4 Lymphatic System
4.1 Overview of the Lymphatic System
A Lymphatic organs and vessels
The lymphatic system, which is widely distributed throughout most of the body, consists of lymphatic organs and lymphatic vessels. It has three main functions:
• Immunological defense (lymphatic organs and vessels). The main function of the immune response is to distinguish “self” from “nonself” (or foreign) substances (such as pathogens, or transplanted tissues) and destroy the “nonself” substances.
• Transport of interstitial fluid to venous blood (lymphatic vessels)
• Removal of lipids from the small intestine while bypassing the hepatic portal system. This allows triglycerides to avoid liver metabolism and to be transported directly to organs that can utilize them.
a Lymphatic organs: All lymphatic organs have a stroma that is populated by lymphocytes that originated in bone marrow. They are directly or indirectly responsible for eliminating antigens (immune response). Antigens are molecules (proteins, carbohydrates, lipids), which the immune system recognizes as foreign and mounts a defense against. There are two types of lymphocytes, which can be further subdivided. (For more details see immunology textbooks).
• B lymphocytes (“B” stands for bone marrow, where the cells are produced) differentiate into plasma cells, which produce antibodies. Antibodies are essential components of the humoral immune response. Humoral immunity refers to antibodies dissolved in blood and interstitial fluid that bind to antigens. Thus, the plasma cells are not directly involved in the immune response.
• T lymphocytes (“T” stands for thymus, where the cells mature) attack and destroy foreign substances (e.g., virus-infected cells) on direct contact (cellular immune response).
There are both primary (red organs in a) and secondary (green organs in a) lymphatic organs:
• In the primary lymphatic organs, lymphocytes derived from stem cells mature and become immunocompetent cells (meaning they are capable of distinguishing between self and nonself substances).
• From these primary lymphatic organs, lymphocytes migrate to the secondary lymphatic organs where they continue to proliferate and mature. They are then able to fulfill their specific roles in the immune response. Lymphocytes can leave an organ and enter the bloodstream.
The structure and function of the individual lymphatic organs will be discussed in the respective organ chapters.
b Lymphatic vessels: Lymphatic vessels (green in a) are part of a tubular system that is distributed to all parts of the body (except for the CNS and renal medulla). The vessels are responsible for absorbing fluid from the interstitial spaces (it is now called lymph) and transporting it to the venous blood. Lymphatic vessels start out as tiny, thin-walled capillaries, which drain into larger pre-collecting and collecting vessels (b). These eventually coalesce into lymphatic trunks. These trunks join to form two larger ducts that end at each of the two venous angles (the junction of the internal jugular vein and subclavian vein), (see p. 30). Lymph nodes are incorporated into the system of peripheral lymphatic vessels. Lymphatic vessels converge in the lymph nodes, where the lymph is filtered and checked for pathogens as it passes through
B Overview of the lymphatic pathways
a The major blood vessels (arteries and veins) generally consist of three layers:
Lymphatic pathways play a clinically significant role in the classification of tumors and their cells that metastasize to lymph nodes. Since lymph node metastases are sometimes discovered before the primary tumor, the organ where the cancer initiated can be determined from the affected lymph nodes. Thus it is crucial to know the lymphatic pathways of organs and regions. The classification of lymph vessels and the lymph nodes associated with them is illustrated below. If one follows the pathway the lymph travels from the site of origin until it flows into the venous blood stream, the basic classification becomes apparent:
• Lymph is formed by ultrafiltration from capillaries in the connective tissue (C).
• There is a superficial and deep lymphatic network (D).
• 5 major lymphatic trunks drain lymph from all areas of the body (see p. 30).
• The lymph nodes incorporated into the lymphatic system can be classified according to their location (see p. 31)
C Lymph formation
Lymph forms as a clear fluid in the capillaries by blood ultrafiltration. Blood passes through capillaries from the arterial to the venous side of the circulatory system. The internal capillary blood pressure is greater than the colloid osmotic pressure in the capillary. As a result, 10% of the fluid from the capillaries remains as interstitial fluid in the interstitial space. This 1.8–2 liters of interstitial fluid (over 24 hours) that is not returned to the blood capillaries is absorbed by lymph capillaries (see Ab), and then collected into larger lymphatic vessels and trunks before it drains into venous blood. Eventually all the lymph in the body drains into two lymphatic trunks (thoracic duct and right lymphatic duct, which drain into the left and right venous angles, respectively, at the junction of the neck and thorax [see page 28, Fig. Aa]). The lymphatic vessels direct lymph through lymph nodes, and the nodes check the lymph for germs and toxins. In cases of purulent inflammation caused by bacteria, reddened superficial lymphatic pathways are visible, which in layman terms is referred to as “blood poisoning.”
Note: After a fat-rich meal, lymph from the small intestine is rich in emulsified lipoprotein particles (chylomicrons) and thus has a milky appearance. Lymph flowing from the small intestine is called chyle and the lymph vessels of the small intestine are sometimes referred to as chyle vessels.
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