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PART 4 Regulation and Maintenance

22.1 Functions of the Lymphatic

System

LEARNING OUTCOME

D. Distinguish between lymphatic tissue and a lymphaticorgan.

E. Describe the structure and function of tonsils, lymphnodes, the spleen, and the thymus.

After reading this section, you should be able to

A. Describe the functions of the lymphatic system.

The lymphatic (lim-fat′ik) system includes lymph, lymphatic ves-sels, lymphatic tissue, lymphatic nodules, lymph nodes, the ton-sils, the spleen, and the thymus (figure 22.1).

The lymphatic system has three main functions:

1. Fluid balance . Approximately 30 L of fluid pass from theblood capillaries into the interstitial fluid each day, whereasonly 27 L pass from the interstitial fluid back into the capil-laries. The difference is 3 L of fluid in the interstitial fluid. Ifthe extra 3 L of fluid were to remain in the interstitial fluid,edema would result, causing tissue damage and eventual death.Instead, lymphatic capillaries collect the 3 L of fluid, where itis called lymph (limf). Lymph passes through the lymphaticvessels back to the blood. In addition to water, lymph containssolutes derived from (a) substances in plasma, such as ions,nutrients, gases, and some proteins, which pass from blood cap-illaries into the interstitial fluid, and (b) substances derivedfrom cells, such as hormones, enzymes, and waste products.2. Lipid absorption . Lacteals (lak′t ē - ă lz) are lymphatic vesselslocated in the lining of the digestive tract. The lymphatic sys-tem absorbs lipids and other substances from the digestive tract(see chapter 24) through the lacteals. Lipids enter the lactealsand pass through the lymphatic vessels to the venous circula-tion. The lymph passing through these lymphatic vessels,called chyle (k ī l), appears white because of its lipid content.3. Defense . Microorganisms and other foreign substances arefiltered from lymph by lymph nodes and from blood by thespleen. In addition, lymphocytes and other cells are capableof destroying microorganisms and other foreign substances.Because the lymphatic system fights infections, and filtersblood and lymph to remove microorganisms, many infectiousdiseases produce symptoms associated with the lymphaticsystem (see the Diseases and Disorders table, later in thischapter).

ASSESS YOUR PROGRESS

1. Describe the three functions of the lymphatic system.

Lymphatic Vessels

The lymphatic vessels are essential for the maintenance of fluidbalance. Lymphatic vessels originate as small, dead-end tubescalled lymphatic capillaries (figure 22.2 a ). Fluids tend to moveout of blood capillaries into tissue spaces (see “Capillary Exchangeand Regulation of Interstitial Fluid Volume,” section 21.7).Excess fluid passes through the tissue spaces and enters lymphaticcapillaries to become lymph. Lymphatic capillaries are in mosttissues of the body. Areas of the body that lack lymphatic vesselsare the central nervous system, the bone marrow, and tissues with-out blood vessels, such as cartilage, epidermis, and the cornea. Asuperficial group of lymphatic capillaries is in the dermis of theskin and the subcutaneous tissue. A deep group of lymphatic cap-illaries drains the muscles, joints, viscera, and other deepstructures.Lymphatic capillary structure is different from that of bloodcapillaries. Lymphatic capillaries lack a basement membrane.Also, the cells of the simple squamous epithelium slightly overlapand are loosely attached to one another (figure 22.2 b ). This struc-ture facilitates their function in two ways: First, the lymphaticcapillaries are far more permeable than blood capillaries, so noth-ing in the interstitial fluid is excluded from entering the lymphaticcapillaries. Second, the overlapping cells of the lymphatic capil-lary epithelium function as a series of one-way valves that allowfluid to enter the capillary but prevent it from passing back into theinterstitial spaces.The lymphatic capillaries join to form larger lymphatic vessels, which resemble small veins. The inner layer of the lymphatic vesselconsists of endothelium surrounded by an elastic membrane; themiddle layer consists of smooth muscle cells and elastic fibers; andthe outer layer is a thin layer of fibrous connective tissue.Small lymphatic vessels have one-way valves along theirlengths that are similar to the valves of veins (see figure 22.2 b ). Thepresence of these valves causes small lymphatic vessels to have abeaded appearance. When a lymphatic vessel is compressed, thevalves close and prevent backward movement of lymph; as a conse-quence, the lymph moves forward through the lymphatic vessel.Lymph moves through lymphatic vessels by threemechanisms:1. Contraction of lymphatic vessels . In many parts of the body,lymphatic vessels pump lymph. The unidirectional valvesdivide lymphatic vessels into a series of chambers, whichfunction as “primitive hearts.” Lymph moves into a chamber,smooth muscle in the chamber wall contracts, and lymphmoves into the next chamber. Some of the smooth musclecells in the walls of lymphatic vessels are pacemaker cells(see “Electrical Properties of Smooth Muscle,” chapter 9).

22.2 Anatomy of the Lymphatic

System

LEARNING OUTCOMES

After reading this section, you should be able to

A. List the parts of the lymphatic system.

B. Describe the structure of lymphatic vessels.

C. Explain how lymph is formed and transported throughlymphatic vessels.