Clinical Hematology: Introduction to Blood Components and Diagnostic Procedures

Introduction to Hematology and the Nature of Blood

Blood Definition and Composition

Blood is a complex biological mixture consisting of various distinct cell types that circulate within a fluid medium known as plasma.

Primary Physiological Role

It serves as the main transport medium of the body, facilitating the movement of nutrients, gases, and waste products.

Physical and Chemical Properties

• Volume: The average human body contains approximately $5 \, L$ of blood.

• Weight: Blood accounts for roughly $8\%$ of the total body weight.

• Viscosity and Density: Blood is denser and approximately $5 imes$ more viscous than water.

• Temperature: The normal physiological temperature of blood is $37.4^{ ext{°C}}$.

• pH Level: The normal pH range of blood is slightly alkaline, maintained between $7.35 - 7.45$.

Three Main Components of Blood

• Plasma: Comprised of water, proteins, and various other solutes.

• Buffy Coat: A thin layer containing White Blood Cells (WBCs) and platelets.

• Packed Red Blood Cells (RBCs): The lowermost layer containing erythrocytes.

Blood Fractions and Clinical Deviations

Hematocrit (Hct)

• Represents the volume percentage of red blood cells in the blood.

• Normal Male Range: $42\% - 52\%$

• Normal Female Range: $37\% - 47\%$

Clinical Conditions Related to Hematocrit

• Anemia: Characterized by a depressed or low hematocrit percentage.

• Polycythemia: Characterized by an elevated or high hematocrit percentage.

Serum vs. Plasma

• Plasma ($55\% - 60\%$ of blood): The liquid portion of whole blood treated with an anticoagulant.

• Serum: The liquid portion of blood that remains after the blood has been allowed to clot. Unlike plasma, serum does not contain fibrinogen and other clotting factors used during the coagulation process.

Detailed Composition of Plasma

• Water Content: Approximately 90\ ext{ extbackslash%} of plasma is water.

• Plasma Proteins ($5-7\% - 8\%$):

  • Albumin: The primary protein responsible for maintaining osmotic concentration (oncotic pressure).

  • Globulin: Involved in acute-phase reactants (APRs) and adaptive immunity (antibodies).

  • Fibrinogen: Essential for the process of hemostasis (blood clotting).

• Regulatory Proteins ($<1 ext{ extbackslash%}$): Includes various hormones and enzymes.

• Other Solutes ($1 ext{ extbackslash%}$): Nutrients, dissolved gases ($O<em>2$, $CO</em>2$), electrolytes, metabolic waste products.

Red Blood Cells (Erythrocytes)

• Morphology: Flat, biconcave disks; they are anucleate (contain no nucleus).

• Color: Pale red or salmon pink in appearance.

• Size: Approximately $7-8 \, ext{ extmu m}$ diameter.

• Lifespan: Roughly $120$ days.

• Structure: Features a central zone of pallor that occupies about $1/3$ the cell's center, reflecting its shape.

• Function: Contains hemoglobin which facilitates transport of oxygen ($O<em>2$) and carbon dioxide ($CO</em>2$) between lungs and body tissues.

• Normal Values (NV):

  • Males: $4.2-6.0 imes 10^{12}/L$

  • Females: $3.8-5.2 imes 10^{12}/L$

Leukocytes: Granulocytes (Neutrophils, Eosinophils, Basophils)

• General Reference Value for Total WBC Count: $4.5-11.0 imes 10^9/L$

Neutrophils

• Prevalence: The most common type of leukocyte ($50\% - 70\%$ differential count).

• Nucleus: Lobulated and segmented, typically containing $3-5$ lobes.

• Cytoplasm: Fine pink-lavender-staining granules filled with bactericidal substances. Phagocytic cells that engulf and destroy microorganisms and foreign debris.

• Clinical Significance:

  • Increased (Neutrophilia): Indicates bacterial or fungal infections (sepsis) and myelogenous leukemia.

  • Decreased (Neutropenia): Bone marrow (BM) suppression or viral infection.

