The system of lymph vessels begins as dead-end lymph capillaries found in most tissue spaces (Fig. 14–2). Lymph capillaries are very permeable and col-lect tissue fluid and proteins. Lacteals are specialized lymph capillaries in the villi of the small intestine; they absorb the fat-soluble end products of digestion, such as fatty acids and vitamins A, D, E, and K.
Lymph capillaries unite to form larger lymph ves-sels, whose structure is very much like that of veins. There is no pump for lymph (as the heart is the pump for blood), but the lymph is kept moving within lymph vessels by the same mechanisms that promote venous return. The smooth muscle layer of the larger lymph vessels constricts, and the one-way valves (just like those of veins) prevent backflow of lymph. Lymph ves-sels in the extremities, especially the legs, are com-pressed by the skeletal muscles that surround them; this is the skeletal muscle pump. The respiratory pump alternately expands and compresses the lymph vessels in the chest cavity and keeps the lymph moving.
Where is the lymph going? Back to the blood to become plasma again. Refer to Fig. 14–3 as you read the following. The lymph vessels from the lower body unite in front of the lumbar vertebrae to form a vessel called the cisterna chyli, which continues upward in front of the backbone as the thoracic duct. Lymph vessels from the upper left quadrant of the body join the thoracic duct, which empties lymph into the left subclavian vein. Lymph vessels from the upper right quadrant of the body unite to form the right lymphatic duct, which empties lymph into the right subclavian vein. Flaps in both subclavian veins permit the entry of lymph but prevent blood from flowing into the lymph vessels.
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