decreased osmolarity. Increased osmolality in the blood will stimulate the secretion of antidiuretic hormone (adh), increasing water reabsorption, causing more concentrated urine, and less concentrated blood plasma. So, if you have ADH (vasopressin) secretion, you will have the opposite effect over time -- remember it is in constant change and balance.
When tumors secrete high levels of vasopressin, what happens to plasma osmolarity, urine osmolarity and urine volume?
SIADH. Plasma osmolarity will decrease, urine osmolarity will increase, and urine volume will decrease.
Tricky concept. ADH / vasopressin makes the body reabsorb water. However, the atrial natriuretic peptide then kicks in and rids the body of that water by forcing excretion of sodium in the urine. Low plasma sodium / lowish osmolarity; urine sodium will be inappropriately high.