Like the venous system, the lymphatic system transports fluids throughout the body. The lymphatic system consists of thin-walled lymphatic vessels, lymph nodes, and two collecting ducts (see Biology of the Immune System: Lymphatic System: Helping Defend Against Infection). Lymphatic vessels, located throughout the body, are larger than capillaries, 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 fluids from tissues throughout the body that have diffused through the very thin walls of capillaries. The fluids contain proteins, minerals, nutrients, and other substances, which provide nourishment to tissues. However, most of the fluid is reabsorbed into the capillaries. The rest of the fluid (lymph) is drained from the spaces surrounding the cells into the lymphatic vessels, which eventually return it to the veins. Lymphatic vessels also collect and transport damaged cells, cancer cells, and foreign particles (such as bacteria and viruses) that may have entered the tissue fluids.

All lymph passes through strategically placed lymph nodes, which filter damaged cells, cancer cells, and foreign particles out of the lymph. Lymph nodes also produce specialized blood cells 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 lymph 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.

The lymphatic system may not perform its function adequately when the quantity of fluid is excessive or when the lymph vessels or nodes are damaged or removed during surgery, become blocked by a tumor, or become inflamed.

Last full review/revision August 2012 by James D. Douketis, MD