The Anatomy and Physiology of the Breast
The breasts comprise glandular, connective and fatty tissue. Within these tissues are milk-producing cells. Tiny openings in the nipple allow milk to flow. Surrounding the nipple is the areola, an area of darker skin that becomes both larger and darker during pregnancy. Montgomery glands located on the areola secrete a lubricant to help clean the area.
The nipple is stimulated when the baby sucks; muscular tissue surrounding the nipple causes it to become erect. When the nipple is stimulated, the brain's pituitary gland secretes the hormone prolactin, which triggers the breast's milk gland cells to produce milk. This does not occur until the baby and placenta are delivered.
The hormone oxytocin is then released, helping to relax the mother, and causing the milk gland to contract and push milk out of the nipple. This is called letdown or milk ejection reflex. Letdown may take several minutes, especially when the mother is tired or tense.