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Overview of the Lymphatic System

By James D. Douketis, MD, Professor, Divisions of General Internal Medicine, Hematology and Thromboembolism, Department of Medicine; Director, Vascular Medicine Research Program, McMaster University; St. Joseph's Healthcare Hamilton

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, 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 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.

Lymphatic System: Helping Defend Against Infection

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.

The lymphatic system is a network of lymph nodes connected by lymphatic vessels. This system transports lymph throughout the body.

Lymph is formed from fluid that seeps through the thin walls of capillaries into the body's tissues. This fluid contains oxygen, proteins, and other nutrients that nourish the tissues. Some of this fluid reenters the capillaries and some of it enters the lymphatic vessels (becoming lymph). Small lymphatic vessels connect to larger ones and eventually form the thoracic duct. The thoracic duct is the largest lymphatic vessel. It joins with the subclavian vein and thus returns lymph to the bloodstream.

Lymph also transports foreign substances (such as bacteria), cancer cells, and dead or damaged cells that may be present in tissues into the lymphatic vessels and to lymph organs for disposal. Lymph contains many white blood cells.

All substances transported by the lymph pass through at least one lymph node, where foreign substances can be filtered out and destroyed before fluid is returned to the bloodstream. In the lymph nodes, white blood cells can collect, interact with each other and with antigens, and generate immune responses to foreign substances. Lymph nodes contain a mesh of tissue that is tightly packed with B cells, T cells, dendritic cells, and macrophages. Harmful microorganisms are filtered through the mesh, then identified and attacked by B cells and T cells.

Lymph nodes are often clustered in areas where the lymphatic vessels branch off, such as the neck, armpits, and groin.

Disorders of the lymphatic system

The lymphatic system may not carry out its function adequately due to

  • Blockage (obstruction): Obstruction in the lymphatic system leads to an accumulation of fluid (lymphedema). Obstruction may result from scar tissue that develops when the lymph vessels or nodes are damaged or removed during surgery, by radiation therapy, by injury, or in tropical countries, by infection with a threadworm (filariasis) that blocks the lymphatic ducts.

  • Infection: Infection may cause swollen lymph nodes because the lymph nodes are inflamed. Sometimes the lymph nodes themselves may become infected (lymphadenitis) by organisms that spread through the lymphatic system from the original site of infection.

  • Cancer: Tumors may block the lymphatic ducts or may travel (metastasize) to lymph nodes near a tumor, interfering with flow of lymphatic fluid through the node. Rarely, a tumor (lymphangiosarcoma) may develop in the lymphatic system.

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