How Do Boobs Make Milk?

a woman holding a baby in her lap

During pregnancy, supportive fat and milk-making glandular tissue grow and develop. They form channels called ducts that carry milk to the nipple.

Your baby’s sucking stimulates the pituitary gland to release oxytocin and prolactin into your bloodstream. These hormones cause the milk-filled alveoli to contract, triggering the letdown reflex.

The Breast

The breast consists of milk-producing glands, tubes (ducts), and fatty tissue that covers the outer surface, called the areola. The nipple has holes for the milk to flow through, and Montgomery glands secrete oil that helps lubricate the nipple and areola. Breast milk is a liquid that contains fat, protein, water, sugar, and vitamins. The amount of milk made depends on how often and thoroughly the baby nurses or pumps. Frequent breastfeeding or pumping (8-12 times every 24 hours) increases the amount of milk that is produced.

When the baby suckles, it sends a signal to the brain to release hormones called prolactin and oxytocin. These cause the alveoli to start making milk and cause the muscles around the ducts to squeeze milk out through them. This releases the letdown reflex, which causes milk to flow into the nipple and then the nipple to empty into the baby’s mouth.

The size of a woman’s breasts doesn’t affect her ability to make milk. The same amount of glandular breast tissue is present in women with small and large-breasted breasts.

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The Lobes

Each breast contains a complex network of fatty and glandular tissue – This section is the work of the portal’s author The fatty tissue fills the spaces between the glandular tissue, which doctors call mammary tissue. There are 15 to 20 overlapping sections of this glandular tissue, called lobes. Each lobe has dozens of tiny “bulbs” that make milk. These bulbs are linked by milk ducts that carry milk to the nipple opening when you nurse your baby. The lobes and ducts are arranged in groups that look like bunches of grapes. Each breast is attached to a dark area of skin called the areola. Muscles lie underneath the breast and against the chest wall.

The Ducts

A network of milk ducts, which start at each lobe and end at the tip of the nipple (areola), makes up your breast milk-making system. Each lobe has up to 20 of these milk ducts. Each duct branches off into smaller channels called ductules, which then lead to the small, grapelike clusters of cells that make your milk. These are called alveoli.

Your boobs have been preparing to nurse since you were in the womb. When you begin nursing, your baby’s sucking stimulates the pituitary gland to release the hormones prolactin and oxytocin into your bloodstream. Prolactin tells the milk-making glands to switch into high gear, while oxytocin causes the alveoli to contract and squeeze their nutrient-rich contents into the ducts.

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The ducts carry the milk from the alveoli through nerves that lead from the nipple and areola, then out of your nipple and into your baby’s mouth. The more your baby sucks, the more you make — a process called supply and demand. This explains why it’s important to feed your baby often and at the right times.

The Alveoli

The lungs’ blood vessels, called capillaries, connect to tiny air sacs called alveoli. Alveoli are cup-shaped and have thin walls, and they’re grouped together in bunches that look like tiny grapes (alveolar sacs). Oxygen from the air we breathe in diffuses into your blood through these alveoli. Carbon dioxide from the blood passes through the alveoli into the bronchioles and out of your body.

The walls of each alveolus contain a fluid extracellular matrix that houses the enmeshed network of capillaries. There are also small pores between alveoli called Pores of Kohn that allow gases to pass through the walls and into and out of the capillaries. The walls of the alveoli contain a special type of cell called type II cells that secrete a pulmonary surfactant, which lowers water surface tension so the walls of the alveolus can stretch when filled with oxygen during inhalation and then spring back to expel carbon dioxide during exhalation.

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When your baby suckles, nerves from the nipple and areola send a message to the brain that stimulates the release of the milk-producing hormone prolactin. This causes the cells around the alveoli to contract and squeeze, causing milk to flow out through the ducts and into your baby’s mouth.

The Nipple

In an onslaught of breast augmentation commercials and boob-lifting bras, it’s easy to forget that a woman’s nipples serve more than just an evolutionary purpose (i.e., to feed her offspring). Nipples, also called teats or nipples, are the fleshy area in the center of each breast that contains nerves and milk duct openings for breastfeeding. They are surrounded by the areola, a round patch of skin covered in Montgomery glands that secrete an oil to help prevent the nipple and areola from chafing during lactation.

During pregnancy, the nipples and areola become larger and darker as the body prepares to make milk. When a baby suckles, it stimulates the nerves to release the hormones prolactin and oxytocin, which stimulate the alveoli to make milk and suck down through the milk ducts.

Milk production is a demand-and-supply process; the more the nipples are used to nurse, pump or express milk, the more they will produce. This is why it’s important to see a doctor any time you experience changes in your breast or nipple, especially if they are painful or red.