Chapter 18: Blood

Blood is a dynamic connective tissue composed of cellular and acellular components suspended in plasma that performs vital transport, regulatory, and protective functions throughout the body. This chapter examines the composition and physiology of blood, beginning with plasma constituents including water, electrolytes, proteins such as albumin and globulins, and dissolved substances like glucose and hormones that maintain osmotic pressure and facilitate nutrient distribution. Red blood cells, or erythrocytes, are explored in terms of their biconcave morphology, hemoglobin structure and function in oxygen transport, and the lifespan of approximately 120 days before removal by the reticuloendothelial system. The chapter discusses erythropoiesis, the process of red blood cell production in bone marrow regulated by erythropoietin in response to tissue oxygen availability, and describes how nutritional factors including iron, vitamin B12, and folate are essential for proper cell formation. White blood cells, or leukocytes, are categorized into granulocytes and agranulocytes, with detailed examination of their origins, functions, and roles in immune defense including neutrophil phagocytosis, lymphocyte-mediated immunity, and monocyte differentiation into macrophages. Platelets and hemostasis are explained through the cascade of events initiating blood clotting, including platelet adhesion and aggregation, intrinsic and extrinsic coagulation pathways, and fibrin formation that seals vessel wounds. The chapter addresses blood typing and transfusion compatibility based on antigen-antibody interactions, hemolytic reactions, and the clinical significance of major and minor blood group systems. Pathological conditions are integrated throughout, including anemia from various causes, polycythemia, leukemia, and coagulation disorders such as hemophilia and thrombotic disease, demonstrating how deviations from normal blood physiology compromise oxygen delivery, immune function, and hemostatic balance.