The Lymphatic System

Closely connected with the blood and circulatory system, the lymphatic system is an extensive drainage system that returns water and proteins from various tissues back to the bloodstream. It is comprised of a network of ducts, called lymph vessels and carries lymph, a clear, watery fluid that resembles the plasma of blood. Some scientists consider this system to be part of the blood and circulatory system because lymph comes from blood and returns to blood, and because its vessels are very similar to the veins and capillaries of the blood system. Throughout the body, wherever there are blood vessels, there are lymph vessels, and the two systems work together.

How Are the Spleen and Lymphatic System Necessary for Living?

The entire lymphatic system flows toward the bloodstream, returning fluid from body tissues to the blood. If there were no way for excess fluid to return to the blood, our body tissues would become swollen. For example, when a body part swells, it may be because there is too much fluid in the tissues in that area. The lymph vessels collect that excess fluid and carry it to the veins through the lymphatic system.

This process is crucial because water, proteins, and other molecules continuously leak out of tiny blood capillaries into the surrounding body tissues. This lymph fluid has to be drained, and so it returns to the blood via the lymphatic vessels. These vessels also prevent the back flow of lymph fluid into the tissues.

The lymphatic system also helps defend the body against invasion by disease-causing agents such as viruses, bacteria, or fungi. Harmful foreign materials are filtered out by small masses of tissue called lymph nodes that lie along the network of lymphatic vessels. These nodes house lymphocytes (white blood cells), some of which produce antibodies, special proteins that fight off infection. They also stop infections from spreading through the body by trapping disease-causing germs and destroying them.

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.

Basic Anatomy

The lymphatic system is a network of very fine vessels or tubes called lymphatics that drain lymph from all over the body. Lymph is composed of water, protein molecules, salts, glucose, urea, lymphocytes, and other substances.

Lymphatics are found in every part of the body except the central nervous system. The major parts of the 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.

Normal Physiology

Lymph drains into open-ended, one-way lymph capillaries. It moves more slowly than blood, pushed along mainly by a person's breathing and contractions of the skeletal muscles. The walls of blood capillaries are very thin, and they have many tiny openings to allow gases, water, and chemicals to pass through to nourish cells and to take away waste products. Interstitial fluid passes out of these openings to bathe the body tissues.

Lymph vessels recycle the interstitial fluid and return it to the bloodstream in the circulatory system. They collect the fluid and carry it from all of the body's tissues and then empty it into large veins in the upper chest, near the neck.

Lymph nodes are made of a mesh like network of tissue. Lymph enters the lymph node and works its way through passages called sinuses. The nodes contain macrophages, phagocytic cells that engulf (phagocytize) and destroy bacteria, dead tissue, and other foreign matter, removing them from the bloodstream. After these substances have been filtered out, the lymph then leaves the nodes and returns to the veins, where it reenters the bloodstream.

When a person has an infection, germs collect in great numbers in the lymph nodes. If the throat is infected, for example, the lymph nodes of the neck may swell. Sometimes the phagocytic cells may not be able to destroy all of the germs, and a local infection in the nodes may result.

Because the lymphatic system extends to the far reaches of the body, it also plays a role in the spread of cancer. This is why lymph nodes near a cancerous growth are usually removed with the growth.

Diseases, Conditions, Disorders, and Dysfunction's

Because the lymphatic system branches through most of the parts of the body, it may be involved in a wide range of conditions. Diseases may affect the lymph nodes, the spleen, or the collections of lymphoid tissue that occur in certain areas of the body.

Disorders of the lymph nodes

Lymphadenopathy. Most lymph nodes in the body can't be felt easily unless they become swollen or enlarged. Lymphadenopathy is an increase in the size of a lymph node or nodes, most often as the result of a nearby infection (for example, lymphadenopathy in the neck might be the result of an infection of the throat). Less commonly (particularly in children), swelling of the lymph nodes can be due to an infiltration of cancerous cells. If lymphadenopathy is generalized (meaning that the swelling is present in several lymph node groups throughout the body), it usually indicates that the person has a systemic disease.

Lymphadenitis, or adenitis, is an inflammation (swelling, tenderness, and sometimes redness and warmth of the overlying skin) of the lymph node due to an infection of the tissue in the node itself. In children, this condition most commonly involves the lymph nodes of the neck.

Lymphomas. A group of cancers that arise from the lymph nodes, these diseases result when lymphocytes undergo changes and start to multiply out of control. The involved lymph nodes enlarge, and the cancer cells crowd out healthy cells and may form tumors (solid growths) in other parts of the body.

Disorders of the spleen

Splenomegaly (enlarged spleen). In children, the spleen is usually small enough that it can't be felt by pressing on the abdomen, but the spleen can enlarge to several times its normal size with certain diseases. There are many possible reasons for this including various blood diseases and cancers, but the most common cause in children is infection (particularly viral infections). Infectious mononucleosis, a condition usually caused by the Epstein-Barr virus (EBV), is one of many viral infections associated with an enlarged spleen. Children and teens with an enlarged spleen should avoid contact sports because they can have a life-threatening loss of blood if their spleen is ruptured.

Disorders of other lymphoid tissue

Tonsillitis. An extremely common condition, particularly in children, tonsillitis occurs when the tonsils, the collections of lymphoid tissue in the back of the mouth at the top of the throat, are involved in a bacterial or viral infection that causes them to become swollen and inflamed. The tonsils normally help to filter out bacteria and other microorganisms to aid the body in fighting infection. Symptoms include sore throat, high fever, and difficulty swallowing. The infection may also spread to the throat and surrounding areas, causing pain and inflammation (pharyngitis).

Glossary

antibodies:

chemicals produced by white blood cells to fight bacteria, viruses, and other foreign substances

immunoblasts:

Lymphocytes that become stimulated and enlarged when they encounter foreign substances

interstitial fluid:

fluid that leaks out of capillaries (the tiniest blood vessels) and bathes body tissues

lymph vessels:

channels or ducts that contain and convey lymph; also called lymphatics

lymph:

pale fluid that bathes the body tissues, passes into lymphatic vessels, and is discharged into the blood by way of the thoracic duct; it consists of a liquid resembling blood plasma and contains white blood cells

lymph nodes:

organized masses of lymphoid tissue that are distributed along the branching system of lymphatic vessels; they contain numerous lymphocytes and other cells that filter bacteria, dead tissue, and foreign matter from the lymph that flows through them

lymphocytes:

white blood cells

macrophages:

white blood cells that remove damaged cells from the bloodstream

spleen:

organ found on the left side of the abdomen; it helps control the amount of blood and blood cells that circulate through the body and helps destroy damaged cells

thoracic duct:

major lymphatic vessel, which begins near the lower part of the spine and collects lymph from the lower limbs, pelvis, abdomen, and lower chest; lymph flowing through the duct eventually empties into a large vein in the upper chest and returns to the bloodstream.