Lymph and the Lymphatic Vessels

Lymph is usually a clear, colorless fluid, similar to blood plasma but low in protein. Its composition varies substantially from place to place. After a meal, for example, lymph draining from the small intestine has a milky appearance because of its high lipid content. Lymph leaving the lymph nodes contains a large number of lymphocytes—indeed, this is the main supply of lymphocytes to the bloodstream.

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Obstruction Lymphatic Vessels

Figure 21.2 Elephantiasis, a Tropical Disease Caused by Lymphatic Obstruction. Mosquito-borne roundworms infect the lymph nodes and block the flow of lymph and recovery of tissue fluid. The resulting chronic edema leads to fibrosis and elephant-like thickening of the skin. The extremities are typically affected as shown here; the scrotum of men and breasts of women are often similarly affected.

Figure 21.2 Elephantiasis, a Tropical Disease Caused by Lymphatic Obstruction. Mosquito-borne roundworms infect the lymph nodes and block the flow of lymph and recovery of tissue fluid. The resulting chronic edema leads to fibrosis and elephant-like thickening of the skin. The extremities are typically affected as shown here; the scrotum of men and breasts of women are often similarly affected.

Lymph may also contain bacteria, viruses, cellular debris, or even traveling cancer cells.

Origin of Lymph

Lymph originates in microscopic vessels called lymphatic capillaries. These vessels penetrate nearly every tissue of the body but are absent from the central nervous system, cartilage, bone, and bone marrow. They are closely associated with blood capillaries, but unlike them, they are closed at one end (fig. 21.3). A lymphatic capillary consists of a sac of thin endothelial cells that loosely overlap each other like the shingles of a roof. The cells are tethered to surrounding tissue by protein filaments that prevent the sac from collapsing. Unlike the endothelial cells of blood capillaries, lymphatic endothelial cells are not joined by tight junctions. The gaps between them are so large that bacteria and other cells can enter along with the fluid. The overlapping edges of the endothelial cells act as valvelike flaps that can open and close. When tissue fluid pressure is high, it pushes the flaps inward (open) and fluid flows into the lymphatic capillary. When pressure is higher in the lymphatic capillary than in the tissue fluid, the flaps are pressed outward (closed).

Chapter 21 The Lymphatic and Immune Systems 801

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Lymphatic Capillaries Valves

capillary

Figure 21.3 Lymphatic Capillaries. (a) Relationship of the lymphatic capillaries to a bed of blood capillaries. (b) Uptake of tissue fluid by a lymphatic capillary.

capillary

Figure 21.3 Lymphatic Capillaries. (a) Relationship of the lymphatic capillaries to a bed of blood capillaries. (b) Uptake of tissue fluid by a lymphatic capillary.

_Think About It_

Contrast the structure of a lymphatic capillary with that of a continuous blood capillary. Explain why their structural difference is related to their functional difference.

Lymphatic Vessels

Lymphatic vessels form in the embryo by budding from the veins, so it is not surprising that the larger ones have a similar histology. They have a tunica interna with an endothelium and valves (fig. 21.4), a tunica media with elastic fibers and smooth muscle, and a thin outer tunica externa. Their walls are thinner and their valves are more numerous than those of the veins.

Lymph takes the following route from the tissues back to the bloodstream: lymphatic capillaries ^ collecting

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Lymphatic Vessel Histology

Figure 21.4 Valve in a Lymphatic Vessel.

What would be the consequence if these valves did not exist?

Figure 21.4 Valve in a Lymphatic Vessel.

What would be the consequence if these valves did not exist?

vessels ^ six lymphatic trunks ^ two collecting ducts ^ subclavian veins. Thus, there is a continual recycling of fluid from blood to tissue fluid to lymph and back to the blood (fig. 21.5).

The lymphatic capillaries converge to form collecting vessels. These often travel alongside veins and arteries and share a common connective tissue sheath with them. Numerous lymph nodes occur along the course of the collecting vessels, receiving and filtering the lymph. The collecting vessels converge to form larger lymphatic trunks, each of which drains a major portion of the body. The principal lymphatic trunks are the lumbar, intestinal, intercostal, bronchomediastinal, subclavian, and jugular trunks. Their names indicate their locations and parts of the body they drain; the lumbar trunk also drains the lower extremities.

The lymphatic trunks converge to form two collecting ducts, the largest of the lymphatic vessels: (1) The right lymphatic duct begins in the right thoracic cavity with the union of the right jugular, subclavian, and bron-chomediastinal trunks. It receives lymphatic drainage from the right arm and right side of the thorax and head and empties into the right subclavian vein (fig. 21.6a).

Heart Lymph System Capillaries
Figure 21.5 Fluid Exchange Between the Circulatory and Lymphatic Systems. Blood capillaries lose fluid to the tissue spaces. The lymphatic system picks up excess tissue fluid and returns it to the bloodstream.

(2) The thoracic duct, on the left, is larger and longer. It begins as a prominent sac in the abdominal cavity called the cisterna chyli (sis-TUR-nuh KY-lye) and then passes through the diaphragm and up the mediastinum. It receives lymph from all parts of the body below the diaphragm and from the left arm and left side of the head, neck, and thorax (fig. 21.6b). It empties into the left subclavian vein.

Flow of Lymph

Lymph flows under forces similar to those that govern venous return, except that the lymphatic system has no pump like the heart. Lymph flows at even lower pressure and speed than venous blood; it is moved primarily by rhythmic contractions of the lymphatic vessels themselves, which contract when stretched by lymph. The lymphatic vessels, like the veins, are also aided by a skeletal

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Blood Vessel Veins Mcgraw HillLymphatic Drainage Abdominal Wall

Figure 21.6 Lymphatic Drainage of the Thoracic Region. (a) Drainage of the right mammary and axillary regions. (b) Drainage of the right lymphatic duct and thoracic duct into the subclavian veins.

Figure 21.6 Lymphatic Drainage of the Thoracic Region. (a) Drainage of the right mammary and axillary regions. (b) Drainage of the right lymphatic duct and thoracic duct into the subclavian veins.

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804 Part Four Regulation and Maintenance muscle pump that squeezes them and moves the lymph along. Also like the medium veins, lymphatic vessels have valves that prevent lymph from flowing backward. Since lymphatic vessels are often wrapped with an artery in a common sheath, arterial pulsation may also rhythmically squeeze the lymphatic vessels and contribute to lymph flow. A thoracic (respiratory) pump aids the flow of lymph from the abdominal to the thoracic cavity as one inhales, just as it does in venous return. Finally, at the point where the collecting ducts join the subclavian veins, the rapidly flowing bloodstream draws the lymph into it. Considering these mechanisms of lymph flow, it should be apparent that physical exercise significantly increases the rate of lymphatic return.

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Responses

  • folco goldworthy
    How does the lymphatic system return tissue fluid to the blood stream?
    4 years ago
  • sophie
    Where does lymph return to the bloodstream?
    4 years ago
  • NIKOLA
    What is thoracic duct?
    4 years ago
  • dana
    Do lymph capillary converge with blood capillary?
    3 years ago
  • Evelyn
    Which vein empties lymph into the blood?
    2 years ago

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