Lymphatic System PDF

Page 835 Page 836 21.1 Lymph and Lymph Vessels The lymphatic (lim-fat′ik) system is composed of (a) lymph vessels and (b) lymphoid structures ( figure 21.1). Lymph is the fluid transported within lymph vessels. We begin by describing the formation and characteristics of lymph. We then discuss its tr...

Page 835 Page 836 21.1 Lymph and Lymph Vessels The lymphatic (lim-fat′ik) system is composed of (a) lymph vessels and (b) lymphoid structures ( figure 21.1). Lymph is the fluid transported within lymph vessels. We begin by describing the formation and characteristics of lymph. We then discuss its transport through progressively larger lymph vessels until the lymph is returned to the cardiovascular system. Note that the term lymph vessel is used to indicate any type of vessel of the lymphatic system (e.g., lymphatic capillary, lymphatic vessel, lymphatic trunk), whereas the term lymphatic vessels is used when referring specifically to the vessels between the lymphatic capillaries and lymphatic trunks. Figure 21.1 Lymphatic System. The lymphatic system is composed of both lymph vessels (labeled on the left side of the image) and lymphoid structures (labeled on the right). Lymphoid structures are organized into primary lymphoid structures, which are involved in the formation and maturation of lymphocytes and secondary lymphoid structures, which house lymphocytes. Watch Video: Lymphatic System 21.1a Lymph and Lymphatic Capillaries LEARNING OBJECTIVES 1. Describe lymph and its contents. 2. Discuss the location and anatomic structure of lymphatic capillaries and the process of fluid entry. Lymph originates as interstitial fluid surrounding tissue cells. Lymph is formed as fluid in the interstitial space moves passively into the lymphatic capillaries due to a hydrostatic pressure gradient. Lymphatic capillaries merge to form larger lymph vessels. Characteristics of Lymph Approximately 15% of the fluid that enters the interstitial space surrounding the cells is not reabsorbed back into the blood capillaries during capillary exchange (see section 20.3d). This interstitial fluid amounts to about 3 liters daily and is normally absorbed into lymphatic capillaries. Once inside the lymph vessels, this fluid is called lymph (limf; lympha = clear spring water). The components of lymph include water, dissolved solutes (e.g., ions), a small amount of protein (approximately 100 to 200 grams that leaked into the interstitial space during capillary exchange), sometimes foreign material that includes both cell debris and pathogens, and perhaps metastasized cancer cells (see Clinical View 21.1: “Metastasis”). INTEGRATE CLINICAL VIEW 21.1 Metastasis Although the lymph vessels provide an essential function by rerouting excess interstitial fluid back into the blood, these vessels sometimes can participate in the spread of pathogens or cancerous cells. For example, cancerous cells can break free from a primary tumor (see section 5.6b) and be transported in the lymph. These cancerous cells may establish secondary tumors that develop in other locations within the body, a process referred to as metastasis (mĕ-tas΄tă-sis). For example, breast cancer may metastasize to the lungs, and this cancer in the lungs is referred to as metastasized breast cancer, not lung cancer. A biopsy of a lymph node that is positive for the presence of cancer cells from another body organ verifies that the cancer has metastasized. Lymphatic Capillaries The lymph vessel network begins with lymphatic capillaries, which are the smallest lymph vessels ( figure 21.2). Lymphatic capillaries are microscopic, closed-ended vessels that absorb interstitial fluid. They are interspersed throughout areolar connective tissue among capillary networks, except those within the red bone marrow and avascular tissues (such as epithelia). In addition, recent research has found lymph vessels associated with the dural venous sinuses that drain blood away from the brain. Figure 21.2 Lymphatic Capillaries. (a) Lymphatic capillaries begin as closed-end vessels within connective tissue among most blood capillary networks and absorb excess interstitial fluid left during capillary exchange. (b) A lymphatic capillary takes up excess interstitial fluid through overlapping endothelial cells. The fluid is then called lymph. Here, the black arrows show blood flow and the green arrows show lymph flow. APR Module 10: Lymphatic: Animations: Lymphatic vascular network (3D) A lymphatic capillary resembles the anatomic structure of a blood capillary in that its wall is composed of an endothelium ( figure 21.2b). However, lymphatic capillaries are typically larger in diameter than blood capillaries, lack a basement membrane, and have overlapping endothelial cells. These overlapping endothelial cells act as one-way flaps to allow fluid to enter the lymphatic capillary but prevent its release. Anchoring filaments help hold these endothelial cells to the nearby structures. Lymphatic capillaries located within the gastrointestinal (GI) tract, called lacteals (lak′tē-ăl; lactis = milk), allow for the absorption