Anatomy & Physiology

Anatomy & Physiology Study Guide the epithelia lining of the respiratory, digestive, and urinary tracts. They are also located in more complex lymphoid organs, such as lymph nodes or the spleen. A single nodule averages about a millimeter in diameter, but the boundaries are not distinct because no fibrous capsule surrounds them. Each nodule often has a central zone called a germinal center , which contains dividing lymphocytes. 21.6 Lymph Flow The lymphatic system absorbs the fluid that is filtered from our blood capillaries. The capillaries will reabsorb most of it, but what is left is filtered into the lymphatic capillaries, which flows into the lymphatic vessels. They move to the lymph nodes and the lymphatic trunks . They pour into two final collecting ducts, and then into the subclavian veins, where they are returned into the bloodstream. Tonsils The tonsils are large lymphoid nodules in the walls of the pharynx. Most people have five tonsils. Right and left palatine tonsils are located at the posterior, inferior margin of the oral cavity at the boundary of the pharynx. A single pharyngeal tonsil often called the adenoid, lies in the superior posterior wall of the nasopharynx. A pair of lingual tonsils lies deep to the mucous epithelium covering the base (pharyngeal portion) of the tongue. Due to their location, the latter are usually not visible unless they become infected and swollen. This condition is known as tonsillitis . Lymphoid Organs There are three primary lymphoid organs—the lymph nodes, the thymus, and the spleen. Lymph Nodes Lymph nodes are small lymphoid organs ranging in diameter from 1 mm to 25 mm (1 inch = 25.4 mm). Each lymph node is covered by a dense connective tissue capsule. Bundles of collagen fibers extend from the capsule into the interior of the node. These fibrous partitions are called trabeculae. Nerves and blood vessels reach the lymph node at the hilum, a shallow indentation. Two sets of lymphatic vessels, afferent lymphatics and efferent lymphatics, are connected to each lymph node. Afferent lymphatics carry lymph to the lymph node from peripheral tissues. Opposite the hilum, the afferent lymphatics penetrate the capsule of the node. Then starting at the hilum, efferent lymphatics leave the lymph node carrying away lymph towards the venous circulation. This process removes at least 99 percent of the antigens present in the lymph. In addition to filtering, lymph nodes provide an early-warning system. Any infection or other abnormality in a peripheral tissue will introduce abnormal antigens into the interstitial fluid and, thus, into the lymph that leaves the area. Thymus Located in the mediastinum, generally just posterior to the sternum is an organ called the thymus. It is at its largest in children and gradually decreases in size over time. The capsule that covers the thymus divides it into two thymic lobes. Fibrous partitions, called septa , originate at the capsule and divide the lobes into lobules. Each lobule consists of a densely packed outer cortex and a paler, central medulla . Lymphocytes in the cortex are arranged in clusters that are surrounded by reticular epithelial cells. These cells maintain the blood-thymus barrier. Achieve Page 282 of 368 ©2018