THE RESPIRATORY SYSTEM

the interstitial fluid enters the lymphatic capillaries, the fluid is called lymph. Lymphatic Vessels Lymphatic vessels are formed from the merging of lymphatic capillaries (fig. 1-37). Lymphatic vessels, also known simply as lymphatics, are similar to veins in structure. The vessel walls are composed of three layers: an inner layer of endothelial tissue, a middle layer of smooth muscle and elastic fibers, and an outer layer of connective tissue. Like a vein, the lymphatic vessel has valves to prevent backflow of lymph. The larger lymphatic vessels lead to specialized organs called lymph nodes. After leaving these structures, the vessels merge to form still larger lymphatic trunks. Lymphatic Trunks and Ducts Lymphatic trunks drain lymph from large regions in the body. The lymphatic trunks are usually named after the region they serve, such as the subclavian trunk that drains the arm. There are many lymphatic trunks through out the body. These lymphatic trunks then join one of two collecting ducts, the thoracic duct and the right lymphatic duct (fig. 1-38). Lymphatic trunks from the upper half of the right side of the body converge to form the right lymphatic duct, which empties into the right subclavian vein. Drainage from the remainder of the body is by way of the thoracic duct, which empties into the left subclavian vein. LYMPH NODES Lymph nodes, which are frequently called glands but are not true glands, are small bean-shaped bodies of lymphatic tissue found in groups of two to fifteen along the course of the lymph vessels (fig. 1-38). Major locations of lymph nodes are in the following regions: cervical, axillary, inguinal, pelvic cavity, abdominal cavity, and thoracic cavity. Lymph nodes vary in size and act as filters to remove bacteria and particles from the lymph stream. Lymph nodes produce lymphocytes, which help defend the body against harmful foreign particles, such as bacteria, cells, and viruses. Lymph nodes also contain macrophages, which engulf and destroy foreign substances, damaged cells, and cellular debris. THE RESPIRATORY SYSTEM LEARNING OBJECTIVE: Identify the location and function of each part of the respiratory system, and recall the process of respiration. Respiration is the exchange of oxygen and carbon dioxide between the atmosphere and the cells of the body. There are two phases of respiration: � Physical, or mechanical, respiration involves the motion of the diaphragm and rib cage. The 1-32 LYMPH CAPILLARY TISSUE CELLS VENULE LYMPHATIC VESSEL ARTERIOLE HM3F0137 Figure 1-37.—Lymphatic capillary and vessel.