of lipid-soluble substances from the GI tract, a concept detailed in section 26.4c. Page 837 INTEGRATE CONCEPT CONNECTION Lacteals are lymphatic capillaries within the small intestine that absorb dietary lipids and lipid-soluble vitamins that are unable to enter the blood directly from the gastrointestinal (GI) tract (see section 26.4c). The lipids are absorbed as part of chylomicrons (lipid droplets enveloped within protein). Movement of Lymph into Lymphatic Capillaries The driving force to move fluid into the lymphatic capillaries is an increase in hydrostatic pressure (see section 4.3b) within the interstitial space. Interstitial hydrostatic pressure rises as additional fluid is filtered from the blood capillaries (see section 20.3b). This pressure exerted by interstitial fluid at the margins of the lymphatic capillary endothelial cells “pushes” interstitial fluid into the lymphatic capillary lumen when the interstitial fluid hydrostatic pressure becomes greater than the lymph hydrostatic pressure. The higher the interstitial fluid pressure, the greater the amount of fluid that enters the lymphatic capillary. The anchoring filaments extending between lymphatic capillary cells and the surrounding tissue prevent the collapse of the lymphatic capillaries as pressure exerted by the interstitial fluid increases. The pressure exerted by lymph after it enters the lymphatic capillary forces the endothelial cells of these vessels to close. Thus, lymph becomes “trapped” within the lymphatic capillary and cannot move back into the interstitial space. Lymph is then transported through a network of increasingly larger vessels that include (in order) lymphatic capillaries, lymphatic vessels, lymphatic trunks, and lymphatic ducts. WHAT DID YOU LEARN? 1 What substances typically are absorbed from the interstitial space into lymphatic capillaries? 2 How does fluid enter and become “trapped” in the lymphatic capillaries? INTEGRATE LEARNING STRATEGY 21.1 Fluid entry into a lymphatic capillary is analogous to the movement of the entryway door to your house or apartment. Imagine that the door is unlocked and the knob is turned. Putting 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 to the inside so you can enter. Once inside, pressure applied to the inside surface of the door (or fluid pressure against the inside of the lymphatic capillary surface) causes it to close. 21.1b Lymphatic Vessels, Trunks, and Ducts LEARNING OBJECTIVES 3. Explain the mechanisms that move lymph through lymphatic vessels, trunks, and ducts. 4. Describe the flow of lymph from lymphatic capillaries through the various types of lymph vessels until it is returned to the cardiovascular system. After lymph enters the lymphatic capillaries, it continues to flow into increasingly larger lymphatic vessels, trunks, and ducts. Ultimately, the lymph is returned to the cardiovascular system as it empties into the venous circulation. Lymphatic Vessels Lymphatic capillaries merge to form larger structures that are called lymphatic vessels ( figure 21.1). Superficial lymphatic vessels are generally positioned adjacent to the superficial veins of the body; in contrast, deep lymphatic vessels are next to deep arteries and veins. Lymphatic vessels resemble small veins because both contain all three vessel tunics (intima, media, and externa) and have valves within their lumen. Valves are required to prevent lymph from pooling in these vessels and help prevent lymph backflow because the lymphatic vessel network is a low-pressure system. These valves are especially important in areas where lymph flow is against the direction of gravity, such as in the lower limbs. The lymphatic system lacks a pump and, thus, relies on other mechanisms to move lymph through its vessels. These mechanisms to move lymph include: contraction of nearby skeletal muscles in the limbs (skeletal muscle pump) and the respiratory pump in the torso, which is similar to how blood movement is assisted through the venous circulation (see section 20.5a); rhythmic contraction of smooth muscle within the walls of larger lymph vessels (trunks and ducts), which narrows the lumen and squeezes the lymph within the lymph vessel; and pulsatile movement of blood in nearby arteries. All of these mechanisms are dependent upon valves within lymph vessels, which prevent the backflow of lymph, causing the lymph to move in one direction to be returned to venous blood circulation. Some lymphatic vessels connect directly to lymphoid organs called lymph nodes. Foreign or pathogenic material is filtered as lymph passes through lymph nodes. They are arranged in a series along lymph vessels and are described in more detail in section 21.4a. Lymphatic Trunks Lymphatic vessels drain into lymphatic trunks on both the right and left sides of the body ( figure 21.3). Each lymphatic trunk removes lymph from a specific major body region: Jugular trunks drain lymph from both the head and neck. Subclavian trunks remove lymph from the upper limbs, breasts, and superficial thoracic wall. Bronchomediastinal trunks drain lymph from deep thoracic structures. A