Oxytocin also has an antidiuretic action, but it is weaker in this respect than ADH. In addition, it can cause contractions of the smooth muscles in the uterine wall.
This hormone is released near the end of pregnancy and may play a role in childbirth by stimulating uterine contractions.
The mechanism that triggers the release of oxytocin is not clearly understood. It is known, however, that the uterus becomes more and more sensitive to oxytocin's effects during pregnancy.
Also, it is believed that stretching of liter me and vaginal tissues late in pregnancy, caused by the growing fetus, may initiate nerve impulses to the hypothalamus.
The hypothalamus may then signal the posterior lobe to release oxytocin, which in turn, enhances uterine wall contractions during labor.
Oxytocin also has an effect upon the breasts, causing contractions in certain cells associated with milk-producing glands and their ducts.
In lactating breasts, this action forces liquid from the milk glands into the milk ducts and causes the milk to be ejected from the breasts—an effect that is necessary before milk can be removed by sucking.
Sucking the nipple of a breast initiates nerve impulses that travel to the mother's hypothalamus. The hypothalamus responds by signaling the posterior lobe to release oxytocin, which in turn stimulates the release of milk. Thus, milk is normally not ejected from the milk glands and ducts until it is needed.
Oxytocin has no established function in males.
If the uterus is not contracting sufficiently to expel a fully developed fetus, commercial preparations of oxytocin are sometimes used to stimulate uterine contractions. Such preparations are often administered to the mother following childbirth to ensure that the uterine muscles contract enough to squeeze broken blood vessels closed, minimizing the danger of hemorrhage.
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