Muscular System Physiology
Right foot, Superior and Inferior views. The filtration endothelial-capsular membrane, Details of the filtration membrane. Internal anatomy of the spinal cord: Lithograph by Pierre Roche Vigneron. Sacral plexus in anterior view.
The differentiation of T cells occurs in the cortex of the thymus. In humans the thymus appears early in fetal development and continues to grow until puberty , after which it begins to shrink. The decline of the thymus is thought to be the reason T-cell production decreases with age. The thymocytes then move to the medulla of the thymus, where further differentiation occurs.
Positive and negative selection destroy a great number of thymocytes; only about 5 to 10 percent survive to exit the thymus. Those that survive leave the thymus through specialized passages called efferent outgoing lymphatics, which drain to the blood and secondary lymphoid organs. The thymus has no afferent incoming lymphatics, which supports the idea that the thymus is a T-cell factory rather than a rest stop for circulating lymphocytes.
In birds B cells mature in the bursa of Fabricius. The process of B-cell maturation was elucidated in birds—hence B for bursa. In mammals the primary organ for B-lymphocyte development is the bone marrow, although the prenatal site of B-cell differentiation is the fetal liver.
Unlike the thymus, the bone marrow does not atrophy at puberty, and therefore there is no concomitant decrease in the production of B lymphocytes with age. The secondary lymphoid organs serve two basic functions: The lymph nodes, or lymph glands, are small, encapsulated bean-shaped structures composed of lymphatic tissue. Thousands of lymph nodes are found throughout the body along the lymphatic routes, and they are especially prevalent in areas around the armpits axillary nodes , groin inguinal nodes , neck cervical nodes , and knees popliteal nodes.
The nodes contain lymphocytes, which enter from the bloodstream via specialized vessels called the high endothelial venules. T cells congregate in the inner cortex paracortex , and B cells are organized in germinal centres in the outer cortex. Lymph, along with antigens, drains into the node through afferent incoming lymphatic vessels and percolates through the lymph node , where it comes in contact with and activates lymphocytes.
Activated lymphocytes, carried in the lymph, exit the node through the efferent outgoing vessels and eventually enter the bloodstream, which distributes them throughout the body. The spleen is found in the abdominal cavity behind the stomach. Although structurally similar to a lymph node, the spleen filters blood rather than lymph. One of its main functions is to bring blood into contact with lymphocytes. The functional tissue of the spleen is made up of two types of cells: The splenic artery enters the red pulp through a web of small blood vessels, and blood-borne microorganisms are trapped in this loose collection of cells until they are gradually washed out through the splenic vein.
The white pulp contains both B and T lymphocytes. T cells congregate around the tiny arterioles that enter the spleen, while B cells are located in regions called germinal centres, where the lymphocytes are exposed to antigens and induced to differentiate into antibody -secreting plasma cells.
Another group of important secondary lymphoid structures is the mucosa-associated lymphoid tissues. These tissues are associated with mucosal surfaces of almost any organ, but especially those of the digestive, genitourinary, and respiratory tracts, which are constantly exposed to a wide variety of potentially harmful microorganisms and therefore require their own system of antigen capture and presentation to lymphocytes.
Other, less-organized regions of the gut also play a role as secondary lymphoid tissue. The host of secondary lymphoid organs provides a system of redundancy for antigen sampling by the cells of the immune system. Removal of the spleen, selected lymph nodes, tonsils, or appendix does not generally result in an excessive increase in disease caused by pathogenic microorganisms.
However, the importance of the primary lymphoid organs is clear. For example, two autoimmune diseases, DiGeorge syndrome and Nezelof disease, result in the failure of the thymus to develop and in the subsequent reduction in T-cell numbers, and removal of the bursa from chickens results in a decrease in B-cell counts. The destruction of bone marrow also has devastating effects on the immune system , not only because of its role as the site of B-cell development but also because it is the source of the stem cells that are the precursors for lymphocyte differentiation.
Thus the capsule, the trabeculae, and the hilum make up the framework of the node. Within this framework, a delicate arrangement of connective tissue forms the lymph sinuses, within which lymph and free lymphoid elements circulate. A subcapsular or marginal sinus exists between the capsule and the cortex of the lymph node. Lymph passes from the subcapsular sinus into the cortical sinus toward the medulla of the lymph node. Medullary sinuses represent a broad network of lymph channels that drain toward the hilum of the node; from there, lymph is collected into several efferent vessels that run to other lymph nodes and eventually drain into their respective lymphatic ducts see the image below.
