Chapter 9: Blood Composition, Function, & Hemostasis

Blood Composition, Function, & Hemostasis from Medical Physiology: Principles for Clinical Medicine provides an in-depth analysis of blood as a dynamic connective tissue, exploring its four primary functions of transport, hemostasis, homeostatic regulation of temperature and pH, and immunological defense. The text dissects the composition of whole blood, distinguishing between the liquid plasma matrix—rich in albumins for oncotic pressure, globulins for transport, and fibrinogen for clotting—and the formed elements, which include red blood cells, white blood cells, and platelets. Considerable attention is given to the physiology of erythrocytes, detailing their biconcave structure, the oxygen-carrying capacity of hemoglobin (including HbA and fetal hemoglobin HbF), and the regulation of erythropoiesis by the renal hormone erythropoietin in response to hypoxia. The chapter further categorizes various anemias based on morphology and cause, such as microcytic iron deficiency anemia, macrocytic pernicious anemia, and genetic hemoglobinopathies like sickle cell disease and thalassemia. The discussion on leukocytes classifies them into granulocytes (neutrophils, eosinophils, and basophils) and agranulocytes (monocytes and lymphocytes), elucidating their distinct roles in phagocytosis, parasitic defense, inflammation, and adaptive immunity. A major section is dedicated to hemostasis, outlining the four sequential steps: vascular constriction, the formation of a platelet plug via adhesion and degranulation, the coagulation cascade where intrinsic and extrinsic pathways converge to convert prothrombin to thrombin and fibrinogen to a stable fibrin clot, and finally fibrinolysis mediated by plasmin. The summary also covers essential clinical diagnostics, including the Complete Blood Count (CBC), hematocrit, metabolic panels (BMP and CMP), and coagulation profiles like Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT). Finally, the chapter explains the principles of blood typing based on ABO and Rh antigens to prevent fatal agglutination reactions during transfusions and addresses the recycling of blood components, specifically the metabolism of heme into bilirubin and the storage of iron via ferritin and transferrin.