The breast is usually pear shaped, but shape and size vary widely among women. Externally, each breast has a nipple that is the site of milk duct openings. The nipple is surrounded by an area of darker tissue called the areola.

Internally, the breast is composed of breast tissue, which reaches from the collarbone to the lower ribs. It’s cushioned in fat which makes up about one third of the breast. The breast rests against the pectoral or chest muscles.

Inside the breast, there are lobules that produce milk and ducts that transport milk to the nipple. Each nipple has about 10 duct openings, each leading to a ductal system that goes from the nipple to the back of the breast, spreading like a vine into different branches that end in lobes. Lobes are made up of smaller lobules.

All of the ductal systems in the breast work together to provide milk but are independent of each other.