The Major Lymphatic Organs Help (page 2)
Introduction to The Major Lymphatic Organs
The lymphatic system consists of a collection of lymphatic organs, as well as lymphatic vessels.
The Lymph Nodes
The most widespread lymphatic organs are the lymph nodes ( NOADS ). The lymph nodes are a group of small, bean-like organs scattered in clusters in various parts of the body (Figure 17.3). Afferent ( AF -fer- ent ) lymphatic vessels “carry” (fer) dirty lymph “towards” (af-) the lymph nodes. Efferent ( EE -fer- ent ) lymphatic vessels carry clean lymph “away from” (ef-) them.
The Thymus Gland
A most unusual lymphatic organ is the thymus ( THIGH -mus) gland. The word, thymus, comes from the Ancient Greek for “warty outgrowth,” reflecting the bumpy appearance of this endocrine gland. The thymus is a thin, flat gland lying just deep to the sternum ( STER -num) or “breastplate.” This gland consists of two bumpy-looking lobes. The thymus gland secretes the hormone, thymosin (thigh- MOH -sin), which stimulates the activity of the lymphocytes and other parts of the body’s immune system. It also produces thymic ( THIGH -mik) lymphocytes (introduced as T-cells or T-lymphocytes, earlier).
The thymus gland is most prominent in young humans and other mammals. (In lambs, it is called the “throat sweetbread,” because it is often eaten as sweet-tasting meat.) It reaches its maximum size at puberty, then progressively decreases in size. In most adults, the thymus is completely gone, having been replaced by fatty connective tissue. The thymus is thought to play an important role in the development of immune competence in youngsters – a growing ability to ward off various diseases. The disappearance of the thymus in adults may be related to the gradual decline of immune competence seen in older persons, thereby making them more susceptible to cancer and pneumonia.
Red Bone Marrow and Spleen
Red Bone Marrow
The red bone marrow found within spongy bone (Chapter 13) is a third important type of lymphatic organ. Besides its role in hematopoiesis (blood cell formation), the red bone marrow is a source of B-lymphocytes. As you may recall, the B-lymphocytes differentiate into plasma cells when they are chemically signaled by the T-lymphocytes that a foreign invader (antigen) is present. And these plasma cells, in turn, produce antibodies.
In humans, the spleen is a dark red organ attached to the left side of the stomach. It looks somewhat like a thick, crescent-shaped-roll (croissant), and it rather feels like one, being soft and spongy to the touch. Besides hematopoiesis, the spleen (like the red bone marrow) is involved in the recycling and destruction of old, beaten-up erythrocytes. It also stores blood and contains a lot of lymphatic tissue. The spleen is rich in lymphocytes, plasma cells, and wandering macrophages (mostly derived from monocytes). It is therefore an important helper in phagocytosis of foreign invaders, such as bacteria, as well as devouring fragments of broken erythrocytes.
Tonsils and Tonsillitis
In addition to full-blown lymphatic organs, there are smaller masses of lymphatic tissue scattered here and there around the body. Prominent among these are the tonsils ( TAHN -sils). The tonsils are literally “little almonds” ( tonsils ) – oval, somewhat almond-shaped clusters of lymphatic tissue – lying in the back of the throat (Figure 17.4).
In humans, there are five tonsils. The pharyngeal (fah- RIN -jee-al) tonsil is the single uppermost mass, located in the portion of the pharynx ( FAIR -inks) or “throat” just behind the nose. The pharyngeal tonsil is also called the adenoids ( AD -uh- noyds ), because it is rather big and “gland” (aden) “like” (-oid).
The two palatine ( PAL -ah- tyn ) tonsils, as their name indicates, are a pair of tonsils lying on either side of the throat, just below the palate ( PAL -aht) or “roof of the mouth.” Finally, there is a pair of lingual ( LING -gwal) tonsils, attached way back at the base of the “tongue” (lingu).
Since they are composed of lymphatic tissue, these five tonsils play minor roles in body defense. They contain lymphocytes and macrophages that phagocytose foreign invaders entering the nose, mouth, or throat. These invaders naturally include various airborne bacteria and viruses.
Sometimes, however, the lymphatic tissue of the tonsils becomes overwhelmed by a huge number of bacteria or viruses. In such cases, tonsillitis ( tahn -sihl- EYE -tis) may result. Tonsillitis is “an inflammation and swelling of” (-itis) the tonsils. This inflammation may be accompanied by a dangerously high fever. The operation of tonsillectomy ( tahn -sihl- EK -toh-mee) or “removal of” (-ectomy) the tonsils is then frequently performed.
Study suggestion: Using your growing knowledge, write a single term that literally means, “inflammation of the adenoids.” Why do you think that a person afflicted with this condition might have trouble breathing?
Practice problems for these concepts can be found at: Immune and Lymphatic Systems Test
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