A The human lymphatic system
This system includes the lymphatic vessels and the lymphatic organs (immune organs, see B). The lymphatic vascular system runs parallel to the venous system and performs several functions:
• Its primary function is to clear the interstitial spaces of tissue fluid and substances that cannot be reabsorbed in the venous capillary bed. The composition of the lymph varies in different regions and is similar to that of the surrounding interstitial fluid.
• It carries away food lipids (chylomicrons) that are absorbed in the bowel.
• It returns lymphocytes from the lymphatic organs to the blood.
The lymphatic vascular system consists of
• lymphatic capillaries, which begin peripherally as blind-ended vessels,
• the lymphatic vessels and interposed lymph nodes, and
• the major lymphatic trunks (thoracic duct and right lymphatic duct).
The lymphatic capillaries collect fluid from the interstitium and transport it via the lymphatic vessels and lymph nodes to the major lymphatic trunks. The fluid reenters the venous system from these trunks at the junctions of the left and right subclavian and internal jugular veins. The lymph drained from three body quadrants enters the left jugulo-subclavian venous junction, while only lymph from the right upper quadrant enters the right jugulo-subclavian venous junction. The lymphatic organs are part of the specific immune system and, as such, are situated at likely portals of entry for infectious microorganisms. The spleen is the only immune organ that is directly integrated into the bloodstream.
B Primary and secondary lymphatic organs
The functions of the lymphatic organs include mounting a specific immune response. A distinction is drawn between primary and secondary lymphatic organs. The primary lymphatic organs are concerned with the production, maturation, and selection of immune cells. The secondary lymphatic organs are subsequently populated by the immunocompetent lymphocytes and are sites for various processes, such as antigen presentation, lymphocyte proliferation, and antibody formation.
• Primary lymphatic organs:
– Thymus (selection of T-lymphocytes)
– Bone marrow (selection of B-lymphocytes)
• Secondary lymphatic organs:
– Lymph nodes
– Mucosa-associated lymphatic tissue (MALT) and the pharyngeal lymphatic (Waldeyer’s) ring—the pharyngeal, palatine, and lingual tonsils.
– Bronchus-associated lymphatic tissue (BALT)
C Organization of the lymphatic vascular system (after Kubik)
Three compartments can be distinguished in the lymphatic vascular system based on topographical and functional criteria:
1. A superficial system → drains the skin and subcutaneous tissue.
2. A deep system → drains lymph from the muscles, joints, tendon sheaths, and nerves.
3. An organ-specific system → drains the organs and shows organ-specific differences.
A system of perforator vessels interconnects the superficial and deep systems, conveying lymphatic fluid toward the surface from deeper tissues.
The lymphatic vascular system can be subdivided into four different regions based on the histologic structure of the vessel walls:
1. Lymphatic capillaries
4. Lymphatic trunks
Lymphatic capillaries and precollectors are also known as initial lymphatics.
D Organization and structure of the different lymphatic regions (after Kubik)
a Lymphatics in the skin and muscles.
b Detail from a, showing the structure and function of a collector segment.
Both the superficial and deep lymphatics originate with the extremely thin-walled lymphatic capillaries, which are approximately 50 µm in diameter. Their endothelium is bounded by an incomplete basal lamina, and they are attached by collagenous “anchoring filaments” to elastic fibers and collagen fibers in their surroundings. The network of lymphatic capillaries opens into larger precollectors approximately 100 μm in diameter. Unlike the lymphatic capillaries, these vessels contain valve cusps, and their walls are reinforced by a layer of connective tissue. They open into collectors, which also contain valves and have a transverse diameter of 150 to 600 μm. Like the larger lymphatic vessels and the lymphatic trunks, the collectors have a venous-type wall structure divided indistinctly into an intima (endothelium and basement membrane), a smooth-muscle media, and a fibrous adventitia. Lymph transport is effected by a series of rhythmic contractile waves (10–12/min) that are generated in the smooth-muscle, valveless collector segments. The direction of lymph flow is controlled by closing the distal valves and opening the proximal valves of the precollectors and collectors.