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Components of the Hematologic System
- Blood consists of a variety of components: about 90% water and 10% solutes. In adults, the total blood volume is approximately 5.5 L.
- Plasma, a complex aqueous liquid, contains two major groups of plasma proteins: (a) albumins and (b) globulins.
- The cellular elements of blood are the red blood cells (erythrocytes), white blood cells (leukocytes), and platelets.
- Erythrocytes are the most abundant cells of the blood, occupying approximately 48% of the blood volume in men and approximately 42% in women. Erythrocytes are responsibly for tissue oxygenation.
- Leukocytes are fewer in number than erythrocytes and constitutes approximately 5,000 to 10,000 cells/mm3 of blood. Leukocytes defend the body against infection and remove dead or injured host cells.
- Leukocytes are classified as either granulocytes (neutrophils, basophils, eosinophils) or agranulocytes (monocytes/macrophages, lymphocytes).
- Platelets are not cells but disk-shaped cytoplasmic fragments. Platelets are essential for blood coagulation and control of bleeding.
- The lymphoid organs are sites of residence, proliferation, differentiation, or function of lymphocytes and mononuclear phagocytes.
- The spleen is one of the largest lymphoid organs and functions as the site of fetal hematopoiesis, filters and cleanses the blood, and acts as a reservoir for lymphocytes and other blood cells.
- The lymph nodes are the site of development or activity of large numbers of lymphocytes, monocytes, and macrophages.
- The mononuclear macrophage system (MPS) is composed of monocytes in bone marrow and peripheral blood and macrophages in tissue.
- The MPS is the main line of defense against bacteria in the bloodstream and cleanses the blood by removing old, injured, or dead blood cells; antigen-antibody complexes; and macromolecules.
Development of Blood Cells
- Hematopoiesis, or blood cell production, occurs in the liver and spleen of the fetus and in the bone marrow after birth.
- Hematopoiesis involves two stages: (a) proliferation and (b) differentiation, or maturation. Each type of blood cell has parent cells called stem cells.
- Hematopoiesis continues throughout life to replace blood cells that grow old and die, are killed by disease, or are lost through bleeding.
- Bone marrow consists of blood vessels, nerves, mononuclear phagocytes, stem cells, blood cells in various stages of differentiation, and fatty tissue.
- Hemoglobin, the oxygen-carrying protein of the erythrocyte, enables the blood to transport 100 times more oxygen than could be transported dissolved in plasma alone.
- Erythropoiesis depends on the presence of vitamins (especially vitamin B12, folate vitamin, vitamin B6, riboflavin, pantothenic acid, niacin, ascorbic acid, and vitamin E).
- Regulation of erythropoiesis is mediated by erythropoietin. Erythropoietin is secreted by the kidneys in response to tissue hypoxia and causes a compensatory increase in erythrocyte production if the oxygen content of the blood decreases because of anemia, high altitude, or pulmonary disease.
- Maintenance of optimal levels of granulocytes and monocytes in the blood depends on the availability of pluripotential stem cells in the marrow, induction of these into committed stem cells, and timely release of new cells from the marrow.
- Specific humoral colony-stimulating factors (CSF-s) are necessary for the adequate growth of myeloid, erythroid, lymphoid, and megakaryocytic lineages.
- Platelets develop from megakaryocytes by a process called endomitosis. In endomitosis, the megakaryocytes undergo DNA replication but not cell division; thus, the cell does not divide into two daughter cells.
Mechanisms of Homeostasis
- Hemostasis, or arrest of bleeding, involves (a) vasoconstriction (vasospasm), (b) formation of a platelet plug, (c) activation of the clotting cascade, (d) formation of a blood clot, and (e) clot retraction and clot dissolution.
- The normal vascular endothelium prevents clotting by producing factors such as nitric oxide (NO) and prostacyclin (PGI2) that relax the vessels and prevent platelet activation.
- Lysis of blood clots is the function of the fibrinolytic system. Plasmin, a proteolytic enzyme, splits fibrin and fibrinogen into fibrin degradation products that dissolve the clot.
Pediatrics & Hematologic Value Changes
- Blood cell counts tend to rise above adult levels at birth and then decline gradually throughout childhood.
- The lymphocytes of children tend to have more cytoplasm and less compact nuclear chromatin than do the lymphocytes of adults.
Aging & Hematologic Value Changes
- Blood composition changes little with age. Erythrocyte replenishment may be delayed after bleeding, presumable because of iron deficiency.
- Lymphocyte function appears to decrease with age. Particularly affected is a decrease in cellular immunity.
- Platelet adhesiveness probably increases with age.