Английский язык для медиков: конспект лекций
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Blood is considered a modified type of connective tissue. Mesoder-mal in origin, it is composed of cells and cell frag ments (erythrocytes, leukocytes, platelets), fibrous proteins (fibrinogen – fibrin during clotting), and an extracellular amorphous ground substance of fluid and proteins (plasma). Blood carries oxygen and nutrients to all cells of the body and waste materials away from cells to the kidney and lungs. It also contains cellular elements of the immune system as well as humoral factors. This chapter will discuss the differ ent elements of blood and the processes by which they are formed.
Formed elements of the blood
The formed elements of the blood include erythrocytes, leukocytes, and platelets.
Erythrocytes, or red blood cells, are important in trans porting oxygen from the lungs to tissues and in returning carbon dioxide to the lungs. Oxygen and carbon dioxide carried in the RBC combine with hemoglobin to form oxyhemoglobin and carbaminohemoglobin, respectively.
Mature erythrocytes are denucleated, biconcave disks with a diameter of 7-8 mm. The biconcave shape results in a 20-30% increase in sur face area compared to a sphere.
Erythrocytes have a very large surface area: volume ratio that allows for efficient gas transfer. Erythrocyte membranes are remarkably pliable, enabling the cells to squeeze through the narrowest capillaries. In sickle cell anemia, this plasticity is lost, and the subsequent clogging of capillaries leads to sickle crisis. The normal concentration of erythrocytes in blood is 3,5-5,5 million/mm3 in women and 4,3– 5,9 million/mm3 in men. Higher counts in men are attributed to the erythrogenic androgens. The packed volume of blood cells per total volume of known as the hematocrit. Normal hematocrit values are 46% for women and 41-53% for men.
When aging RBCs develop subtle changes, macrophages in the bone marrow, spleen, and liver engulf and digest them. The iron is carried by transferring in the blood to certain tissues, where it combines with apoferritin to form ferritin. The heme is catabolized into biliver-din, which is converted to bilirubin. The latter is secreted with bile salts.
Leukocytes, or white blood cells, are primarily with the cellular and humoral defense of the organism foreign materials. Leukocytes are classified as granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lympmonocytes).
Granulocytes are named according to the staining properties of their specific granules. Neutrophils sare 10-16 mm in diameter.
They have 3-5 nuclear lobes and contain azurophilic granules (ly-sosomes), which contain hydrolytic enzymes for bacterial destruction, in their cytoplasm. Specific granules contain bactericidal enzymes (e. g., lysozyme). Neutrophils are phagocytes that are drawn (chemo-taxis) to bacterial chemoattractants. They are the primary cells involved in the acute inflammatory response and represent 54-62% of leukocytes.
Eosinophils: they have a bilobed nucleus and possess acid granulations in their cytoplasm. These granules contain hydrolytic enzymes and peroxidase, which a discharged into phagocytic vacuoles.
Eosinophils are more numerous in the blood durii asitic infections and allergic diseases; they norma asent onlyi – 3% of leukocytes.
Basophils: they possess large spheroid granules, which are basophi-lic and metachromatic, due to heparin, a glycosaminoglycan. Their granules also contain histamine.
Basophils degranulate in certain immune reaction, releasing hepa-rin and histamine into their surroundings. They also release additional vasoactive amines and slow reacting substance of anaphylaxis (SRS-A) consisting of leukotrienes LTC4, LTD4, and LTE4. They represent less than 1% – of leukocytes.
Agranulocytes are named according to their lack of specific granules. Lymphocytes are generally small cells measuring 7-10 mm in diameter and constitute 25-33% of leukocytes. They con tain circular dark-stained nuclei and scanty clear blue cyto plasm. Circulating lymphocytes enter the blood from the lymphatic tissues. Two principal types of immunocompetent lymphocytes can be identified using im-munologic and bio chemical techniques: T lymphocytes and
T cells differentiate in the thymus and then circulate in the peripheral blood, where they are the principal effec tors of cell-mediated immunity. They also function as helper and suppressor cells, by modulating the immune response through their effect on В cells, plasma cells, macrophages, and other T Cells.
В cells differentiate in bone marrow and possibly in the gut-associated lymphatic tissues (GALT). They are the principal mediators of humoral immunity through their production of antibodies. Once activated by contact with an antigen, they differentiate into plasma cells, which synthesize antibodies that are secreted into the blood, intercellular fluid, and lymph. В lymphocytes also give rise to memory cells, which differentiate into plas ma cells only after the second exposure to the antigen. They are responsible for the secondary, or amnestic response that occurs when the body is exposed to an antigen for a second time. Monocytes vary in diameter from 15-18 mm and are the largest of the peripheral blood cells. They constitute 3-7% of leukocytes.
Monocytes possess an eccentric U-shaped or kidney-shaped nucleus. The cytoplasm has a ground-glass appearance and fine azurophi-lic granules.
Their nuclei stain lighter than lymphocyte nuclei because of their loosely arranged chromatin.
Monocytes are the precursors for members of the mononuclear phagocyte system, including tissue macrophages (histiocytes), osteoclasts, alveolar macrophages, and Kupffer cells of the liver.
Platelets (thromboplastids) are 2-3 mm in diameter.
They are a nuclear, membrane-bound cellular fragments derived by cytoplasmic fragmentation of giant cells, called megakaryocytes, in the bone marrow.
They have a short life span of approximately 10 days.
There are normally 150 000-400 000 platelets per mm3 of blood. Ultrastructurally, platelets contain two portions: a peripheral, light-staining hyalomere that sends out fine cytoplasmic processes, and a central, dark-staining granulomere that con tains mitochondria, va-cuoles, glycogen granules, and granules. Platelets seal minute breaks in blood vessels and maintain endothelial integrity by adhering to the damaged vessel in a process known as platelet aggregation. Platelets are able to form a plug at the rupture site of a vessel because their mem brane permits them to agglutinate and adhere to surfaces.