Corpus: Lymphatic vessels

Lymphatic vessels are formed during embryonic development under the influence of various growth factors. The VEGF receptor 3 and its ligands, VEGF-C and VEGF-D, play a crucial role in this process. The formation of new lymphatic vessels is referred to as lymphangiogenesis.

Structurally, lymphatic vessels resemble veins. They also exhibit a three-layered structure consisting of:

• tunica externa (longitudinally arranged elastic fibers and smooth muscle cells, connective tissue)
• tunica media (circularly arranged elastic fibers and smooth muscle cells)
• tunica intima (endothelial cells).

In smaller lymphatic vessels, elastic fibers and smooth muscle are absent, resulting in a two-layered structure of endothelium and surrounding connective tissue. In larger lymphatic vessels, one also encounters vascular valves that prevent the backflow of lymph, referred to as valvulae lymphaticae. The section of a lymphatic vessel between two valves forms a so-called lymphangion.

Classification

The lymphatic vessel system is hierarchically organised like the blood vessel system. The vessel caliber and wall thickness of the lymphatic vessels increase in the direction of lymph flow. The largest lymphatic vessel in the human body is the thoracic duct. From peripheral to central, one distinguishes:

• lymph capillaries ("initial lymphatic vessels")
• precollectors
• collectors
• lymphatic trunks (trunci lymphatici)
• lymphatic ducts (ductus lymphatici)

Lymphatic ducts are sometimes also included in the lymphatic collecting trunks in literature. Lymph nodes are integrated as passage stations in the lymphatic vessel system, playing a crucial role in the immune response.

Lymphatic Trunks

The collectors unite to form mostly paired lymphatic trunks:

• The right and left lumbar trunks drain lymph from the leg and pelvic area.
• The right and left intestinal trunks collect lymph from the intestine and unpaired abdominal organs.
• The right and left bronchomediastinal trunks direct lymph from the right or left thoracic cavity.
• The right and left subclavian trunks drain lymph from the upper extremities.
• The right and left jugular trunks direct lymph from the head and neck region.

Lymphatic Ducts

The above lymphatic trunks join to form the two main lymphatic ducts. The thoracic duct receives the following lymphatic trunks:

• left jugular trunk
• left subclavian trunk
• left bronchomediastinal trunk
• right and left intestinal trunks
• right and left lumbar trunks

The right lymphatic duct receives the remaining lymphatic trunks:

• right jugular trunk
• right bronchomediastinal trunk
• right subclavian trunk

Simplifying, the thoracic duct drains all lymph from the lower extremities and the upper left side of the body, while the right lymphatic duct drains lymph from the upper right side of the body. The main lymphatic trunks empty into the left or right venous angle, thereby introducing lymph and lymphocytes into the bloodstream.

To enable lymph flow against hydrostatic pressure, the larger lymphatic vessels are capable of active contraction. The lymphangions possess a mesh-like, circularly arranged musculature, whose contraction waves propel the lymph forward. This active transport is ensured by 6-12 contraction waves per minute. The flow rate is approximately 10 µm per second. The activity of the lymphangions can dynamically adjust to the volume of fluid to be handled.

The normal transport capacity of the lymphatic system is relatively low compared to the venous system, at two to four litres of fluid per day, disposing of approximately 10% of the fluid filtered into the tissue by the lymphatic system.

Lymph is coagulable, similar to blood plasma in its content of clotting factors and fibrinogen.

Lymphatic vessels perform several important functions. Their primary function is the return of fluid and proteins from the interstitial space to the bloodstream. Without this return, increased colloid osmotic pressure in the interstitium would cause edema and intravascular fluid loss. Accordingly, drainage disorders of lymphatic vessels in their tributary area lead to lymphedema. Other functions include:

• Transport of antigens and lymphocytes to the lymph nodes
• Transport of chylomicrons from the gastrointestinal tract; this fat-rich lymph is also referred to as chyle.

Diseases

Diseases affecting the lymphatic vessels or caused by changes in the lymphatic vessels include:

• lymphangitis
• lymphangiectasis
• lymphedema
• lymphangiomatosis
• lymphangiosis carcinomatosa
• lymphangioma
• lymphocele
• lymphatic filariasis

Diagnostics

Techniques for imaging lymphatic vessels include lymphography and, more commonly today, lymphoscintigraphy. In these procedures, a contrast agent or a radioactive tracer is injected into the lymphatic system or the interstitium.