Final answer:
Oxytocin is the hormone that controls milk ejection during breastfeeding and uterine contractions during labor, and it is released by the posterior pituitary gland. It is also influenced by signals from the hypothalamus triggered by infant suckling.
Step-by-step explanation:
Hormone that Controls Milk Ejection and Uterine Contractions
The hormone responsible for controlling the ejection of milk during breastfeeding and the contractions of the uterus during labor and delivery is oxytocin. This endocrine hormone is secreted by the posterior pituitary gland.
During childbirth, oxytocin stimulates uterine contractions, which increase in intensity through a positive feedback mechanism until delivery is complete.
In breastfeeding, the suckling of an infant triggers oxytocin release, which leads to the contraction of myoepithelial cells, forcing milk from the mammary glands into ducts to facilitate the milk ejection or "let-down" reflex.
Oxytocin's role extends beyond just childbirth and breastfeeding; it also helps stimulate contractions in other parts of the body, such as the ductus deferens and prostate gland during emission in males.
Thus, oxytocin is a versatile and critical hormone for reproductive and postnatal processes. Another important hormone in lactation is prolactin, which is released from the anterior pituitary gland in response to infant suckling and is primarily responsible for milk production in the mammary glands.