Each breast has 15 to 20 sections, or lobes, that surround the nipple in a radial manner, like spokes on a wheel. Inside these lobes are smaller sections, called lobules. At the end of each lobule are tiny "bulbs" that produce milk. These structures are linked together by small tubes called ducts, which carry milk to the nipples. Fat fills the spaces between the lobes and ducts.
The nipple is in the center of a dark area of skin called the areola. The areola contains small glands that lubricate the nipple during breastfeeding. There are no muscles in the breasts, but muscles lie under each breast to cover the ribs.
Each breast also contains blood vessels and vessels that transport lymph. Lymph is a fluid that travels through a network of channels called the lymphatic system and carries cells that help the body fight infections. The lymph vessels lead to the lymph nodes which are small, bean-shaped glands that are part of the infection-fighting lymphatic system. Lymph nodes are located in the armpits, above the collarbone, and in the chest. If a cancer has reached these nodes, it may mean that cancer cells have spread to other parts of the body. Lymph nodes are also found in many other parts of the body including inside the chest, abdominal cavity, and the groin.
Breast development and function depend on hormones produced by the ovaries, namely estrogen and progesterone. Estrogen elongates the ducts and causes them to create side branches. Progesterone increases the number and size of the lobules in order to prepare the breast for nourishing a baby. After ovulation, progesterone makes the breast cells grow, and blood vessels enlarge and fill with blood. At this time, the breasts often become engorged with fluid and may be tender and swollen.