Clinical meaning
Hematology encompasses the study of blood, blood-forming organs, and blood disorders. Blood is a specialized connective tissue composed of formed elements (red blood cells, white blood cells, platelets) suspended in plasma (the liquid component containing water, proteins, electrolytes, nutrients, hormones, and waste products). Hematopoiesis -- the production of blood cells -- occurs primarily in the red bone marrow (found in flat bones: sternum, ribs, pelvis, vertebrae, skull) from pluripotent hematopoietic stem cells that differentiate into all blood cell lineages. Red blood cells (erythrocytes) contain hemoglobin for oxygen transport; normal RBC lifespan is approximately 120 days, after which senescent cells are removed by the spleen and their iron is recycled. White blood cells (leukocytes) provide immune defense: neutrophils (60-70%, first responders to bacterial infection), lymphocytes (20-25%, adaptive immunity -- T cells and B cells), monocytes (3-8%, become tissue macrophages), eosinophils (2-4%, parasitic infections and allergic responses), and basophils (<1%, histamine release in allergic reactions). Platelets (thrombocytes) are cell fragments from megakaryocytes essential for primary hemostasis (platelet plug formation). The coagulation cascade involves a complex series of clotting factor activations (intrinsic and extrinsic pathways converging on the common pathway) that generate fibrin to stabilize the platelet plug.