Blood: Structure, Function, and Typing

Introduction

Blood is a specialized connective tissue essential for transporting gases, nutrients, hormones, and waste products throughout the body. It also plays a critical role in immune defense and hemostasis. Understanding the cellular components and blood typing is fundamental in Anatomy & Physiology.

Components of Blood

Main Components

• Plasma: The liquid matrix of blood, making up about 55% of total blood volume. It contains water, proteins (such as albumin, globulins, and fibrinogen), electrolytes, nutrients, hormones, and waste products.

• Formed Elements: The cellular portion of blood, including erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).

Formed Elements

• Erythrocytes (Red Blood Cells, RBCs)

  • Shape: Discoid, biconcave cells.

  • Function: Transport oxygen (O2) and carbon dioxide (CO2).

  • Normal Count: 4.2 million – 6.1 million RBCs/mm3.

  • Surface Markers: Contain antigens (agglutinogens) that determine blood type.

• Leukocytes (White Blood Cells, WBCs)

  • Function: Immune defense—identify and destroy pathogens.

  • Normal Count: 4,800 – 10,800 WBCs/mm3.

  • Types: Divided into granulocytes and agranulocytes (see below).

• Thrombocytes (Platelets)

  • Function: Essential for blood clotting (hemostasis).

  • Origin: Fragments of megakaryocytes in bone marrow.

Leukocytes: Classification and Functions

Types of Leukocytes

• Granulocytes (contain visible cytoplasmic granules)

  • Neutrophils

    • Description: Multilobed nucleus, very small cytoplasmic granules, diameter 10–12 μm.

    • Function: Phagocytize and destroy bacteria.

    • Abundance: ~7,000/mm3 (most abundant WBC).

  • Eosinophils

    • Description: Bilobed nucleus, red cytoplasmic granules, diameter 10–14 μm.

    • Function: Kill parasitic worms; involved in allergy and asthma.

    • Abundance: ~100–400/mm3.

  • Basophils

    • Description: Bilobed nucleus, large purple-black granules, diameter 10–14 μm.

    • Function: Release histamine and other mediators of inflammation; contain heparin (anticoagulant).

    • Abundance: ~20–50/mm3 (least abundant WBC).

• Agranulocytes (lack visible cytoplasmic granules)

  • Lymphocytes

    • Description: Spherical or indented nucleus, diameter 5–17 μm.

    • Function: Mount immune response by direct cell attack or via antibodies.

    • Abundance: ~1,500–3,000/mm3.

  • Monocytes

    • Description: U- or kidney-shaped nucleus, abundant blue cytoplasm, diameter 14–24 μm.

    • Function: Phagocytosis; develop into macrophages in tissues.

    • Abundance: ~100–700/mm3.

Summary Table: Leukocyte Types

Blood Typing and Antigen-Antibody Reactions

Blood Types: Antigens and Antibodies

• Antigens (Agglutinogens): Proteins on the surface of RBCs that determine blood type (e.g., A, B, Rh antigens).

• Antibodies (Agglutinins): Proteins in plasma that react against specific antigens not present on the individual's own RBCs.

• Blood type is determined by the presence or absence of A and B antigens on RBCs and corresponding antibodies in plasma.

ABO Blood Group System

Rh (D) Factor

• Rh+: D antigen present on RBCs.

• Rh-: D antigen absent. Anti-D antibodies are produced only after exposure to Rh+ blood (e.g., transfusion or pregnancy).

• Clinical importance: Hemolytic disease of the newborn can occur if an Rh- mother carries an Rh+ fetus.

Blood Typing Procedure

• Mix patient blood with anti-A, anti-B, and anti-D (Rh) sera.

• Observe for agglutination (clumping), which indicates the presence of the corresponding antigen.

• Correct blood typing is critical for safe transfusions and to prevent life-threatening reactions.

Clinical Relevance: Blood Disorders and Transfusion Reactions

• Leukocytosis: Increased WBC count above normal; may indicate infection or inflammation.

• Leukopenia: Decreased WBC count; may result from certain infections (e.g., AIDS) or toxins.

• Polycythemia: Increased RBC count; may be due to cancer or physiological adaptation (e.g., high altitude, exercise).

• Anemia: Decreased RBC count or hemoglobin; causes include blood loss, inadequate erythropoiesis, or hemolysis.

• Transfusion Reactions: Occur when donor RBC antigens are attacked by recipient antibodies, leading to agglutination and hemolysis. Can be fatal if not managed promptly.

Key Rules for Blood Transfusions

• Donor RBC antigens must not react with recipient plasma antibodies.

• Universal donor: Type O- (no A, B, or Rh antigens).

• Universal recipient: Type AB+ (no anti-A, anti-B, or anti-D antibodies).

• Special consideration for Rh factor in women of childbearing age to prevent hemolytic disease of the newborn.

Summary

• Blood is composed of plasma and formed elements (RBCs, WBCs, platelets).

• Leukocytes are classified as granulocytes or agranulocytes, each with distinct functions.

• Blood typing is based on antigen-antibody interactions; correct matching is essential for safe transfusions.

• Disorders of blood cell counts can indicate underlying disease or physiological adaptation.

Additional info: Some details, such as the precise mechanisms of hemolytic disease of the newborn and the full list of plasma proteins, were inferred for completeness and academic context.