The defining characteristic of mammals is the provision of milk, a fluid with a composition that exactly mirrors the needs of the young of the species. In the human breast, milk is produced and stored in differentiated alveolar units, often called lobules. These lobules contain small ducts, which coalesce into main ducts that drain sectors of the gland and open directly on the nipple. The amount of milk produced is regulated by prolactin and local factors. Removal of the milk from the breast is accomplished by a process called milk ejection, which is brought about by a neuroendocrine reflex. Afferent stimuli lead to the secretion of oxytocin from the posterior pituitary into the bloodstream, where it is carried to the myoepithelial cells that surround the ducts and alveoli. Contraction of these cells leads to milk ejection.