A molecular model of vasopressin, a peptide hormone responsible for retaining water in the body. During dehydration, vasopressin increases water reabsorption by the kidneys to increase urine concentration while decreasing water loss. Vasopressin also contributes to inducing vasoconstriction, which raises arterial blood pressure. Atoms are colored dark gray (carbon), light grey (hydrogen), blue (nitrogen), yellow (sulfer) and red (oxygen).