Pathophysiology
Lymph is formed from the fluid that filters out of the blood circulation to nourish cells. This fluid returns through venous capillaries to the blood circulation through the force of osmosis in the venous blood; however, a portion of the fluid which contains proteins, cellular debris, bacteria, etc. must return through the lymphatic collection system to maintain tissue fluid balance. The collection of this prelymph fluid is carried out by the initial lymph collectors which are blind-ended epithelial-lined vessels with fenestrated openings that allow fluids and particles as large as cells to enter. Once inside the lumen of the lymphatic vessels, the fluid is guided along increasingly larger vessels, first with rudimentary valves to prevent backflow, which later develop into complete valves similar to the venous valve. Once the lymph enters the fully valved lymphatic vessels, it is pumped by a rhythmic peristaltic-like action by smooth muscle cells within the lymphatic vessel walls. This peristaltic action is the primary driving force, moving lymph within its vessel walls. The regulation of the frequency and power of contraction is regulated by the sympathetic nervous system. The movement of lymph can also be influenced by the pressure of nearby muscle contraction, arterial pulse pressure, and the vacuum created in the chest cavity during respiration, but these passive forces contribute only a minor percentage of lymph transport. The fluids collected are pumped into continually larger vessels and through lymph nodes, which clean out debris and police the fluid for potential threats from dangerous microbes. The lymph ends its journey in the thoracic duct, where it re-enters the blood circulation.
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