OUTLINE 24.1 Functions of the Lymphatic System 725 24.2 Lymph and Lymph Vessels 726 24 24.2a Lymphatic Capillaries 726 24.2b Lymphatic Vessels 726 24.2c Lymphatic Trunks 727 24.2d Lymphatic Ducts 727 Lymphatic 24.3 Lymphatic Cells 729 24.3a Types and Functions of Lymphocytes 729 24.3b Lymphopoiesis 734 24.4 Lymphatic Structures 735 System 24.4a Lymphatic Nodules 735 24.4b Lymphatic Organs 736 24.5 Aging and the Lymphatic System 741 24.6 Development of the Lymphatic System 741 MODULE 10: LYMPHATIC SYSTEM mck78097_ch24_724-746.indd 724 2/25/11 2:16 PM Chapter Twenty-Four Lymphatic System 725 e have seen in chapters 21–23 how the cardiovascular system W transports blood throughout the body, where it exchanges gases and nutrients with the tissues. Another body system, called the lymphatic system, aids the cardiovascular system by transporting excess interstitial fluid through lymph vessels assisting in maintain- ing fluid homeostasis. Once this fluid enters the vessels, the fluid is Tonsils renamed lymph. Along the way, lymph is filtered and checked for Cervical lymph nodes foreign or pathologic material, such as bacteria and cancer cells. Right lymphatic duct Lymphatic structures contain certain cells that initiate an immune Axillary response to abnormal materials. Without the primary immune Thymus response by the lymphatic system, the body would be unable to fight lymph nodes infection and keep itself healthy. In this chapter we examine the lymph vessels, lymphatic Thoracic duct structures, and lymphatic organs of the body, and learn how each Spleen Cisterna chyli of these components plays an important role in keeping us healthy. Mucosa- associated lymphatic tissue (MALT) (in small 24.1 Functions of the Lymphatic intestine) System Inguinal Learning Objective: lymph nodes 1. Explain how the lymphatic system aids fluid homeostasis and guards the health of body cells and tissues. The lymphatic (lim-fat ́ik) system involves several organs Red bone marrow as well as a system of lymphatic cells and lymph vessels located throughout the body (figure 24.1). Together, these structures Lymph vessels transport fluids and help the body fight infection. However, not all of these components of the lymphatic system are involved in each function. At the arterial end of a capillary bed, blood pressure forces fluid from the blood into the interstitial spaces around cells. This fluid is called interstitial fluid (not to be confused with extracel- lular fluid, a term that encompasses both interstitial fluid and plasma [see chapter 2]). Most of this fluid is reabsorbed at the venous end of the capillaries, but an excess of about 3 liters of fluid per day remains in the interstitial spaces. A network of lymph vessels (figure 24.1) reabsorbs this excess fluid and returns it to the venous circulation. If this excess fluid were not removed, body tissues would swell, a condition called edema (e-dē ḿă ; oidema = a swelling). Further, this excess fluid would accumulate outside the bloodstream, causing blood levels to drop precipitously. Thus, the lymphatic system prevents interstitial fluid levels from rising out Figure 24.1 of control and helps maintain blood volume levels. Lymphatic System. The lymphatic system consists of lymph vessels, Lymph vessels also transport dietary lipids. Although most lymphatic cells, and lymphatic organs that work together to pick up nutrients are absorbed directly into the bloodstream, some larger and transport interstitial fluid back to the blood and to mount an materials, such as lipids and lipid-soluble vitamins, are unable to immune response when needed. enter the bloodstream directly from the gastrointestinal (GI) tract. These materials are transported through tiny lymph vessels called producing), which are any substances perceived as abnormal to the lacteals, which drain into larger lymph vessels and eventually into body, such as bacteria, viruses, and even cancer cells. If antigens the bloodstream. are discovered, lymphatic cells initiate a systematic defense against ū ́ Lymphatic organs house lymphocytes, a type of leukocyte (see the antigens, called an immune (i-m n ) response. Some of the cells chapter 21). While some lymphocytes circulate in the bloodstream, produce soluble proteins called antibodies that bind to the foreign or most are located in the lymphatic structures and organs. Some lym- abnormal agent, thus damaging it or identifying it to other elements phatic organs assist in these cells’ maturation, while others serve as of the immune system. Other cells attack and destroy the antigen a site for lymphocyte replication (mitosis). directly. Still other cells become memory cells, which remember the Finally, the lymphatic system cells generate an immune past antigen encounters and initiate an even faster and more power- response and increase the lymphocyte population when necessary. ful response should the same antigen appear again. Lymphatic structures contain T-lymphocytes, B-lymphocytes, and macrophages (monocytes that have migrated from the bloodstream WHATW DID YOU LEARN? into other tissues). These cells are constantly monitoring the blood ●1 What is the “immune response,” and how does the lymphatic and the interstitial fluid for antigens (an ti-gen;́ anti(body) + gen = system initiate it? mck78097_ch24_724-746.indd 725 2/25/11 2:16 PM 726 Chapter Twenty-Four Lymphatic System When the pressure increases in the lymphatic capillary, the cell wall 24.2 Lymph and Lymph Vessels margin pushes back into place next to the adjacent endothelial cell. The fluid that is now “trapped” in the lymph capillary cannot be Learning Objectives: released back into the interstitial spaces. This process is analogous 1. Identify the components of lymph. to the movement of the entryway door to your house or apartment. 