1. Structure of the Blood System: Hematopoietic Tissues and Hematopoietic Function

Hematology is an independent branch of medical science that takes blood and hematopoietic tissues as its primary objects of study. The blood system mainly consists of hematopoietic tissues and blood.

1. Structure of the Blood System: Hematopoietic Tissues and Hematopoietic Function
   Hematopoietic tissues refer to the tissues that produce blood cells, including bone marrow, thymus, lymph nodes, liver, spleen, as well as the hematopoietic tissues of embryos and fetuses. The sites of hematopoiesis vary at different stages, which can be divided into three phases: embryonic stage, fetal stage, and postnatal stage. Specifically, these include the mesodermal hematopoietic phase, the hepato-splenic hematopoietic phase, and the bone-marrow hematopoietic phase. The yolk sac is the earliest site of hematopoiesis during the embryonic stage. After two months, the yolk sac regresses, and the liver and spleen take over its hematopoietic function. Starting from the fourth or fifth month of fetal development, the hematopoietic functions of the liver and spleen gradually decline, while the bone marrow, thymus, and lymph nodes begin to exhibit hematopoietic activity. Even after birth, the thymus continues to perform hematopoietic functions. After puberty, the thymus gradually atrophies, and the bone marrow becomes the primary organ for hematopoiesis. When the bone marrow lacks sufficient reserve capacity, other organs outside the bone marrow (such as the liver and spleen) will step in to participate in hematopoiesis, resulting in what is known as extramedullary hematopoiesis.
2. Production of Blood Cells and Regulation of Hematopoiesis
   It is now widely recognized that all types of blood cells and immune cells originate from the same bone-marrow hematopoietic stem cell (HSC). Self-renewal and multi-directional differentiation are the two major characteristics of HSC. In addition to HSC, the production of blood cells also requires a normal hematopoietic microenvironment and the presence of both positive and negative regulatory factors for hematopoiesis. Non-hematopoietic components within the hematopoietic tissues, including the microvascular system, neural components, reticular cells, stroma, and other connective tissues, are collectively referred to as the hematopoietic microenvironment. The hematopoietic microenvironment can directly contact blood cells or release certain factors that influence or induce the production of blood cells.
   The humoral factors that regulate hematopoietic function include positive regulators that stimulate the proliferation of various progenitor cells, such as erythropoietin (EPO), colony-stimulating factor (CSF), and interleukin-3 (IL-3), as well as negative regulators for various systems, such as tumor necrosis factor (TNF-α) and interferon-γ (IFN-γ). These two types of factors mutually restrain each other, maintaining the stability of hematopoietic function within the body.
3. Classification of Blood System Diseases
   Blood system diseases refer to primary diseases (such as leukemia) or diseases that primarily affect the blood and hematopoietic organs (such as iron deficiency anemia). The classification of blood system diseases is as follows:
4. Red Blood Cell Diseases
   Such as various types of anemia and polycythemia.
5. Granulocyte Diseases
   Such as granulocytopenia, neutrophil dysmorphism, and leukemoid reactions.
6. Monocyte and Macrophage Diseases
   Such as inflammatory histiocytosis.
7. Lymphocyte and Plasma Cell Diseases
   Such as various types of lymphoma, acute and chronic lymphocytic leukemia, hemophagocytic lymphohistiocytosis, and multiple myeloma.
8. Hematopoietic Stem Cell Diseases
   Such as aplastic anemia, paroxysmal nocturnal hemoglobinuria, myelodysplastic syndromes (MDS), myeloproliferative neoplasms (MPNs), and acute myeloid leukemia (AML).
9. Splenic Hyperfunction
   Such as infectious mononucleosis, subacute infective endocarditis, miliary tuberculosis, brucellosis, schistosomiasis, kala-azar, and malaria.
10. Bleeding and Thrombotic Diseases
    Such as vascular purpura, thrombocytopenic purpura, coagulation disorders, disseminated intravascular coagulation, and thrombotic diseases.
    In addition to blood system diseases, hematology also includes transfusion medicine and hematopoietic stem cell transplantation.
11. Diagnostic Methods for Blood System Diseases
    Since blood exists in liquid form and continuously circulates throughout the body, perfusing the microcirculation of every organ, the manifestations of blood diseases are often systemic. At the same time, because blood is a complex composed of blood cells and plasma components that perform various physiological functions, and together with hematopoietic tissues forms a complete dynamic equilibrium system, the symptoms and signs of blood diseases are diverse and often lack specificity. Laboratory tests play a prominent role in the diagnosis of blood diseases. Secondary abnormalities in blood counts are more common than primary blood diseases, and almost any lesion in the body or tissues can cause changes in blood counts, even some can lead to severe or persistent abnormalities that closely resemble primary blood diseases.
    (---) Medical History Taking
    Common symptoms of blood diseases include anemia, bleeding, fever, masses, enlargement of the liver, spleen, and lymph nodes, as well as bone pain. For each patient, it is necessary to understand whether these symptoms are present and their characteristics, as well as to inquire about any history of exposure to drugs, toxins, or radioactive substances, nutritional and dietary habits, surgical history, menstrual and pregnancy history, and family history.
    (2) Physical Examination
    Whether there are changes in skin and mucous membrane color, jaundice, petechiae, nodules, or plaques; whether the tongue papillae are normal; whether there is tenderness in the sternum; whether superficial lymph nodes, liver, and spleen are enlarged; whether there are masses in the abdomen, etc.
    (3) Laboratory Tests
    (1) Complete blood count, hemoglobin measurement, and detailed observation of blood smear morphology are the most basic diagnostic methods, often reflecting pathological changes in bone-marrow hematopoiesis.
    (2) Reticulocyte count reflects the bone-marrow’s red blood cell production function.
    (3) Bone-marrow examination and cytochemical staining, including bone-marrow aspiration smears and bone-marrow biopsies, have diagnostic value for certain blood diseases (such as leukemia, myeloma, and bone-marrow fibrosis) and reference value for others (such as hyperplastic anemia). Cytochemical staining is indispensable for the differential diagnosis of acute leukemia, such as peroxidase staining, alkaline phosphatase staining, and non-specific esterase staining.
    (4) Examination for Bleeding Diseases: Basic tests include bleeding time, clotting time, prothrombin time, partial thromboplastin time, and fibrinogen quantification. Platelet aggregation and adhesion tests are used to assess platelet function, and coagulation factor tests are conducted to evaluate the activity of coagulation factors in the body.
    (5) Examination for Hemolytic Diseases: Common tests include free hemoglobin measurement, plasma-bound haptoglobin measurement, Rous test (urine iron-containing hemosiderin test), urine occult blood test (intravascular hemolysis); acid hemolysis test, sucrose hemolysis test (paroxysmal nocturnal hemoglobinuria); osmotic fragility test (hereditary spherocytosis); high hemoglobin reduction test (glucose-6-phosphate dehydrogenase deficiency in red blood cells); anti-human globulin test (autoimmune hemolytic anemia), etc., to determine the cause of hemolysis.
    (6) Biochemical and Immunological Examinations: Such as iron metabolism tests for iron deficiency anemia, and autoimmune blood diseases and lymphatic system diseases often involve abnormalities in immunoglobulins.

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