Lymphatic System Bio 13.2 PDF

Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 13: Circulation 481 Lesson 13.2 **Lymphatic System** Introduction The **lymphatic system** is composed of lymphoid organs, lymphatic vessels (ie, lymphatics), and fluid within the lymphatic vessels (ie, lymph). Lymph...

Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 13: Circulation 481 Lesson 13.2 **Lymphatic System** Introduction The **lymphatic system** is composed of lymphoid organs, lymphatic vessels (ie, lymphatics), and fluid within the lymphatic vessels (ie, lymph). Lymphoid organs, such as the [red bone marrow](javascript:void(0)), thymus, spleen, and lymph nodes, play crucial roles in immune defense of the body, including in the production of cells (ie, lymphocytes) that constitute the immune system. In addition to immune defense of the body (discussed in Lesson 20.1), the lymphatic system functions to move fluid and proteins that leak into the interstitial space from blood capillaries back to the blood. Furthermore, following the absorption of lipids from food in the digestive system, the lymphatic system transports lipids to the blood to be distributed throughout the body. This lesson covers the structure and function of lymphatic vessels, movement of lymph in the body, and major functions of the lymphatic system. 13.2.01 Lymphatic Vessels **Lymphatic vessels** carry lymph in one direction only, from the tissues toward the heart. Interstitial fluid initially enters the lymphatic system via blind-ended (ie, closed at one end) **lymphatic capillaries**, which are located among blood capillaries and tissue cells. Like blood capillaries (see Concept 13.1.10), the walls of lymphatic capillaries are composed of a single layer of endothelial cells. However, unlike blood capillaries, lymph capillaries lack a continuous basement membrane, and lymphatic endothelial cells loosely overlap one another to form flaplike valves (Figure 13.27). The flaplike valves in the walls of lymph capillaries permit easy entry of interstitial fluid (containing proteins, lymphocytes, and, potentially, pathogens) into lymph capillaries but prevent outward leakage. In addition, lymphatic endothelial cells are anchored to the surrounding extracellular matrix by protein filaments that help pull the overlapping cells apart to facilitate fluid entry. Chapter 13: Circulation 482 **Figure 13.27** Structure of a lymphatic capillary. After entering lymphatic capillaries, lymph moves through a series of lymph vessels that have progressively larger diameters and thicker walls containing [smooth muscle](javascript:void(0)). Lymphatic capillaries merge to form **collecting vessels**, which, as shown in Figure 13.28, contain **valves** (similar to those present in cardiovascular system veins) to prevent backflow of lymph. Collecting vessels carry lymph to lymph nodes and eventually merge to form **lymphatic trunks**. Each lymphatic trunk carries lymph to one of two **lymphatic ducts** (ie, right lymphatic duct, thoracic duct), which empty the lymph into veins of the cardiovascular system near the heart. A diagram of a body Description automatically generated Chapter 13: Circulation 483 **Figure 13.28** Sequence of structures through which lymph passes after entering lymphatic capillaries. As shown in Figure 13.29, the **right lymphatic duct** receives lymph that drains from the right arm, the right side of the head, and the right side of the thorax. The **thoracic duct** drains lymph from all other body regions; consequently, the thoracic duct transfers the majority of the body\'s lymph to the blood.  Chapter 13: Circulation 484 **Figure 13.29** Lymphatic drainage. A diagram of a person\'s body Description automatically generated Chapter 13: Circulation 485 **Concept Check 13.3** The following diagram depicts steps in a pathway along which a plasma protein would likely travel after the protein has leaked into the interstitial space from a blood vessel. Complete the diagram by determining which of the following terms belongs in each blank box: \"Blood capillary,\" \"Cardiovascular system vein,\" \"Collecting vessel,\" \"Lymphatic capillary,\" \"Lymphatic duct,\" \"Lymphatic trunk,\" \"Lymph node.\" [**Solution**](javascript:void(0))  A white rectangular object with a shadow Description automatically generated with medium confidence Chapter 13: Circulation 486 13.2.02 Lymphatic System Function As discussed in Concept 13.1.10, bulk flow causes fluid to move out of blood capillaries into the interstitial space at the arterial end of a capillary bed and reenter the capillaries at the venous end of the capillary bed. However, less fluid typically reenters the capillaries than leaves the capillaries, which could result in harmful fluid accumulation in the interstitial space (ie, edema) if interstitial fluid was not continuously returned to the blood. Consequently, to maintain appropriate fluid balance, a primary lymphatic system function is to collect and transport excess interstitial fluid (along with constituent proteins) to the blood via the lymphatic vessels. In addition to its role in balancing fluid, the lymphatic system functions to transport [chylomicrons](javascript:void(0)) (ie, triglyceride-rich lipoproteins) to the blood from cells in the small intestine that absorb lipids during food digestion (see Concept 15.1.03). Chylomicrons are too large to enter blood capillaries, so they are taken into specialized lymphatic capillaries called **lacteals**, which are present within [villi](javascript:void(0)) of the small intestine. The fluid within lacteals, which becomes incorporated into the lymph transported by the lymphatic system, is called **chyle**. Unlike the cardiovascular system, in which the heart is the dedicated pump, the lymphatic system lacks a dedicated pump to move lymph through lymphatic vessels. Instead, skeletal muscle contraction results in compression of lymphatic vessels, and the presence of valves within lymphatic vessels prevents backflow of lymph. Consequently, lymph is propelled in only one direction (ie, toward the heart) as skeletal muscle contraction occurs. In addition, the walls of lympphatic vessels larger than lymphatic capillaries contain smooth muscle, which contracts rhythmically to facilitate lymph movement