The hormones released by the posterior pituitary are actually synthesized in the hypothalamus. They migrate along nerve fibers to the posterior pituitary, and are stored until stimulated to be released into the general circulation.
Oxytocin is responsible for uterine contractions, both before and after delivery. The muscle layers of the uterus (myometrium) become more sensitive to oxytocin near term. Towards the end of a term pregnancy, levels of progesterone decline, and contractions that were previously suppressed by progesterone begin to be more frequent and stronger. This change in the oxytocin/progesterone ratio is believed to be one of the initiators of labor.
Oxytocin is responsible for the contractions that bring about delivery, by thinning and dilating the cervix, and applying pressure that helps the baby descend in the pelvis. It is also important after delivery, as it continues to cause the myometrium to contract. These contractions help constrict the blood vessels that are sending blood to the uterus at the time of childbirth at the rate of a liter a minute!
Oxytocin is also responsible for milk ejection during breastfeeding, by contraction of the myoepithelial cells in the lactating mammary gland. The uterine "cramping" that often occurs with breastfeeding is a signal that oxytocin is still causing the uterus to contract after delivery. These contractions help the uterine muscle to continue to constrict the uterine blood vessels, and bring about a decrease in the amount of vaginal bleeding after delivery.
Antidiuretic Hormone (ADH)
The function of ADH is to inhibit or prevent the formation of urine. Osmoreceptors monitor the solute concentrations in the blood. During pregnancy the osmoreceptors are "reset" to deal with the increased blood volume of pregnancy. If the osmoreceptors send excitatory messages to the "ADH secreting neurones," less urine is produced, leaving more volume in the circulating blood.
The actions of the hormones of the posterior pituitary are especially important to consider in the pregnant woman who is at risk for preterm labor. Maternal dehydration may trigger the secretion of ADH by the posterior pituitary. It is thought that oxytocin may also be released at the same time, bringing about uterine contraction before the optimum time. These uterine contractions, or uterine "irritability" (low intensity, high frequency contractions) of preterm labor are often treated with maternal hydration. Women at risk for preterm labor are encouraged to drink copious amounts of water throughout the day. And, if hospitalized for contractions, hydration with a bolus of IV fluid is often effective to "quiet" the uterus.
Secretion of ADH is also stimulated by pain, low blood pressure and drugs such as nicotine, morphine and barbiturates. In trauma situations, a great deal of ADH is released, to counteract blood loss. The result is constriction of smooth muscles of the blood vessels, in order to raise the arterial blood pressure. (As a result, of this "pressor" effect, ADH is sometimes referred to as vasopressin.) Very little blood is getting to the baby through the constricted blood vessels.
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