Lymphatic system

The lymphatic system and the immune system are terms that are used interchangeably to refer to the body's ability to defend against pathogens. The lymphatic system is comprised of three interrelated functions: (1) Removal of excess fluids, lymph, from body tissues, (2) Absorption of fatty acids and subsequent transport of fat, chyle, to the circulatory system and (3) Formation of white blood cells (WBCs), and initiation of immunity through the formation of antibodies, lending specific resistance to pathogens.

Lymph originates as blood plasma that leaks from the capillaries of the circulatory system, becoming interstitial fluid, filling the space between individual cells of tissue. Plasma is forced out of the capillaries by hydrostatic pressure, and as it mixes with the interstitial fluid, the volume of fluid accumulates slowly. Most of the fluid is returned to the capillaries by osmosis. The proportion of interstitial fluid that is returned to the circulatory system by osmosis is about 90% of the former plasma, with about 10% accumulating as overfill. The excess interstitial fluid is collected by the lymphatic system by diffusion into lymph capillaries, and is processed by lymph nodes prior to being returned to the circulatory system. Once within the lymphatic system the fluid is called lymph, and has almost the same composition as the original interstitial fluid.

Lymphatic circulation
The lymphatic system acts as a secondary circulatory system, except it collaborates with white blood cells in lymph nodes to protect the body from being infected by cancer cells, fungi, viruses or bacteria. Unlike the circulatory system, the lymphatic system is not closed and has no central pump; the lymph moves slowly and under low pressure due to peristalsis, the operation of semilunar valves in the lymph veins, and the milking action of skeletal muscles. Like veins, lymph vessels have one-way, semilunar valves and depend mainly on the movement of skeletal muscles to squeeze fluid through them. Rhythmic contraction of the vessel walls may also help draw fluid into the lymphatic capillaries. This fluid is then transported to progressively larger lymphatic vessels culminating in the right lymphatic duct (for lymph from the right upper body) and the thoracic duct (for the rest of the body); these ducts drain into the circulatory system at the right and left subclavian veins.

Function of the Fatty Acid Transport System
Lymph vessels, called lacteals, are present in the lining of the gastrointestinal tract. While most other nutrients absorbed by the small intestine are passed on to the portal venous system to drain, via the portal vein, into the liver for processing, fats are passed on to the lymphatic system, to be transported to the blood circulation via the thoracic duct. The enriched lymph originating in the lymphatics of the small intestine is called chyle (not chyme). The nutrients that are released to the circulatory system are processed by the liver, having passed through the systemic circulation. The lymph system is a one-way system, transporting interstitial fluid back to blood.

Lymphoid organs
The thymus, spleen, lymph nodes, peyer's patches, tonsils, vermiform appendix, and red bone marrow are accessory lymphoid tissues that comprise the lymphoid organs. These organs contain a scaffolding that support circulating B- and T-lymphocytes and other immune cells like macrophages and dendritic cells. Another sub-component of the lymphatic system is the reticuloendothelial system. When micro-organisms invade the body or the body encounters other antigens (such as pollen), the antigens are transported from the tissue to the lymph. The lymph is carried in the lymph vessels to regional lymph nodes. In the lymph nodes, the macrophages and dendritic cells phagocytose the antigens, process them, and present the antigens to lymphocytes, which can then start producing antibodies or serve as memory cells to recognize the antigens again in the future.