Lymphatic vessels—often simply called lymphatics—originate as microscopic blind-end vessels called lymphatic capillaries. (Those originating in the villi of the small intestine are called lacteals; see Chapter 29.) The wall of each lymphatic capillary consists of a single layer of flattened endothelial cells. Each blindly ending capillary is attached, or fixed, to surrounding cells by tiny connective tissue filaments. Networks of lymphatic capillaries branch and anastomose extensively. Lymphatic networks are located in the intercellular (interstitial) spaces and are widely distributed throughout the body. As a rule, lymphatic and blood capillary networks lie side by side but are always independent of each other.

As twigs of a tree join to form branches, branches join to form larger branches, and larger branches join to form the tree trunk, so do lymphatic capillaries merge, forming slightly larger lymphatics that join other lymphatics to form still larger vessels, which merge to form the main lymphatic trunks: the right lymphatic duct and the thoracic duct (see Figure 23-2).

Lymph from the entire body, except for the upper right quadrant, drains eventually into the thoracic duct, which drains into the left subclavian vein at the point where it joins the left internal jugular vein (Figure 23-3). Lymph from the upper right quadrant of the body empties into the right lymphatic duct and then into the right subclavian vein. However, this structure can be vary among individuals. For example, in most people there are three separate lymphatic ducts that drain into the right subclavian vein rather than a single right lymphatic duct.

Because most of the lymph of the body returns to the bloodstream by way of the thoracic duct, this vessel is considerably larger than the other main lymph channels, the right lymphatic ducts, but is much smaller than the large veins, which it resembles in structure. It has an average diameter of about 5 mm (1.5 inches) and a length of about 40 cm (16 inches). It originates as a dilated structure, the cisterna chyli or chyle cistern, in the lumbar region of the abdominal cavity—where fatty lymph called chyle from the intestinal tract collects. The thoracic duct then ascends a curving pathway to the root of the neck, where it joins the subclavian vein as just described (see Figures 23-2 and 23-3).

Lymphatics resemble veins in structure with these exceptions:

The lymphatic capillary wall is formed by a single layer of large but very thin and flat endothelial cells (Figure 23-4). Although the openings (clefts) between endothelial cells of lymphatic capillary walls are small, they are larger than those found in blood capillaries—a fact that explains the remarkable permeability of this system.

As lymph flows from the thin-walled capillaries into vessels with a larger diameter (0.2 to 0.3 mm), the walls become thicker and exhibit the three coats, or layers, typical of arteries and veins (see Table 21-1, p. 646). Interlacing elastic fibers and several strata of circular smooth muscle bundles are found in both the tunica media and the tunica externa of the large lymphatic vessel wall. Boundaries between layers are less distinct in the thinner lymphatic vessel walls than in arteries or veins.

One-way valves are extremely numerous in lymphatics of all sizes and give the vessels a somewhat varicose and beaded appearance. Valves are present every few millimeters in large lymphatics and are even more numerous in the smaller vessels. Formed from folds of the tunica intima, each valve projects into the vessel lumen in a slightly expanded area circled by bundles of smooth muscle fibers.

Experimental evidence suggests that most lymph vessels have the capacity for repair or regeneration when damaged. Formation of new lymphatic vessels occurs by extension of solid cellular cores, or sprouts, formed by mitotic division of endothelial cells in existing vessels, which later become “canalized.”

The lymphatics play a critical role in numerous interrelated homeostatic mechanisms. The high degree of permeability of the lymphatic capillary wall permits very large molecules and even particulate matter, which cannot be absorbed into a blood capillary, to be removed from the interstitial spaces. Proteins that accumulate in the tissue spaces can return to blood only by way of lymphatics. This fact has great clinical importance. For instance, if anything blocks lymphatic return, blood protein concentration and blood osmotic pressure soon fall below normal, and fluid imbalance and death will result (discussed in Chapter 32).

Lacteals (lymphatics in the villi of the small intestine) serve an important function in the absorption of fats and other nutrients. The milky lymph found in lacteals after digestion contains 1% to 2% fat and is called chyle. Interstitial fluid has a much lower lipid content than chyle (see Chapter 29).