how is oxytocin regulated by the the endocrine system?

In short oxytocin is regulated via a positive feedback loop.
Oxytocin is produced by the posterior pituitary gland and is one one of the few hormones that is positively regulated, which means once it released by the posterior pituitary it continues to be released until the signal dissipates.  
Oxytocin plays a large role in birth and lactation.  Once the fetus begins to descend causing the cervix to dilate, oxytocin is released and causes smooth muscle contractions of the uterus.  Pitocin is a generic drug given to women to help induce labor.  It's half-life in the blood is about 3 minutes.  Once the infant is released, oxytocin is no longer carried to the uterus, but once the baby begins to suck oxytocin is released and sent to the mammary glands of the breasts causing smooth muscle contractions and excretion of milk via the nipples.