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Lymph capillaries, the smallest lymphatic vessels, begin as dead‐end vessels. They resemble blood capillaries, but are much more porous to surrounding fluids due to the following two features:
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Valvelike openings form at the juncture of adjacent endothelial cells. Unlike the tightly joined endothelial cells that make up the walls of blood capillaries, those of lymph capillaries loosely overlap. When fluid pressure increases in surrounding regions, the overlapped cells separate, allowing fluids to enter the lymph capillary. When pressure inside the capillary exceeds the pressure outside, the spaces between the endothelial cells close, holding fluids inside the capillary.
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Anchoring filaments attach the endothelial cells of the lymphatic vessels to surrounding collagen. When interstitial fluid pressure increases, the anchoring filaments prevent the endothelial cells from collapsing, keeping the spaces between the endothelial cells open.
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Lacteals are specialized lymph capillaries that occur in the fingerlike projections (villi) that extend into the small intestine. Lacteals absorb lipids from the intestinal tract. The lymph within these capillaries, called chyle, has a creamy white color (rather than clear) due to the presence of fats.
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Lymphatic‐collecting vessels form as lymph capillaries merge. Collecting vessels have the following characteristics:
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Valves are present to prevent the backward flow of lymph (as in veins).
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The walls of collecting vessels consist of the same three tunics (layers) that characterize veins, but the layers are thinner and poorly defined.
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Lymphatic trunks form from the union of collecting vessels. The nine major trunks, draining lymph from regions for which they are named, are the lumbar, jugular, subclavian, and bronchomediastinal trunks, each of which occurs in pairs (left and right, for each side of the body), and a single intestinal trunk.
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Lymphatic ducts are the largest lymphatic vessels. These two ducts drain lymph into veins in the neck (the right and left subclavian veins at their junctures with the internal jugular veins). Valves in the lymphatic ducts at their junctures with the veins prevent the entrance of blood into the lymphatic vessels.
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The thoracic duct collects lymph from the left side of the body and regions of the right side of the body below the thorax. It ultimately drains lymph into the left subclavian vein. It begins at the cisterna chili, an enlarged region of the lymphatic vessel that forms following the union of the intestinal trunk and right and left lumbar trunks.
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The right thoracic duct collects lymph from the upper right side of the body (right arm and right regions of thorax, neck, and head), a much smaller area than that serviced by the thoracic duct. It ultimately drains lymph into the right subclavian vein.
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Figure 1 illustrates the location of the thoracic duct and the left thoracic duct.
Figure 1. Lymphatic trunks and ducts.