Chapter 18: The Cardiovascular System: The Heart

Blood consists of two major components: plasma and formed elements. Plasma, composed primarily of water, serves as the transport medium for proteins including albumin, globulins, and fibrinogen, along with hormones, nutrients, and metabolic waste products. The formed elements comprise erythrocytes specialized for oxygen binding and transport via hemoglobin, leukocytes that serve immune and protective functions, and platelets crucial for blood clotting. The chapter details hematopoiesis, the process of blood cell formation occurring in red bone marrow, with erythropoiesis controlled by the hormone erythropoietin and progressing through multiple differentiation stages from hematopoietic stem cells to mature cells. Clinical conditions including anemia resulting from blood loss, reduced production, or accelerated destruction, and polycythemia from abnormally elevated erythrocyte counts are discussed as examples of dysfunction. Leukocytes are categorized into granulocytes—neutrophils, eosinophils, and basophils—and agranulocytes including lymphocytes and monocytes, each performing distinct roles in the immune response. The chapter explores hemostasis, the mechanism preventing blood loss through three coordinated phases: vascular constriction, formation of platelet aggregates, and activation of the coagulation cascade resulting in fibrin mesh formation. Blood typing based on surface antigens of the ABO system and Rh factor is presented with applications to transfusion compatibility and hemolytic disease of the newborn. Together these concepts establish blood's critical roles in maintaining homeostasis, delivering oxygen, defending against infection, and preventing hemorrhage.