Lymph is fluid that drains from the extracellular space of tissues. This fluid normally forms as capillary fluid passes out of the vessels, and is composed of water, electrolytes, and lesser amounts of plasma proteins. Lymph vessels return this fluid to the circulating blood. It is a one “circuit system” until the two interconnected circulatory system comprised by the arteries and veins.
Lymph flows at extremely low pressure and filters through lymph nodes which act as the sentinels for immune stimulation. Because of the low pressure, the walls of lymphatics are exceedingly thin and are readily collapsible (often not apparent in routine histologic sections). The basement membrane is largely absent and there are no pericytes. As a result, lymphatic endothelium is considerably more permeable than continuous blood capillary endothelium. Anchoring filaments attach the endothelial cells to surrounding connective tissue. Valves in lymphatic vessels prevent retrograde flow. The major conduit for lymph in the body is the thoracic duct, which drains into the left brachiocephalic vein (canine; species variations exist). Elevations in central venous pressure have potential to disrupt lymphatic flow, resulting in chylothorax. Tissues that notably lack lymphatics include the central nervous system, cartilage, bone, bone marrow, placenta, cornea, and teeth.
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