The thymus is a bilobed lymphoid organ located in the superior mediastinum of the thorax, posterior to the sternum. After puberty, it begins to decrease in size; it is small and fatty in adults after degeneration. The primary function of the thymus is the processing and maturation of T lymphocytes. While in the thymus, T lymphocytes do not respond to pathogens and foreign organisms. After maturation, they enter the blood and go to other lymphatic organs, where they help provide defense.
Structurally, the thymus is similar to the spleen and lymph nodes, with numerous lobules and cortical and medullary elements. It also produces thymosin, a hormone that helps stimulate maturation of T lymphocytes in other lymphatic organs.
The spleen, the largest lymphatic organ, is a convex lymphoid structure located below the diaphragm and behind the stomach. It is surrounded by a connective tissue capsule that extends inward to divide the organ into lobules consisting of cells, small blood vessels, and 2 types of tissue known as red and white pulp. Red pulp consists of venous sinuses filled with blood and cords of lymphocytes and macrophages; white pulp is lymphatic tissue consisting of lymphocytes around the arteries. Lymphocytes are densely packed within the cortex of the spleen.
The spleen filters blood in much the same way that lymph nodes filter lymph. Lymphocytes in the spleen react to pathogens in the blood and attempt to destroy them. Macrophages then engulf and phagocytose damaged cells and cellular debris. The spleen, along with the liver, eradicates damaged and old erythrocytes from the blood circulation. Like other lymphatic tissue, it produces lymphocytes in an immunologic response to offending pathogens.
It plays an important role in red blood cell and iron metabolism through macrophage phagocytosis of old and damaged red blood cells. Tonsils are aggregates of lymph node tissue located under the epithelial lining of the oral and pharyngeal areas.
The main areas are the palatine tonsils on the sides of the oropharynx , the pharyngeal tonsils on the roof of the nasopharynx; also known as adenoids , and the lingual tonsils on the base of the posterior surface of the tongue.
Because these tonsils are so closely related to the oral and pharyngeal airways, they may interfere with breathing when they become enlarged. The predominance of lymphocytes and macrophages in these tonsillar tissues offers protection against harmful pathogens and substances that may enter through the oral cavity or airway. Diseases of the lymphatic system include lymphedema, lymphoma, lymphadenopathy, lymphadenitis, filariasis, splenomegaly, and tonsillitis.
Lymphedema results when the lymphatic system cannot adequately drain lymph, resulting in an accumulation of fluid that causes swelling. It may be either primary or secondary.
Primary lymphedema is an inherited condition that occurs as a result of impaired or missing lymphatic vessels; it may be present at birth, may develop with the onset of puberty, or may occur in adulthood, with no apparent causes. Secondary lymphedema is basically acquired regional lymphatic insufficiency, which may occur as a consequence of any trauma, infection, or surgical procedure that disrupts the lymphatic vessels or results in the loss of lymph nodes [ 8 ]. Treatment consists of compression bandages or pneumatic stockings to alleviate the swelling after appropriate diagnosis is made.
Lymphoma is a medical term used for a group of cancers that originate in the lymphatic system. Lymphomas usually begin with malignant transformation of the lymphocytes in lymph nodes or bunches of lymphatic tissue in organs like the stomach or intestines. Hodgkin lymphoma and non-Hodgkin lymphoma are the 2 major categories of lymphoma, characterized by enlargement of lymph nodes, usually present in the neck.
When you eat, food travels into your stomach, and digestion begins. You don't need your stomach working if you aren't eating. The digestive system works with a series of hormones and nervous impulses to stop and start the secretion of acids in your stomach. Another example of negative feedback occurs when your body's temperature begins to rise and a negative feedback response works to counteract and stop the rise in temperature. Sweating is a good example of negative feedback. Positive Feedback Positive feedback is the opposite of negative feedback in that encourages a physiological process or amplifies the action of a system.
Positive feedback is a cyclic process that can continue to amplify your body's response to a stimulus until a negative feedback response takes over. An example of positive feedback also can happen in your stomach. Your stomach normally secretes a compound called pepsinogen that is an inactive enzyme.