![]() 18.6 Blood TypingĪntigens are nonself molecules, usually large proteins, which provoke an immune response. A thrombus is a collection of fibrin, platelets, and erythrocytes that has accumulated along the lining of a blood vessel, whereas an embolus is a thrombus that has broken free from the vessel wall and is circulating in the bloodstream. Excessive clotting, called thrombosis, can be caused by excessive numbers of platelets. Inadequate clotting can result from too few platelets, or inadequate production of clotting factors, for instance, in the genetic disorder hemophilia. They are important in limiting the extent and duration of clotting. Anticoagulants are substances that oppose coagulation. Fibrinolysis is the process in which a clot is degraded in a healing vessel. Hemostasis involves three basic steps: vascular spasm, the formation of a platelet plug, and coagulation, in which clotting factors promote the formation of a fibrin clot. Hemostasis is the physiological process by which bleeding ceases. While many platelets are stored in the spleen, others enter the circulation and are essential for hemostasis they also produce several growth factors important for repair and healing. Platelets are fragments of cells known as megakaryocytes that dwell within the bone marrow. Leukemia and lymphoma are malignancies involving leukocytes. About 20–30 percent of all leukocytes are lymphocytes, which are critical to the body’s defense against specific threats. The most abundant leukocytes are the neutrophils, which are first responders to infections, especially with bacteria. The other agranular leukocytes, NK cells, B cells, and T cells, arise from the lymphoid stem cell line. Granular leukocytes, which include neutrophils, eosinophils, and basophils, originate with myeloid stem cells, as do the agranular monocytes. They squeeze out of the walls of blood vessels through emigration or diapedesis, then may move through tissue fluid or become attached to various organs where they fight against pathogenic organisms, diseased cells, or other threats to health. Anemia is a deficiency of RBCs or hemoglobin, whereas polycythemia is an excess of RBCs. The breakdown products are recycled or removed as wastes: Globin is broken down into amino acids for synthesis of new proteins iron is stored in the liver or spleen or used by the bone marrow for production of new erythrocytes and the remnants of heme are converted into bilirubin, or other waste products that are taken up by the liver and excreted in the bile or removed by the kidneys. Worn-out erythrocytes are phagocytized by macrophages and their hemoglobin is broken down. Erythrocytes live only 120 days on average, and thus must be continually replaced. In the bloodstream, iron picks up oxygen in the lungs and drops it off in the tissues the amino acids in hemoglobin then transport carbon dioxide from the tissues back to the lungs. The hemoglobin molecule contains four globin proteins bound to a pigment molecule called heme, which contains an ion of iron. The most abundant formed elements in blood, erythrocytes are red, biconcave disks packed with an oxygen-carrying compound called hemoglobin. Hemopoietic growth factors, including erythropoietin, thrombopoietin, colony-stimulating factors, and interleukins, promote the proliferation and differentiation of formed elements. ![]() Lymphoid stem cells give rise only to the various lymphocytes designated as B and T cells, and NK cells. Myeloid stem cells give rise to most of the formed elements. Hemopoiesis begins in the red bone marrow, with hematopoietic stem cells that differentiate into myeloid and lymphoid lineages. Through the process of hemopoiesis, the formed elements of blood are continually produced, replacing the relatively short-lived erythrocytes, leukocytes, and platelets. ![]() ![]() It is also slightly alkaline, and its temperature is slightly higher than normal body temperature. Because of the formed elements and the plasma proteins and other solutes, blood is sticky and more viscous than water. The remainder is mostly plasma proteins-mainly albumin, globulins, and fibrinogen-and other dissolved solutes such as glucose, lipids, electrolytes, and dissolved gases. ![]() Blood is composed of formed elements-erythrocytes, leukocytes, and cell fragments called platelets-and a fluid extracellular matrix called plasma. Blood is a fluid connective tissue critical to the transportation of nutrients, gases, and wastes throughout the body to defend the body against infection and other threats and to the homeostatic regulation of pH, temperature, and other internal conditions. ![]()
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