Antidiuretic Hormones released by the Neurohypophysis of all Vertebrates (structure varies with species) to regulate Water balance and Osmolarity. In general, Vasopressin is a nonapeptide consisting of a six-amino-acid ring with a Cysteine 1 to Cysteine 6 disulfide bridge or an octapeptide containing a Cystine. All Mammals have Arginine Vasopressin except the pig with a Lysine at position 8. Vasopressin, a vasoconstrictor, acts on the Kidney Collecting Ducts to increase Water reabsorption, increase Blood Volume and Blood Pressure.