single intestinal trunk drains lymph from most abdominal structures. Lumbar trunks drain lymph from the lower limbs, abdominopelvic wall, and pelvic organs. Figure 21.3 Lymphatic Trunks and Ducts. Lymph drains from lymphatic trunks into two lymphatic ducts (right lymphatic duct and thoracic duct) that empty into the junctions of the right jugular and right subclavian veins and left jugular and left subclavian veins, respectively. (a) An anterior view of the posterior thoracic wall illustrates the major lymphatic trunks and ducts and the site of lymph drainage into the venous circulation of the cardiovascular system. (see lymph drainage into the right lymphatic duct and the thoracic duct. section 20.10b) (b) Areas of APR Module 10: Lymphatic: Dissection: Thoracic: Anterior: Thoracic Duct Page 838 WHAT DO YOU THINK? 1 Predict what may occur with respect to lymphatic drainage if a group of lymph nodes and their associated lymph vessels are surgically removed, as might occur when breast cancer has metastasized. Lymphatic Ducts Lymphatic trunks drain into the largest lymph vessels called lymphatic ducts. There are only two lymphatic ducts: the right lymphatic duct and the thoracic duct. Both of these convey lymph back into the venous circulation. Right Lymphatic Duct The right lymphatic duct is located near the right clavicle. It receives lymph from the lymphatic trunks that drain the following areas: (a) the right side of the head and neck, (b) the right upper limb, and (c) the right side of the thorax. It returns the lymph into the junction of the right subclavian vein and the right internal jugular vein. Thus, the right lymphatic duct drains lymph from the upper right quadrant of the body ( figure 21.3b). Thoracic Duct The larger of the two lymphatic ducts is the thoracic duct. It has a length of about 37.5 to 45 centimeters (15 to 18 inches) and extends from the diaphragm to the junction of the left subclavian and left jugular veins. The thoracic duct drains lymph from the remaining areas of the body (left side of the head and neck, left upper limb, left thorax, all of the abdomen, and both lower limbs). At the base of the thoracic duct and anterior to the L2 vertebra is a rounded, saclike structure called the cisterna chyli (sis-ter′nă kī′lī). The cisterna chyli gets its name from the milky, lipid-rich lymph called chyle (kīl; chylos = juice), which it receives from vessels that drain the small intestine of the gastrointestinal (GI) tract (see section 26.4c). Both the intestinal trunk and the left and right lumbar trunks drain into the cisterna chyli. The thoracic duct extends superiorly from the cisterna chyli and lies directly anterior to the vertebral bodies. It passes through the aortic opening of the diaphragm, then ascends to the left of the vertebral body midline. WHAT DID YOU LEARN? 3 What mechanisms are used to assist lymph movement through lymph vessels? 4 Which major body regions drain lymph to the right lymphatic duct? Page 839 INTEGRATE CLINICAL VIEW 21.2 Lymphedema Lymphedema (limf΄e-dē΄mă; oidema = a swelling) is an accumulation of interstitial fluid that occurs due to interference with lymph drainage in a part of the body. Interstitial fluid increases, and the affected area both swells and becomes painful. If lymphedema is left untreated, the protein-rich interstitial fluid may interfere with wound healing and can even contribute to an infection by acting as a growth medium for bacteria. Most cases of lymphedema are obstructive, meaning they are caused by blockage of lymph vessels. There are several causes of obstructive lymphedema: Trauma or infection of the lymph vessels The spread of malignant tumors within the lymph nodes and lymph vessels Radiation therapy, which may cause scar formation in lymph vessels or nodes Any surgery that requires removal of a group of lymph nodes (e.g., breast cancer surgery when the axillary lymph nodes are removed) Elephantiasis, shown here in the right lower limb, caused by lymphatic filariasis. ©Andy Crumo, TDR, WHO/Science Source Although lymphedema has no cure, it can be controlled. Patients may wear compression stockings or other compression garments, which reduce swelling and assist interstitial fluid return to the circulation. Certain exercise and massage regimens may improve lymph drainage as well. Millions of individuals in Southeast Asia and Africa have developed lymphedema as a result of infection by threadlike, parasitic filarial worms. In lymphatic filariasis (fil-ă-rĪ΄ă-sis; filum = thread), filarial worms lodge in the lymph vessels, live and reproduce there for years, and eventually obstruct lymph drainage. Mosquitoes are the most common vector for transmitting lymphatic filariasis, although some filarial worms gain entrance through cracks in the skin of the foot. An affected body part can swell to many times its normal size. In these extreme cases, the condition is known as elephantiasis (el΄ĕ-fan-tĪ΄ă-sis; elephas = elephant). Patients are treated with antiparasitic medications to kill the filarial worms, although the damage to the lymph vessels may be irreversible.