Lymphatic vessels
Lymphatic vessels consist of a, fine network of thin—walled vessels that drain into progressively larger and progressively thicker—walled collecting trunks. These ultimately drain, via the thoracic duct and right lymphatic duct, into the left and right subclavian veins at their angles of junction with the internal jugular veins, respectively. The lymphatics serve as a one—way (i. e., toward the heart) drainage sys tem for the return of tissue fluid and other diffusible substances, including plasma proteins, which constantly escape from the blood through capillaries. They are also important in serving as a conduit for channeling lymphocytes and antibodies produced in lymph nodes into the blood circulation.
Lymphatic capillaries consist of vessels lined with en—dothelial cells, which begin as blind—ended tubules or sac—cules in most tis sues of the body. Endothelium is attenuated and usually lacks a continuous basal lamina. Lymphatic vessels of large diameter resemble veins in their struc ture but lack a clear—cut separation between layers. Valves are more numerous in lymphatic vessels. Smooth muscle cells in the media layer engage in rhythmic contraction, pumping lymph toward the venous system. Smooth muscle is well—developed in large lymphatic ducts.
Circulation of lymph is slower than that of blood, but it is nonetheless an essential process. It has been estimated that in a single day, 50 % or more of the total circulating protein leaves the blood circulation at the capillary level and is recaptured by the lymphatics.
Distribution of lymphatics is ubiquitous with some notable excep tions, including epithelium, cartilage, bone, central nervous sys tem, and thymus.
New words
plasma – плазма
extracellular – внеклеточный
aqueous – водный
solution – раствор
proteins – белки
inorganic – неорганический
salts – соли
organic – органический
albumin – альбумин
globulins – глобулины
alpha – альфа
beta – бета
gamma – гамма
fibrinogen – фибриноген
lymphatic – лимфатический
vessel – сосуд
endothelium – эндотелий
circulation – кровообращение
lymph – лимфа
ubiquitous – вездесущий
notable – известный
21. Hematopoietic tissue. Erythropoiesis
Hematopoietic tissue is composed of reticular fibers and cells, blood vessels, and sinusoids (thin—walled blood channels). Myeloid, or blood cell—forming tissue, is found in the bone marrow and provides the stem cells that develop into erythrocytes, granulocytes, agranulocytes, and platelets. Red marrow is characterized by active hemato—poiesis; yellow bone marrow is inactive and contains mostly fat cells. In the human adult, hematopoiesis takes place in the mar row of the flat bones of the skull, ribs and sternum, the vertebral column, the pelvis, and the proximal ends of some long bones. Erythropoiesis is the process of RBC formation. Bone marrow stem cells (colony—forming units, CFUs) differentiate into proerythroblasts under the influence of the glycoprotein erythropoietin, which is produced by the kidney.