Final answer:
Oxytocin is the hormone that is responsible for causing uterine contractions during childbirth and the expulsion of milk from the mammary glands during lactation. It is secreted by the posterior pituitary gland and its release is stimulated by both the stretching of the uterus during labor and the suckling of infants during breastfeeding.
Step-by-step explanation:
The Role of Oxytocin in Childbirth and Lactation
The hormone involved in uterine contractions during childbirth and the expulsion of milk from the mammary glands is oxytocin. Produced by the hypothalamus and released by the posterior pituitary gland, oxytocin's release triggers uterine contractions which increase in intensity through a positive feedback mechanism until the baby is born. Additionally, oxytocin stimulates the contraction of myoepithelial cells surrounding the mammary glands, leading to the milk ejection or "let-down" reflex during breast-feeding.
During pregnancy, the uterus develops a heightened sensitivity to oxytocin as the number of oxytocin receptors increases. The stretching of the uterus and cervix during labor also stimulates additional oxytocin release. Oxytocin release further intensifies the contractions required for childbirth. In terms of lactation, the act of suckling by an infant on the breast promotes oxytocin production, leading to the contraction of cells in the mammary glands and expulsion of milk.
Thus, oxytocin plays a crucial role in both the endocrine and reproductive systems by facilitating important processes such as labor and lactation.