The body is mostly composed of fluids. All its cells contain and are surrounded by fluids. In addition, four to five liters of blood circulate through the cardiovascular system at any given time. Some of that blood escapes from the system as it passes through tiny blood vessels called capillaries in the body tissues. Fortunately, there is a "secondary circulatory system" that reabsorbs escaped fluid and returns it to the veins.

That system is the lymphatic system. It runs parallel to the veins and empties into them. Lymph forms at the microscopic level. Small arteries, or arterioles, lead to capillaries, which in turn lead to small veins, or venules. Lymph capillaries lie close to the blood capillaries, but they are not actually connected. The arterioles deliver blood to the capillaries from the heart, and the venules take blood away from the capillaries. As blood flows through the capillaries it is under pressure. This is called hydrostatic pressure. This pressure forces some of the fluid in the blood out of the capillary into surrounding tissue. Oxygen from the red blood cells, and nutrients in the fluid then diffuse into the tissue.

Carbon dioxide and cellular waste products in the tissue diffuse back into the bloodstream. The capillaries reabsorb most of the fluid. The lymph capillaries absorb what fluid is left.

Edema, or swelling, occurs when fluid in or between the cells leaks into the body tissues. It is caused by events that increase the flow of fluid out of the bloodstream or prevent its return. Persistent edema may be a sign of serious health problems and should be checked by a health care professional.

The lymphatic system can play a very worrisome role in the spread of breast cancer.

Lymph nodes filter the lymph as it passes through the system. They are located at specific points throughout the body such as in the armpits and high in the throat.

Lymphatic circulation in breast tissue helps regulate the local fluid balance as well as filter out harmful substances. But the breast's lymphatic system can also spread diseases such as cancer through the body.

Lymphatic vessels provide a highway along which invasive cancerous cells move to other parts of the body.

The process is called metastasis. It can lead to the formation of a secondary cancer mass in another part of the body.

This mammogram shows a tumor and the lymph vessel network it has invaded.

No woman is too young to know that regular breast self-examinations can help to catch tumors earlier in their growth, hopefully before they spread or metastasize.