lymphatic system

The lymphatic system is a complex network of lymphoid organs, lymph nodes, lymph ducts, lymph tissues, lymph capillaries and lymph vessels that produce and transport lymph fluid from tissues to the circulatory system. The lymphatic system is a major component of the immune system.

The lymphatic system has three interrelated functions: (1) removal of excess fluids from body tissues, (2) absorption of fatty acids and subsequent transport of fat,chyle, to the circulatory system and, (3) production of immune cells (such as lymphocytes, monocytes, and antibody producing cells called plasma cells).

Lymph originates as blood plasma that leaks from the capillaries of the circulatory system, becoming interstitial fluid, and filling the space between individual cells of tissue. Plasma is forced out of the capillaries (called filtration) and forced back in (called absorption) due to interactions of hydrostatic pressure (favoring movement out of the capillaries) and oncotic pressure (favoring movement into the capillaries). While out of the capillaries, the fluid mixes with the interstitial fluid, the volume of fluid accumulates slowly. Most of the fluid is returned to the capillaries. 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 that 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. The lymph nodes filter the lymph fluid and remove foreign material, such as bacteria and cancer cells. Specialized cells called macrophages and dendritic cells phagocytose pathogens, process them, and present antigens to lymphocytes. When these pathogens are recognized, the lymph nodes enlarge as additional immune cells are produced to help fight the infection.

Pathology

In elephantiasis, obstructions of the lymphatic vessels cause a thickening of the skin and enlargement of underlying tissues, especially in the legs and genitals. It is most commonly caused by a parasitic disease known as lymphatic filariasis.

Lymphedema also causes abnormal swelling, especially in the appendages (though the face, neck, and abdomen can also be affected). It occurs if the lymphatic system is damaged, or underdeveloped in some way. An estimated 170 million suffer with the disorder. There are three stages:
Stage 1:Pressing the swollen limb leaves a pit that takes a while to fill back in. Because there is little fibrosis (hardening) it is often reversible. Elevation reduces swelling.
Stage 2: Pressure does not leave a pit. Elevation does not help. If left untreated, the limb becomes fibrotic.
Stage 3: This stage of lymphedema is often called elephantiasis. It is generally only in the legs after lymphedema that has gone long untreated. While treatment can help a little, it is not reversible.

Embryology

The lymphatic system begins to develop by the end of the fifth week of gestation. Six primary lymph sacs are formed: two jugular, two iliac, one retroperitoneal and one cisterna chyli. Numerous channels connect the sacs with each other. Eventually, the thoracic duct is formed and lymphatic vessels replace the lymph sacs.