Anatomy & Physiology

Anatomy & Physiology Study Guide contains a single molecule of heme, which also has an iron atom that can reversibly bind an oxygen molecule. Damaged or dead RBCs are recycled by phagocytes. • Damaged RBCs are continuously replaced at a rate of approximately three million new RBCs entering the bloodstream per second. They are replaced before they hemolyze. • The components of hemoglobin are individually recycled; the heme is stripped of its iron and converted to biliverdin, which is converted to bilirubin. If bile ducts are blocked, bilirubin builds up in skin and eyes, resulting in jaundice. • Iron also recycled by being stored in phagocytic cells or transported through the bloodstream. • Erythropoiesis, the development of red blood cells, occurs only in red bone marrow (myeloid tissue). The process speeds up under stimulation by erythropoietin (EPO or erythropoiesis- stimulating hormone). Stages of RBC development include erythroblasts and reticulocytes. The ABO blood types and Rh system are based on antigen–antibody responses: • Blood type is determined by the presence or absence of specific surface antigens (agglutinogens) in the RBC plasma membranes: antigens A, B, and Rh (D). • Antibodies (agglutinins) in the plasma will react with RBCs bearing different surface antigens. When an antibody meets its specific surface antigen, the resulting reaction is a cross-reaction. The different types of white blood cells contribute to the body’s defenses: • White blood cells (leukocytes) have nuclei and other organelles. They defend the body against pathogens and remove toxins, wastes, and abnormal or damaged cells. • White blood cells are capable of margination, amoeboid movement, and positive chemotaxis. • Some WBCs are also capable of phagocytosis. • Granular leukocytes (granulocytes) are subdivided into neutrophils, eosinophils, and basophils. Fifty to seventy percent of circulating WBCs are neutrophils, which are highly mobile phagocytes. The much less common eosinophils are phagocytes attracted to foreign compounds that have reacted with circulating antibodies. The relatively rare basophils move to damaged tissues and release histamine and heparin, assisting with the inflammatory response. • Agranular leukocytes (agranulocytes) include monocytes and lymphocytes. Monocytes that move into peripheral tissues turn into tissue macrophages. Lymphocytes, the chief cells of the lymphatic system, include T cells (which enter peripheral tissues and attack foreign cells directly or influence the activities of other lymphocytes), B cells (which produce antibodies), and natural killer (NK) cells (which destroy abnormal tissue cells). • A differential count of the WBC population can indicate a variety of disorders. Leukemia is indicated by extreme leukocytosis—that is, excessive numbers of WBCs. • Granulocytes and monocytes are produced by myeloid stem cells in the bone marrow that divide to create progenitor cells. Lymphoid stem cells also originate in the bone marrow, but many migrate to peripheral lymphoid tissues. • Factors that regulate lymphocyte maturation are not completely understood. Several colony- stimulating factors (CSFs) are involved in regulating other WBC populations and in coordinating RBC and WBC production. Achieve Page 234 of 368 ©2018