Which of the following hormones is mainly responsible for promoting milk release during breastfeeding?

The hormone that primarily facilitates the release of milk during breastfeeding is oxytocin. Oxytocin is secreted in response to the baby's suckling and plays a crucial role in the milk ejection reflex, also known as "let-down." When a baby suckles, nerve impulses are sent to the mother's hypothalamus, which triggers the release of oxytocin from the posterior pituitary gland. This hormone causes the myoepithelial cells surrounding the milk-producing alveoli in the breast to contract, thereby pushing milk into the ducts and making it available to the nursing infant.

Prolactin is important for milk production, but it is oxytocin that directly causes the milk to be released during breastfeeding. Estrogen and relaxin have roles in pregnancy and preparing the body for lactation but do not directly affect the milk release process itself. Understanding the distinct roles of these hormones highlights the coordinated physiological responses involved in lactation, emphasizing oxytocin's critical function in the breastfeeding process.