Final answer:
Labor contractions are initiated by the hormone oxytocin, which can be administered in its synthetic form, pitocin, to induce labor when necessary. Option a is the answer.
Step-by-step explanation:
Which Substance Initiates Labor Contractions?
Labor contractions can be initiated by the administration of oxytocin. This hormone, produced by the posterior pituitary gland, plays a critical role in stimulating the contractions of labor. The myometrium, which is the smooth muscle layer of the uterus, increases its sensitivity to oxytocin by expressing more receptors for this hormone. During the process of labor, oxytocin begins to stimulate stronger and more frequent uterine contractions, which in turn stimulate the secretion of prostaglandins from fetal membranes. These substances enhance uterine contractile strength, working in tandem to facilitate childbirth.
When a pregnancy is not progressing to labor naturally, a synthetic form of oxytocin, commonly known as pitocin, may be used to induce labor. This is administered intravenously to stimulate the uterine muscles, thereby initiating the labor process. The other options listed, such as nitrous oxide, magnesium sulfate, and epidural anesthesia, do not initiate labor contractions; they serve different roles during labor and delivery.