Lymphatic System

The lymphatic system is a network of vessels and channels that branch throughout the body and bathe cells in a fluid called lymph while also filtering out foreign material from the blood. As the body's second circulatory system, the lymphatic system is the body's transport mechanism for necessary fluid (lymph) and also makes up part of the body's defense system by carrying lymphocytes (specialized white blood cells) that fight infection.

As blood circulates throughout the human body, a tiny fraction of its plasma (the fluid part of the blood) leaks out and collects around body cells and tissues. The lymphatic system exists partly to prevent this fluid, called lymph, from building up in the tissues of the body. It does this by a system of tubes with thin walls that absorb the excess fluid and move it slowly through the body. The lymphatic system does not have a pump like the heart to move its fluid, but instead uses the normal movements of the body to slowly push the fluid in a one-way direction. Lymph vessels do more than just transport excess fluid however, for they also pick up fat from the digestive tract and transport it to the blood where it will be used for energy. It also plays an important role in the movement of white blood cells in the body's immune response.

The complete lymphatic system includes lymph nodes and lymph ducts as well as the thymus and the spleen. The system branches to nearly all parts of the body, excluding the brain or spinal cord, draining and filtering lymph and returning it back into the bloodstream. Lymph is a colorless fluid that, although it is derived from blood, contains no red blood cells. As it seeps out of the capillaries (hairlike blood vessels) and circulates through lymph vessels, lymph is filtered as it passes through lymph nodes that are clustered throughout the body. These lymph nodes are round structures that are about the size of a pea. They act as filters, catching bacteria, toxins, dead cells and other particles, and destroying them. The main groups of lymph nodes are located in the neck, armpit, chest, abdomen, and groin. These nodes often become swollen and painful as they produce and supply extra white blood cells to fight an infection. When this happens, we experience what are called "swollen glands."

The thymus, located behind the breastbone and between the lungs, is also part of the lymphatic system since it secretes a hormone that tells the body's bone marrow (tissue that fills the bone cavities) to produce specialized white blood cells called lymphocytes. It is these cells that attack and destroy invading microorganisms as well as create antibodies (a specific protein targeted to kill a specific invader). The lymphatic system is the main highway for the patrolling lymphocytes that are always on the lookout for foreign bodies. Another part of the lymphatic system, the spleen, filters lymph and removes waste and other materials (like old red blood cells). In humans, the spleen is located below and behind the left side of the stomach. Altogether, the lymphatic system can be thought of as the body's fluid drainage network that filters and destroys foreign particles through the transportation of infection-fighting cells.

[See alsoAntibody and Antigen; Immune System ]