2. Outline the path of lymph from interstitial tissues to the Imagine that the door is unlocked and the knob is turned. Putting bloodstream. pressure on the outside of the door (like the pressure of interstitial fluid on the outside of the lymphatic capillary wall) causes it to open Excess interstitial fluid and solutes are returned to the blood- to the inside so you can enter. Once inside, pressure applied to the stream through a lymph vessel network. When the combination inside surface of the door (or fluid pressure against the inside lym- of interstitial fluid, solutes, and sometimes foreign material enters phatic capillary surface) causes it to close. the lymph vessels, the liquid mixture is called lymph (limf; lym- The small intestine (part of the GI tract) contains special pha = clear spring water). The lymph vessel network is composed types of lymphatic capillaries called lacteals (lak t́ē -ăl; lactis = of increasingly larger vessels, as follows (from smallest to largest milk). Lacteals pick up not only interstitial fluid, but also dietary in diameter): lymphatic capillaries, lymphatic vessels, lymphatic lipids and lipid-soluble vitamins (vitamins that must be dissolved trunks, and lymphatic ducts. Thus, the term “lymph vessel” is a in lipids before they can be absorbed). The lymph from the GI tract general term to describe all of these specific lymphatic capillaries, has a milky color due to the lipid, and for this reason the GI tract lymphatic vessels, trunks, and ducts. lymph is also called chyle (kı̄l; chylos = juice). 24.2a Lymphatic Capillaries The lymph vessel network begins with microscopic vessels called 24.2b Lymphatic Vessels lymphatic capillaries. Lymphatic capillaries are closed-ended tubes Lymphatic capillaries merge to form larger structures called lym- that are interspersed among most blood capillary networks (figure phatic vessels. Lymphatic vessels resemble small veins, in that 24.2), except those within the red bone marrow and central ner- both contain three tunics (intima, media, and externa) and both vous system. In addition, avascular tissues (such as epithelia) lack have valves within the lumen. Since the lymph vessel network is lymphatic capillaries. A lymphatic capillary is similar to a blood a low-pressure system, valves prevent lymph from pooling in the capillary in that its wall is an endothelium. However, lymphatic vessel and help prevent lymph backflow (figure 24.3). These capillaries tend to be larger in diameter, lack a basement mem- valves are especially important in areas where lymph flow is brane, and have overlapping endothelial cells. Anchoring filaments against the direction of gravity. Contraction of nearby skeletal help hold these endothelial cells to the nearby structures. These muscles also helps move lymph through the vessels. overlapping endothelial cells act as one-way flaps; when interstitial Some lymphatic vessels connect directly to lymphatic organs fluid pressure rises, the margins of the endothelial cells push into called lymph nodes. Afferent lymphatic vessels bring lymph to a the lymphatic capillary lumen and allow interstitial fluid to enter. lymph node where it is filtered for foreign or pathogenic material. Interstitial space Capillary bed Venule Endothelium of lymphatic capillary Lymphatic capillary Interstitial fluid Tissue cell Opening Arteriole Lymph Anchoring filament (a) Capillary bed and lymphatic capillaries (b) Lymphatic capillary Figure 24.2 Lymphatic Capillaries. ( a) Lymphatic capillaries arise as blind-ended vessels in connective tissue spaces among most blood capillary networks. Here, the black arrows show blood flow and the green arrows show lymph flow. (b) A lymphatic capillary takes up interstitial fluid through one- way flaps in its endothelial lining. mck78097_ch24_724-746.indd 726 2/25/11 2:16 PM Chapter Twenty-Four Lymphatic System 727 Overlapping endothelial cells Valve open Lymph (lymph flows forward) Direction of Valve closed lymph flow (backflow of lymph is prevented) LM 100x Valve Lymphatic vessel (a) Lymphatic vessel, longitudinal section (b) Lymphatic vessel, cross section Figure 24.3 Lymphatic Vessels and Valves. (a) Lymphatic vessels contain valves to prevent backflow of lymph. (b) Histologic cross section of a lymphatic vessel. Once filtered, the lymph exits the lymph node via efferent lym- right upper limb, and right side of the thorax (figure 24.4b).