Chapter 20: Cardiovascular System: Blood

Cardiovascular System: Blood on Blood, a specialized fluid connective tissue, establishes the critical functions of the cardiovascular system, including the transport and distribution of essential substances like oxygen, nutrients, hormones, and enzymes, alongside the removal and excretion of metabolic wastes. Blood also plays vital roles in maintaining stable pH and electrolyte composition throughout the body, defending tissues against pathogens and toxins, preventing fluid loss through clotting, and stabilizing body temperature via heat absorption and redistribution. Whole blood is characterized by high viscosity (about 5.0 compared to water's 1.0) and is categorized by volume states such as hypovolemic (low), normovolemic (normal), or hypervolemic (excessive). The composition of whole blood includes plasma, the liquid matrix making up about 55 percent of the volume, which is primarily water (92 percent) and contains crucial dissolved proteins. These plasma proteins are divided into three major classes: albumins (60 percent), which contribute significantly to osmotic pressure and transport lipids and steroid hormones; globulins (35 percent), which include immunoglobulins (antibodies) for immune defense and transport globulins; and fibrinogen (4 percent), which is essential for blood clotting and converts into insoluble fibrin. The remaining volume consists of formed elements, which are suspended blood cells and cell fragments. The formed elements are dominated by red blood cells (RBCs), or erythrocytes, which account for slightly less than half of the total blood volume, a proportion measured by the hematocrit. Erythrocytes possess a unique biconcave disc shape, maximizing surface area for rapid gas diffusion, and enabling them to stack into rouleaux for efficient passage through narrow capillaries. Crucially, mature RBCs lack a nucleus and mitochondria, relying on anaerobic metabolism, ensuring they do not consume the oxygen they are transporting. Over 95 percent of an RBC's proteins is hemoglobin (Hb), the red pigment responsible for binding and transporting oxygen (via iron ions within heme units) and carbon dioxide. Blood type is determined by genetically defined surface antigens (agglutinogens), A, B, and D (Rh factor), found on the RBC plasmalemmae. The plasma contains specific antibodies, or agglutinins, that attack foreign surface antigens, and exposure to foreign blood types results in a dangerous cross-reaction involving clumping (agglutination) and cell rupture (hemolysis). White blood cells (WBCs), or leukocytes, defend the body against pathogens and wastes. They exhibit mobility through vessel walls via diapedesis and attraction to chemical stimuli via chemotaxis. Leukocytes are classified as granular or agranular. Granular leukocytes include neutrophils (the most abundant, highly mobile phagocytes specializing in bacteria), eosinophils (phagocytic cells that increase during allergic reactions and parasitic infections), and basophils (rare cells that migrate to injury sites and release histamine and heparin to promote inflammation). Agranular leukocytes include monocytes (the largest WBCs, which enter tissues to become highly mobile free macrophages) and lymphocytes (the primary cells of the lymphatic system responsible for specific immunity, including T cells, B cells, and NK cells). The final formed element is platelets, which are membrane-enclosed cytoplasmic packets continuously shed by enormous bone marrow cells called megakaryocytes. Platelets are vital for hemostasis, the process that halts blood loss through vessel walls. They transport clotting chemicals, form a temporary platelet plug at injury sites, and cause clot contraction after formation. Hemopoiesis, or blood cell formation, begins with pluripotential stem cells (hemocytoblasts). Erythropoiesis (RBC formation) occurs primarily in the red bone marrow and is dramatically accelerated by erythropoietin (EPO), a hormone secreted under hypoxic (low-oxygen) conditions. EPO stimulates cell division and speeds up hemoglobin synthesis. Leukopoiesis (WBC formation) and lymphopoiesis are regulated by colony-stimulating factors (CSFs), with lymphocytes maturing in primary lymphatic organs like the bone marrow and thymus. Clinical conditions related to blood disorders include anemia (reduced oxygen-carrying capacity), polycythemia (abnormally elevated hematocrit), sickle cell anemia (inherited disorder causing rigid, curved cells), and hemophilia (inadequate clotting factors). The chapter also discusses clinical applications such as transfusions of whole blood or packed red blood cells (PRBCs) and the controversial practice of blood doping to artificially increase oxygen capacity.