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. 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.
Lymphatic vessels drain fluids 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 of 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 destroy damaged cells, cancer cells, 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 major parts of the lymphatic system are the bone marrow, spleen, thymus gland, lymph nodes, and the tonsils. Other organs, including the heart, lungs, intestines, liver, and skin also contain lymphatic tissue.
Lymph nodes are round or kidney-shaped, and range in size from very tiny to 1 inch in diameter. They are usually found in groups in different places throughout the body, including the neck, armpit, chest, abdomen, pelvis, and groin. About two thirds of all lymph nodes and lymphatic tissue are within or near the gastrointestinal tract.
Lymphocytes are white blood cells in the lymph nodes that help the body fight infection by producing antibodies that destroy foreign matter such as bacteria or viruses. Two types are T-cells and B-cells. Some lymphocytes become stimulated and enlarged when they encounter foreign substances; these are called immunoblasts.
The major lymphatic vessel is the thoracic duct, which begins near the lower part of the spine and collects lymph from the lower limbs, pelvis, abdomen, and lower chest. It runs up through the chest and empties into the blood through a large vein near the left side of the neck. The right lymphatic duct collects lymph from the right side of the neck, chest, and arm, and empties into a large vein near the right side of the neck.
The spleen is found on the left side of the abdomen. Unlike other lymphoid tissue, red blood cells flow through it. It helps control the amount of blood and blood cells that circulate through the body and helps destroy damaged cells. The spleen also plays an important part in a person's immune system and helps the body fight infection. Like the lymph nodes, the spleen contains antibody-producing lymphocytes. These antibodies weaken or kill bacteria, viruses, and other organisms that cause infection. Also, if the blood passing through the spleen carries damaged cells, white blood cells called macrophages in the spleen will destroy them and clear them from the bloodstream.
