Lymphatic System; lymphatic flow, lymphatic flow in leg

LYMPHATIC SYSTEM

Compared with the vascular systems in the leg, the lymphatic system is the least understood

and its embryologic development remains relatively unknown. Lymphatic vessels are divided into three categories:

• Initial or terminal lymphatic capillaries

• Collecting vessels

• Lymph nodes.

Terminal lymphatic capillaries originate in the superficial layer of the dermis and have no valves. These lymphatic capillaries drain into the deep dermal and subdermal system,  which is the level at which valved lymphatic vessels can be observed (pre-collectors), and vessels ascend the leg into lymph nodes at the popliteal fossa and the inguinal ligament. Generally, the lymphatic system parallels the larger veins of the proximal leg above the knee, with valves operating in much the same way as venous valves. The lymphatic system then drains though the iliac lymph nodes above the inguinal ligament level, eventually coalescing into the periaortic nodes, the cisterna chyli, and thoracic duct, which ascends along the thoracic aorta on the right side of the chest and empties into the left jugular vein slightly above the jugulo-subclavian junction. While the thoracic duct is considered the main terminus of the lymphatic system, some patients have an accessory right lymphatic duct that drains into the right jugular venous system.

Lymphatic System

 

Lymphatic vessels are much smaller than major arteries or veins—between one-seventh and one-tenth the size—because fluid flow is less. In terms of vessel anatomy, the outer adventitial layer is much thinner although the media contains some elastin fibers and smooth muscle striae, the latter being used to propel the lymphatic flow cephalad through contraction. The intimal layer consists of a single layer of endothelium.

Aspects of lymphatic flow

Lymphatic flow is a consequence of three factors:

capillary blood pressure, osmotic pressure, and interstitial fluid pressure (hydrostatic). The intrinsic contractility of the lymphatic vessel wall, coupled with the action of muscular pumps such as the calf, which aids flow in the same way as for the venous system, creates a suction force distal to the major lymph vessels. In addition, the action of deep breathing, which creates a positive abdominal pressure and a negative thoracic pressure, also increases the cephalic lymph flow. Because capillary cell walls are “leaky,” acellular interstitial fluid containing protein and white blood cells accumulates, and thus the lymphatic system provides a means for drainage of this fluid as well as a mechanism for the return of white blood cells to the vasculature. A normal lymphatic system with intact functional architecture is required for unimpaired lymph circulation. Lymphedema results when disruption or injury occurs to the lymphatic system at a local level because interstitial fluid is no longer being drained adequately. The lymphatic system can be considered a one-way transportation system that prevents the body from drowning in its own fluid. However, besides maintaining interstitial tissue fluid balance (volume and pressure), the lymphatic system also performs other key functions. Composed of a tree-like hierarchical network of vessels and organs, including the spleen, thymus, tonsils, bone marrow, and numerous lymph nodes, the lymphatic system biologically filters lymph at the nodes using macrophages and lymphocytes. The lymphatic organs and nodes also provide a means for lymphocyte maturation and transportation that is crucial to immune function; lymphocytes include natural killer cells involved in the innate immune system, whereas T-cells B-cells are associated with the adaptive immune response. In addition, the lymphatic system plays a role in certain kinds of fat absorption. Thus, when portions of the lymphatic system are injured, the local response to inflammation or infection likewise becomes disrupted by disturbing cytokine (growth factor) and cellular circulation in the affected area. In other words, the lymphedematous extremities develop fatty deposits in response to chronic edema and the swollen limb really does become “fatter.”

 

Lymph flow in the leg

Plasma that has escaped from the capillary vasculature mixes with other interstitial materials (forming pre-lymph) and enters the lymphatic capillaries and pre-collectors in a passive process aided by rhythmic contractions of the lymphangions upstream, nearby muscular contractions and arterial pulsation, suction pressure due to breathing, and manual lymph drainage. The lymph then flows into serially larger collectors of which the more proximal are known as trunks. Distention of the trunk wall is the stimulus for lymphangion contraction, which provides the primary propulsion necessary for lymph flow and which occurs at a rate of 6 to 10 beats per minute.

In essence, the lymphangions function like miniature hearts in linear sequence that are also capable of cardiac-like inotropic and chronotropic responses. Under normal conditions, lymph fl ow can increase by an order of magnitude when an increased filtrate volume is present with higher pre-lymph uptake and a faster rate of lymphangion contraction, ensuring a large margin of capacity. In other words, healthy lymphatics, like the heart, can work harder when more fluid is present.

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