Medical brain notes 27

  LYMPH   Lymph  is the name for tissue fluid that enters lymph   capillaries. As you may recall, filtration in capillaries creates tissue fluid from blood plasma, most of which returns almost immediately to the blood in the capillaries by osmosis. Some tissue fluid, however, remains in interstitial spaces and must be returned to the blood by way of the lymphatic ves-sels. Without this return, blood volume and blood pressure would very soon decrease. The relationship of the lymphatic vessels to the cardiovascular system is depicted in Fig. 14–1.

  LYMPH VESSELS   The system of lymph vessels begins as dead-end  lymph capillaries  found in most tissue spaces (Fig.   14–2). Lymph capillaries are very permeable and col-lect tissue fluid and proteins.  Lacteals  are specialized lymph capillaries in the villi of the small intestine; they absorb the fat-soluble end products of digestion, such as fatty acids and vitamins A, D, E, and K.   Lymph capillaries unite to form larger lymph ves-sels, whose structure is very much like that of veins. There is no pump for lymph (as the heart is the pump for blood), but the lymph is kept moving within lymph vessels by the same mechanisms that promote venous return. The smooth muscle layer of the larger lymph vessels constricts, and the one-way valves (just like those of veins) prevent backflow of lymph. Lymph ves-sels in the extremities, especially the legs, are com-pressed by the skeletal muscles that surround them; this is the  skeletal muscle pump . ...

  LYMPHATIC TISSUE   Lymphatic tissue consists mainly of lymphocytes in a mesh-like framework of connective tissue. Recall that most lymphocytes are produced from stem cells in the red bone marrow, then migrate to the lymph nodes and nodules, to the spleen, and to the thymus. In these structures, lymphocytes become activated and prolif-erate in response to infection (this is a function of all lymphatic tissue). The thymus has stem cells that pro-duce a significant portion of the T lymphocytes.   LYMPH NODES AND NODULES   Lymph nodes  and  nodules  are masses of lymphatic   tissue. Nodes and nodules differ with respect to size and location. Nodes are usually larger, 10 to 20 mm in length, and are encapsulated; nodules range from a fraction of a millimeter to several millimeters in length and do not have capsules.   Lymph nodes  are found in groups along the path-ways of lymph vessels, and lymph flows through these nodes on its way to the ...

  LYMPH NODES AND NODULES   Lymph nodes  and  nodules  are masses of lymphatic   tissue. Nodes and nodules differ with respect to size and location. Nodes are usually larger, 10 to 20 mm in length, and are encapsulated; nodules range from a fraction of a millimeter to several millimeters in length and do not have capsules.   Lymph nodes  are found in groups along the path-ways of lymph vessels, and lymph flows through these nodes on its way to the subclavian veins. Lymph enters a node through several afferent lymph vessels and  leaves through one or two efferent vessels (Fig. 14–4).   Figure 14–4.  Lymph node. ( A ) Section through a lymph node, showing the flow of   lymph. ( B ) Microscopic detail of bacteria being destroyed within the lymph node.  QUESTION:  What is the function of the plasma cells in a lymph node? As lymph passes through a lymph node, bacteria and other foreign materials are phagocytized by fixed (...

  SPLEEN   The  spleen  is located in the upper left quadrant of the abdominal cavity, just below the diaphragm, behind the stomach. The lower rib cage protects the spleen from physical trauma (see Fig. 14–3).   In the fetus, the spleen produces red blood cells, a function assumed by the red bone marrow after birth. After birth the spleen is very much like a large lymph node, except that its functions affect the blood that flows through it rather than lymph.   The functions of the spleen after birth are:   1.   Contains plasma cells that produce antibodies to foreign antigens.   2.   Contains fixed macrophages (RE cells) that phago-cytize pathogens or other foreign material in the blood. The macrophages of the spleen also phago-cytize old red blood cells and form bilirubin. By way of portal circulation, the bilirubin is sent to the liver for excretion in bile.   3.   Stores platelets and destroys them when they are no longer us...

  SPLEEN   The  spleen  is located in the upper left quadrant of the abdominal cavity, just below the diaphragm, behind the stomach. The lower rib cage protects the spleen from physical trauma (see Fig. 14–3).   In the fetus, the spleen produces red blood cells, a function assumed by the red bone marrow after birth. After birth the spleen is very much like a large lymph node, except that its functions affect the blood that flows through it rather than lymph.   The functions of the spleen after birth are:   1.   Contains plasma cells that produce antibodies to foreign antigens.   2.   Contains fixed macrophages (RE cells) that phago-cytize pathogens or other foreign material in the blood. The macrophages of the spleen also phago-cytize old red blood cells and form bilirubin. By way of portal circulation, the bilirubin is sent to the liver for excretion in bile.   3.   Stores platelets and destroys them when they are no longer us...

  THYMUS   The  thymus  is located inferior to the thyroid gland. In the fetus and infant, the thymus is large and extends under the sternum (Fig. 14–5). With increasing age, the thymus shrinks, and relatively little thymus tissue is found in adults, though it is still active. Figure 14–5.  Location of the thymus in a young child.   QUESTION:  Which blood cells mature in the thymus? The stem cells of the thymus produce T lympho-cytes or  T cells ; their functions are discussed in the next section. Thymic hormones are necessary for what may be called “immunological competence.” To be competent means to be able to do something well. The thymic hormones enable the T cells to participate in the recognition of foreign antigens and to provide immunity. This capability of T cells is established early in life and then is perpetuated by the lympho cytes themselves. The newborn’s immune system is not yet fully mature, and infants are more susceptible to cer...

  IMMUNITY   Immunity  may be defined as the ability to destroy   pathogens or other foreign material and to prevent further cases of certain infectious diseases. This ability is of vital importance because the body is exposed to pathogens from the moment of birth. Antigens  are chemical markers that identify cells.   Human cells have their own antigens that identify all the cells in an individual as “self ” (recall the HLA types). When antigens are foreign, or “non-self,” they may be recognized as such and destroyed. Bacteria, viruses, fungi, and protozoa are all foreign antigens that activate immune re-sponses, as are cell products such as bacterial toxins.   Malignant cells, which may be formed within the body as a result of mutations of normal cells, are also recognized as foreign and are usually destroyed before they can establish themselves and cause cancer. Unfor-tunately, organ transplants are also foreign tissue, and the immune system may reje...