Overview of the lymphatic system

[Julene]

The lymphatic system is a vital part of the immune system, along with the thymus, bone marrow, spleen, tonsils, appendix, and Peyer patches in the small intestine.
Like the venous system, the lymphatic system transports fluids throughout the body. The lymphatic system consists of

Thin-walled lymphatic vessels
Lymph nodes
Two collecting ducts

Lymphatic vessels, located throughout the body, are larger than capillaries (the smallest blood vessels, which connect arteries and veins), and most are smaller than the smallest veins. Most of the lymphatic vessels have valves like those in veins to keep the lymph, which can clot, flowing in the one direction (toward the heart). Lymphatic vessels drain fluid called lymph from tissues throughout the body.
Lymph begins as fluid that has diffused through the very thin walls of capillaries into the space between cells. Most of the fluid is reabsorbed into the capillaries and the rest is drained into the lymphatic vessels, which eventually return it to the veins. Lymph also contains many other substances including

Proteins, minerals, nutrients, and other substances, which provide nourishment to tissues
Damaged cells, cancer cells, and foreign particles (such as bacteria and viruses) that may have entered the tissue fluids

Lymph nodes are collection centers for lymph. All lymph passes through strategically placed lymph nodes, which filter damaged cells, cancer cells, and foreign particles out of the lymph. Lymph nodes also contain specialized white blood cells (for example, lymphocytes and macrophages) designed to engulf and destroy damaged cells, cancer cells, infectious organisms, and foreign particles. Thus, important functions of the lymphatic system are to remove damaged cells from the body and to provide protection against the spread of infection and cancer.
The lymphatic vessels drain into collecting ducts, which empty their contents into the two subclavian veins, located under the collarbones. These veins join to form the superior vena cava, the large vein that drains blood from the upper body into the heart.