oxytocin

   A short-lived, fast acting hormone, made by the hypothalamus of the brain, along with its close relative vasopressin (anti-diuretic hormone), stored in the posterior pituitary, and released into the blood as needed. It stimulates certain smooth muscle coats, constricts certain blood vessels and facilitates the sensitivity of some tissues to other hormones and nerves. The main tissues affected are the uterus, including endo- and myometriums, vagina, breasts (both sexes), erectile tissue (both sexes), seminal vesicles, and with special-case effects on uterine muscle contractions in both birth and orgasm, the vascular constriction that lessens placental separation bleeding, and the let-down reflex that nursing mothers have when babies cry (or kittens mew...or husbands whine)