Eosinophils

• Count: 1 ext{ extbackslash%} - 3 ext{ extbackslash%}.

• Characteristically bilobed, with coarse orange-red proteins for immune regulation.

• Phagocytic; specifically effective against antigen-antibody complexes, and also release antihistamines.

• Significance: Increased (Eosinophilia) cases of allergies and parasitic infections.

Basophils

• Count: Least, at (0 ext{ extbackslash%} - 2 ext{ extbackslash%}). Often, the bilobed nucleus is obscured by granules.

• Dark purple, irregular, contains histamines, heparin, and cytokines.

• Expresses $IgE$ receptors on the surface and is involved in inflammatory responses.

• Clinical Significance: (Basophilia) Type I hypersensitivity, Chronic Myelogenous Leukemia (CML), and Polycythemia Vera (PV).

Agranulocytes (Lymphocytes, Monocytes)

Lymphocytes

• Count: 18 ext{ extbackslash%} - 42 ext{ extbackslash%}

• Slightly larger than RBCs, with a large purple-staining nucleus and scant, non-granular cytoplasm.

• Types: T cells, B cells, Natural Killer (NK) cells.

• Memory cells form after pathogen exposure to rapidly increase responses during subsequent exposures.

• Clinical Significance:

  • Lymphocytosis: Viral infections, Infectious Mononucleosis (IM), pertussis, toxoplasmosis, Acute/Chronic Lymphocytic (ALL/CLL).

  • Lymphocytopenia: HIV (AIDS), failure, steroids, chemotherapy.

Monocytes

• Count: 2 ext{ extbackslash%} - 11 ext{ extbackslash%}.

• Indented, horseshoe-shaped, blue-gray color, azure giving a "ground glass" appearance.

• Very phagocytic cells; serve as Antigen-Presenting (APCs) to other cells. Once they leave circulation and enter tissues, they are referred to as macrophages (the abundant cell type in the body).

• Clinical Significance: (Monocytosis) chronic autoimmune disorders, monocytic leukemias, Myelodysplastic Syndromes (MDS).

Platelets (Thrombocytes)

• Round, anucleate, slightly granular fragments.

• Small, measuring $2-4 \, ext{ extmu m}$.

• Origin: These fragments are derived from megakaryocytes situated in the marrow.

• Crucial for controlling hemostasis.

• Normal Values (NV): $150-450 \times 10^9/L$

Clinical Significance

• Increased (Thrombocytosis): Can be secondary to other conditions, or primary in Essential Thrombocythemia, Polycythemia Vera (PV), Primary Myelofibrosis (PMF), CML, and megakaryocytic leukemias.

• Decreased (Thrombocytopenia): Caused by bone marrow suppression, immune-mediated destruction, heparin-induced thrombocytopenia, thrombosis, hypersplenism, or massive transfusion.

Tests Performed in the Hematology Section

• Complete Blood Count (CBC): Comprehensive assessment of all blood cell types.

• Peripheral Blood Smear (PBS): Microscopic examination of blood cell morphology.

• Reticulocyte Count: Measure of young, immature red blood cells to assess marrow activity.

• Malarial Parasite Detection: Specialized staining and microscopy to find parasites.

• Erythrocyte Sedimentation Rate (ESR): A non-specific marker for inflammation.

• Clotting Time/Bleeding Time (CT/BT): Basic assessments of the primary and secondary hemostatic response.

• Prothrombin Time (PT): Evaluates the extrinsic and common pathways of coagulation.

• Partial Thromboplastin Time (PTT): Evaluates the intrinsic and common pathways of coagulation.

• Fibrinogen Assay: Quantifies the amount of fibrinogen available for clotting.

• D-dimer Assay: Measures fibrin degradation products to check for active clot breakdown.

• Bone Marrow Examination: Invasive procedure to view the site of blood cell production.

• Flow Cytometry Immunophenotyping: Uses lasers and antibodies to identify specific cell types based on surface markers.

• Cytogenetics and Molecular Diagnostics: Examination of chromosomal and genetic structures associated with blood disorders.