Chapter 18-Overview of Blood

Blood Composition and Function

  • Fluid Connective Tissue: Blood is classified as a fluid connective tissue composed of cellular elements and extracellular matrix (ECM).
  • Cellular Elements:
    • Erythrocytes (Red Blood Cells - RBCs): Primary role in transporting oxygen and carbon dioxide.
    • Leukocytes (White Blood Cells - WBCs): Defend against infection.
    • Platelets: Cell fragments involved in blood clotting.
  • Plasma: Yellow liquid component that carries cellular elements and is approximately 92% water. Contains proteins, electrolytes, gases, organic nutrients, and metabolic wastes.
  • Functions of Blood:
    • Transportation: Oxygen and nutrients to cells; removal of waste like carbon dioxide; hormone transport.
    • Defense: Immune response via leukocytes against pathogens and mutated cells.
    • Homeostasis: Regulates pH, temperature, and water content in the body.

Blood Composition Analysis

  • Hematocrit Measurement: Percentage of erythrocytes in a blood sample.
    • Normal: ~45%.
    • Anemia: low hematocrit level.
    • Polycythemia: high hematocrit level.
  • Viscosity of Blood: Blood is about five times thicker than water, affecting blood pressure and flow.
  • Ideal Blood Temperature and pH:
    • Temperature: ~38°C (100.4°F).
    • pH: Average of 7.4 (range: 7.35-7.45).

Hemopoiesis - Blood Cell Production

  • Definition: Process of forming blood cells and platelets, known as hemopoiesis or hematopoiesis.
  • Location: Most occurs in red bone marrow (adult sites include cranial, pelvic bones, vertebrae).
  • Stem Cells: Hematopoietic stem cells, which differentiate into various blood cell types when stimulated by growth factors such as erythropoietin (EPO).

Erythrocytes (Red Blood Cells)

  • Characteristics:
    • Biconcave shape to increase surface area for gas exchange.
    • High hemoglobin content (300 million molecules per RBC) for oxygen transport.
    • Life span of ~120 days.
  • Function: Picks up oxygen from lungs and transports carbon dioxide from tissues back to lungs.

Leukocytes (White Blood Cells)

  • Types of Leukocytes:
    • Granular Leukocytes:
    1. Neutrophils: 50-70% of WBCs, first responders to bacterial infection, efficient at phagocytosis.
    2. Eosinophils: 1-3% of WBCs, respond to allergies and parasitic infections.
    3. Basophils: <1%, release histamines and heparin during inflammatory responses.
    • Agranular Leukocytes:
    1. Lymphocytes: Develop from lymphoid stem cells; include B cells, T cells, and natural killer cells. Function in immunity.
    2. Monocytes: Largest WBCs, differentiate into macrophages to engulf pathogens and debris.

Platelets and Hemostasis

  • Platelets (Thrombocytes): Fragments of megakaryocytes that are important for blood clotting.
    • Functions include clot formation and secretion of growth factors for tissue repair.
  • Hemostasis Process:
    1. Vascular Spasm: Constriction of blood vessel to reduce blood loss.
    2. Platelet Plug Formation: Platelets adhere to injury site and clump together, forming a temporary seal.
    3. Coagulation Cascade: Complex series of reactions leading to formation of a stable blood clot composed of fibrin, platelets, and blood cells.

Blood Typing (ABO and Rh System)

  • ABO Blood Groups: Defined by the presence of A and B antigens on erythrocytes.
    • Type A: A antigens, anti-B antibodies.
    • Type B: B antigens, anti-A antibodies.
    • Type AB: A and B antigens, no antibodies (universal recipient).
    • Type O: No antigens, anti-A and anti-B antibodies (universal donor).
  • Rh Factor: Presence (Rh positive) or absence (Rh negative) of D antigens. Important in transfusions and pregnancy; Rh negative individuals can develop anti-Rh antibodies if sensitized.
  • Universal Recipient and Donor:
    • Universal Recipient: AB positive (can receive A, B, AB, O).
    • Universal Donor: O negative (can donate to anyone without agglutination risk).