Which structure is specifically responsible for the expulsion of milk during lactation?

The structure specifically responsible for the expulsion of milk during lactation is the nipple. During breastfeeding, the act of suction by the infant at the nipple triggers a series of physiological responses that lead to the release of milk. When the baby suckles, nerve endings in the nipple and areola are stimulated, sending signals to the hypothalamus. This in turn stimulates the pituitary gland to release oxytocin, a hormone that causes the myoepithelial cells around the alveoli and mammary ducts to contract. This contraction pushes the milk through the ducts and into the nipple, where it can be expelled for the infant to feed.

While alveoli are the milk-producing glands and play a crucial role in storing milk, they do not facilitate the actual expulsion. The lactiferous sinus acts as a reservoir for milk but does not directly contribute to expelling the milk. The mammary ducts are essential for transporting milk but, like the alveoli, do not initiate the expulsion process. The function of the nipple is central to the physical act of lactation and is directly engaged when milk is delivered to the infant.