Each breast contains blood vessels, as well as vessels that carry a fluid called lymph. Lymph travels throughout the body through a network called the lymphatic system, carrying cells that help the body fight infections. The lymph vessels lead to the lymph nodes (small, bean-shaped structures).
One group of lymph nodes is located in the armpits, above the collarbone and in the chest. If breast cancer has reached these nodes, it may mean that cancer cells have spread to other parts of the body via the lymphatic system. Lymph nodes are also found in many other parts of the body.
Breast development and function depend on the hormones estrogen and progesterone, which are produced in the ovaries. 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.